14th

International

Symposium on

Ferroic

Domains

Book of AbstractsB

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Index

Exhibitors

Committees

Foreword

Venue and Key Locations

Information for ParticipantsRegistrationOral PresentationPoster PresentationWifi

Social ProgramCultural visit to the modernist area of Hospital Sant PauConference Dinner

Schedule

List of Abstracts

Speakers Abstracts

Wednesday, September 26th

Thursday, September 27th

Friday, September 28th

Poster Abstracts

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EXHIBITORS

COMMITTEES

International Advisory Committee:

Wenwu Cao (USA)

A. Guillermo Castellanos-Guzman (Mexico)

Gustau Catalan (Spain)

Yasuo Cho (Japan)

Lukas Eng (Germany)

Jiri Hlinka (Czech Republic)

Sergei Kalinin (USA)

Wolfgang Kleemann (Germany)

Fredrik Laurell (Sweden)

Hasu Luo (China)

Xiaoqing Pan (USA)

Gil Rosenman (Israel)

Ekhard Salji

Nava Setter (Switzerland)

Yoshiaki Uesu (Japan)

Wilfried Schranz (Austria)

James F. Scott (UK)

Vladimir Shur (Russia)

Alexander Tagantsev (Switzerland)

Zuo-Guang Ye (Canada)

Yong-Yuan Zhu (China)

Marty Gregg (Northern Ireland)

Nazzanin Bhassiri-Gharb (USA)

Beatriz Noheda (Netherlands)

Organizing Committee:

Conference Chair:

Gustau Catalan Institut Català de Nanociència i Nanotecnoloia (ICN2)

Conference co-Chair and Secretary:

Neus Domingo Institut Català de Nanociència i Nanotecnologia (ICN2)

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FOREWORD

Welcome to the 14th edition of the International Symposium of Ferroic

Domains.

Since 1989, this bi-annual series of conferences has been a reference for the

science of ferroelectric domains, and a vibrant harbinger of the trends at

the cutting edge of ferroic and multiferroic research. In this 2018 edition in

Barcelona, we hope to continue success of the series. The exciting program

covers experimental and theoretical advances in domains, domain walls,

phase boundaries and other topological mobile structures, as well as

emerging materials and methods. In keeping with the dynamic spirit of this

research community, we also expect that there will be plenty of stimulating

discussions, both during the talks and in the many opportunities for informal

exchanges that will arise during the scheduled poster sessions, coffee

breaks, meals and conference dinner. Open discussions are as much part of

the program as the presentations themselves; make sure you participate!

We hope that, in addition to interesting presentations and stimulating

discussions, you will have some time to enjoy the beauty and charms of

Barcelona. The conference venue (Casa de Convalescencia) is itself part of

a UNESCO-listed building complex, a fine example of the city’s famous

“modernist” (Art Nouveau) architectural heritage. But, most of all, this is the

Mediterranean in September: enjoy the atmosphere and contribute to it.

Enjoy the conference!

Gustau Catalan and Neus Domingo

Organizing committee of the 14th ISFD

Gustau Catalan

ICN2

Neus Domingo

ICN2

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VENUE AND KEY LOCATIONS

The conference will take place a the Casa de Convalescència, located on

the historical site of the Hospital de la Santa Creu i Sant Pau and managed

by the Universitat Autònoma de Barcelona to take advantage of their

unique offer of a variety of spaces for the organisation of conferences,

conventions, meetings, seminars, courses and other events.

The conference will take place in the main Auditorium, coffee breaks,

posters and exhibitors will be placed in a room beside, and lunch will be

served in the main Restaurant of the building.

Location:

Casa Convalescència

C/ Sant Antoni Maria Claret 171

08041 Barcelona

Hospital de Sant Pau Modernist Area

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How to get there:

The conference place is within a nice walking distance from the Hotel AYRE

Roselló, and very close to three metro strations: Sant Pau i dos de Maig

or Camp de l’Arpa, and Guinardó .The entrance to the building is

located right in the corner.

AYRE HOTEL ROSSELLÓ

CASA CONVALESCENCIA

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L5 L4

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INFORMATION FOR PARTICIPANTS

Registration:

The registration desk will be open from 8:15 am of Wednesday September

26th, at the entrance of the Casa de Convalescencia. Please, come timely

to avoid queues shortly before the beginning of the lectures.

In the following days, you’ll find the secretary office located at the poster

area next to the conference room.

Oral presentations:

Oral presentation slots are 30 min. long for invited speakers (25 + 5 minutes

for discussion and change to next presentation) and 15 min for contributed

speakers (12 + 3 minutes for discussion and change to next presentation).

Session chairs will be asked to enforce these times strictly.

We strongly encourage the speakers to use the available PC and upload

their presentations from a USB stick and check their proper working at the

latest at the beginning of the coffee break preceding their session. For

morning sessions, speakers must upload their presentation files at the latest

at 8:30 am in the morning on the day of the presentation. To avoid

software compatibility problems speakers are advised to additionally bring

a PDF-version of their presentation.

Still, a projector with VGA connector will be available. In addition, a USB

and HDMI to VGA adaptor will also be available, in case your computer

lacks a VGA output. Mac users are requested please to bring an adequate

adaptor for the projector.

Poster presentations:

Poster size is A0 portrait (84 cm wide, 119 cm high). We will supply material

for mounting the posters. Posters should be mounted by Wednesday Sept.

26th, before the first coffee session at 10:30 am, and need to be

dismounted by Friday at 15:00.

Poster sessions are planned to take place all along the conference during

the morning coffee breaks, which are planed to last 45 min. This schedule is

focused to give the maximum chance of interaction between poster

presenters and other participants and extend discussions all over the

conference length.

Wifi:

Free wifi will be available for all attendants to the conference

User: isfd2018

Password: wifi2018

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SOCIAL EVENTS AND CONFERENCE DINNER

Cultural visit to the modernist area of Hospital Sant Pau:

On Thursday Sept. 27th, after the session at 18:30 we will start a guided tour

open to all conference attendants around the Modernist area of Hospital

Sant Pau, just next to the conference center. On the visit to the Art Nouveau

Site, you’ll be able to appreciate the foremost work of Lluís Domènech i

Montaner, one of the most important architects of Modernisme, the

Catalan Art Nouveau, and the product of one of the most outstanding

rehabilitation processes of recent years.

CASA CONVALESCENCIA

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Conference Dinner:

In keeping with the conference’s “Art Nouveau” architectural theme, the

closing Conference Dinner will take place at the Café de la Pedrera on

Friday Sept. 28th, at 20:30. This unique location is inside Gaudí’s world-

famous building “Casa Milà”, most commonly known as “La Pedrera”, one

of Barcelona’s most iconic buildings. We hope that the combination of

beautiful venue, excellent food and relaxed atmosphere will provide a

good send-off to the conference and good memories to all participants.

El cafè de la Pedrera

(Casa Milà, La Pedrera)

Passeig de Gràcia, 92

08008 Barcelona

CASA CONVALESCENCIA

CONFERENCEDINNER

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ISFD'14 PROGRAM Barcelona September 2018

Wednesday 26th Thursday 27th Friday 28th Domain Walls Materials& Properties Vortices and Domains

08:45 Opening/Welcome

Chair: J.F. Scott Chair: P. Paruch Chair: M. Stengel09:00 Exotic Functionality Associated with Charged Domain

Walls in Ferroelectrics

09:00 Bulk and Flexo-Photovoltaic Effects 09:00 First-priciples predictions of domain walls with tailored

functional and topological properties

M. Gregg M. Alexe J. Iñiguez

09:30 Advanced functional properties at improper

ferroelectric domain walls

09:30 Optically-Induced Polarization Switching in

MoS2/BaTiO3 heterostructures

09:30 Bubble domains and topological phase transitions in

ultrathin ferroelectric films

D.Meier A. Gruverman N. Valanoor

10:00 Anomalous Domain Wall Motion in Cu-Cl Boracite:

Negative permittivity in an improper ferroelectric?

10:00 Light-induced Reversible Control of Feroelectric

Domains

10:00 Non-Ising and Chiral ferroelectric domain walls:

insights from non-linear optical microscopy

J. G. M. Guy M-M. Yang S. Cherifi-Hertel

10:15 Electric-field poling of an improper ferroelectric 10:15 Low-Energy polar domain walls in halide perovskites 10:15 Probing the Atomic Structure and Dynamic Behavior of

Polar Vortex by Advanced Electron Microscopy

S. Krohns A. Warwick P. Chen

10:30 COFFEE BREAK (45 min) /Poster Session 10:30 COFFEE BREAK (45 min) /Poster Session 10:30 COFFEE BREAK (45 min) /Poster Session

Chair: E. Salje Chair: B. Noheda Chair: N. Bassari-Gharb11:15 Roles of oxygen vacances at multiferroic interfaces in

magnetic tunnel junctions

11:15 Imaging Current distribution near the M-I transition in

LAO/STO

11:15 Universality of Topological Vortex Domains

J. Santamaria E. Persky S.W. Cheong

11:45 Polarisation and octahedral tilting at La0.7Sr0.3MnO3-

PbTiO3 interfaces in ferroelectric tunnel junctions

11:45 In-situ imaging of electric field-induced ferroelastic

domain motion in SrTiO3

11:45 Controlling vortex-like patterns in ferroelectric-

composite films

J. Peters B. Casals J. Mangeri

12:00 Nanoscale domain clustering in tetragonal BaTiO3:

Hysteresis loops and domain switching

12:00 Enhanced ferroelectric domain wall motion in thin film

hafnia-zirconia (HZO) on inorganic flexible substrates

12:00 MFM imaging of skyrmions at room temperature

J. Očenášek A. Hsain E. Berganza

12:15 The ferroelectric domain wall phonon polarizer 12:15 DFT study of point defects at domain walls in YMnO3 12:15 Skyrmions and Bloch Walls in Ferroelectrics

M. Royo S.M. Selbach J. Hlinka

12:30 New Domain Studies in Ferroelectriccs 12:30 2D magnetic domain wall ratchet: the limit of

submicrometric holes

12:30 Configurable Domain Textures in Strain Graded

Ferroelectric Nanoplates

J.F. Scott J. Herrero-Albillos C.H. Yang

13:00

LUNCH (1 h 45 MIN)

13:00

LUNCH (1 h 45 MIN)

13:00

LUNCH (1 h 45 MIN)

Chair: I. Arias Chair: M. Alexe Chair: Z.G. Ye14:45 14:45 A rhombohedral ferroelectric phase in epitaxially-

strained Hf0.5Zr0.5O2 thin films

14:45 Seeing is believing: Watching Domain Walls @ Work

B. Noheda L. EngP. Paruch

15:15 Selective control of ferroelectric polarization in thin

films using trailing flexoelectric field

15:15 Designer defect stabilization of the super tetragonal

phase in >70-nm-thick BiFeO3 films on LaAlO3

substrates

15:15 Digital Holographic tomograph for three-dimensional

observations of domain strucures in ferroelectric single

crystals

S.M. Park Bulanadi P. Mokry

15:30 Mechanically soft domain walls in ferroelectrics 15:30 The effect of Mg-doping towards the enhancement of

the ferroelectric properties of epitaxially stabilized thin

15:30 Advanced Electron Microscopy and Spectroscopy on

Ferroic Domains

C. Stefani A. Pignolet P.Gao

15:45 Bloch lines, vortices and fast kinks: elements of

functional domain boundaries

15:45 Engineering domain and superdomain architectures in

PTO films

15:45 Domain Shapes of isolated domains in bulk uniaxial

ferroelectrics. From convex polygons to domain

E. Salje E. Langenberg V. Ya-Shur

16:15COFFEE BREAK (30min)

16:15COFFEE BREAK (30min)

16:15COFFEE BREAK (30min)

Chair: D. Meier Chair: A. Gruverman Chair: L. Eng16:45 Macroscopic polarization from antiferrodistortive

cycloids in SrTiO3

16:45 Probing domain wall orientations in ferroelectric

superlattices

16:45 Multi-scale Domain Structure and High Piezoelectricity

in PbZr1-xTixO3 (PZT)

M. Stengel P. Zubko Z.G. Ye

17:15 The role of flexoelectricity in ferroelectric domain wall

fracture

17:15 Nonlinear polarization dynamics of relaxor-PT single

crystals at cryogenic temperatures

17:15 Study of in-plane domain growth and interaction at non-

polar surfaces of Mg-LiNbO3

I. Arias L. Riemer D. Alikin

17:30 Modeling low frequency modes, electrostriction and

flexoelectricity at ferroelectric domain walls

17:30 Electromechanical response of relaxor-ferroelectric

solid solutions across the phase diagram

17:30 Domain stability at high temperatures in periodically

poled Rb-doped KTiOPO4

S. Artyurkin N. Bassari-Gharb H. Kianirad

17:45 Discretization-originated Peierls-Nabarro barriers in

phase-field simulations of ferroelectric domain walls

17:45 PinPoint Piezo Force Microscopy- frictionless imaging

technique

17:45

CLOSING REMARKS

P. Marton L. Weisser

18:30CULTURAL VISIT MODERNIST

ARCHITECTURE20:30 CONFERENCE DINNER

SESSION W I (09:00 - 10:30) SESSION Th I (09:00 - 10:30) SESSION F I (09:00 - 10:30)

SESSION W II(11:15 - 13:00) SESSION Th II (11:15 - 13:00) SESSION F II (11:15 - 13:00)

SESSION W III (15:00 - 16:15) SESSION Th III (15:00 - 16:15) SESSION F III (15:00 - 16:15)

“One sees qualities at a distance and defects at close

range”: local and macroscale effects of defects on

ferroelectric polarisation and domain walls

SESSION W IV (16:45 - 18:00) SESSION Th IV (16:45 - 18:00) SESSION F IV (16:45 - 18:00)

NAME AFFILIATION POSTER TITLE

P1 Alikin Denis Ural Federal University,

EkaterinburgFerroelectric domain mobility in BiFeO3 thin films and bulk ceramics: role of grain

boundaries and initial domain structure

P2 Arredondo Miriam Queens University Belfast Domain compatibility in polycrystalline ferroics

P3 BodnarchukYa.V. Russian Academy of Sciences,

MoscowElectron‐beam domain patterning in the uniaxial relaxor SrxBa1‐xNb2O6

P4 Brunier Alan University of Warwick Real time observation of ferroelectric switching in BiFeO3

P5 Castellanos

-Guzman

A.G. Universidad de Guadalajara Polarised-Light And Electron Microscopy Of The Static Domain Structure Of Ferroic

Co3B7O13Cl Boracite At Room Temperature

P6 Chezganov Dimitry Ural Federal University,

EkaterinburgDomain Patterning by Electron Beam Irradiation in Lithium Niobate and Lithium

Tantalate Crystals

P7 Chezganov Dimitry Ural Federal University,

EkaterinburgLocal Polarization Reversal by Electron and Ion Beam Irradiation in Relaxor SBN Single

Crystals

P8 Cochard Charlotte Queens University Belfast Anomalous Domain Wall Motion in Cu-Cl Boracite: Study of the Dynamics of the Motion

P9 Cordero-

Edwards

Kumara DQMP, University of Geneva Switchable mechanical properties of ferroelectrics due to flexoelectricity

P10 Domingo Neus ICN2 Ferroelectric Surfaces & Adsorbates

P11 Gaponenko Iaroslav DQMP, University of Geneva Local and correlated studies of humidity‐mediated ferroelectric thin film surface charge

dynamics

P12 García-

Muñoz

Jose Luis ALBA Synchrotron Light

Source, SpainCANCELLED

P13 Gautier Brice Institut des Nanotechnologies

de LyonAccurate multi-scale measurement of ferroelectric remnant polarization by nano-PUND

method

P14 Kianirad H Royal Institute of Technology,

SwedenDomain stabilities and domain wall movement in lithium tantalate studied by SEM

P15 Kim Jeong

RaeSeoul National University,

KoreaExperimental realization of atomically flat and AlO2-terminated LaAlO3 (001) substrate

surfaces

P16 Lavrov Sergey MIREA - Russian

Technological UniversityVisualization of LiNbO3 micro-dimensional domain structures by the method of

nonlinear optical microscopy.

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NAME AFFILIATION POSTER TITLE

P17 Marathe Madhura ICMAB Incommensurate phase transition in antiferroelectric materials

P18 Marton Pavel Institute of Physics, Czech

Academy of SciencesFirst-principles-originated Landau-Devonshire-potentials for ferroelectrics

P19 McConville James PV Queens University Belfast Magnetoresistance and synapse behaviour using conducting domain walls

P20 Ponet Louis Italian Institute of Technology,

Genovacoupling of electrostatic field to orbital angular momentum in FE semiconductors

P21 Rodríguez Laura ICN2 Mud-Crack-Like Strain Relaxation and Domain Patterning in Epitaxial VO2 Thin Films

P22 Santdiumenge Felip ICMAB Pretransitional Charge-Order Tweed Pattern In Strained VO2 Films

P23 Shur Vladimir Ya Ural Federal University,

EkaterinburgForward Growth of Ferroelectric Domains

P24 Spasojevic Irena ICN2 Adsorbates and surface screening at the ferroelectric oxide surfaces

P25 Suchanicz Jan Pedagogical University of

KraKowSignificant increment of the dielectric permittivity and domain properties in the

(1-x)PbTiO3-x(Na0.5Bi0.5)TiO3 crystals

P26 Valanoor Nagarajan UNSW Sydney Nonvolatile ferroelectric domain wall memory

P27 Valés Pablo ICN2 Flexoelectricity and Electrocaloric Effect in Antiferroelectrics

P28 Vlasov Eugene O. Ural Federal University,

EkaterinburgCharacterization of Periodical Domain Patterns Created by Electron Beam in MgO-doped

Lithium Niobate by Second Harmonic Generation

P29 Wang Lingfei Seoul National Univerisity Ferroelectrically tunable magnetic skyrmions in ultrathin oxide heterostructures

P30 Yun Shinhee KAIST Flexoelectric polarizations at ferroelastic domain walls in non-ferroelectric WO3 thin films

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ABSTRACTS

Exotic Functionality Associated with Charged Domain Walls in Ferroelectrics

JM Gregg

School of Mathematics and Physics, Queen’s University Belfast

Charged domain walls in ferroelectric materials, resulting from abrupt discontinuities in polarisation, are often associated with enhanced electrical conductivity. This, combined with an inherent thickness of the order of unit cells and the fact that domain walls can be moved, created and destroyed, immediately suggests a potential role for them as interconnects in completely new forms of agile or adaptive nanoscale circuitry. Moreover, the fact that conduction can be mediated by both n and p-type carriers also suggests that domain wall intersections could behave as one dimensional p-n junctions and that eventually entire active electronic devices could be created within the domain walls themselves. If this were to become possible, then completely ephemeral and dynamic nanoscale circuits could be envisaged as a new paradigm in electronics. This talk will describe some of the most recent studies on charged domain walls performed by the nanoscale ferroelectrics research group in Belfast, along with its collaborators. Firstly, new results in which Kelvin Probe Force Microscopy (KPFM) has been used to map the Hall Voltage in current carrying domain walls in ErMnO3 will be discussed, where both p and n-type current-carrying walls are simultaneously imaged and seen to converge at domain wall vertices. Secondly, anomalous charged domain wall motion in Copper-Chlorine boracites will be revealed in which polar states opposing the applied electric field grow at the expense of those aligned with the field, generating counterintuitive polarisation-field hysteresis loops and associated negative capacitance. If time allows, recent phonon scattering experiments using domain walls to control thermal conduction will also be discussed.

Advanced Functional Properties At Improper Ferroelectric Domain Walls

D. Meier 1Department of Materials Science and Engineering, Norwegian University of Science and

Technology (NTNU), Sem Sælandsvei 12, Trondheim, Norway dennis.meier@ntnu.no

Oxide materials exhibit a broad range of tunable phenomena, including magnetism, multiferroicity, and superconductivity. Oxide interfaces are particularly intriguing. The low local symmetry combined with the sensitivity to electrostatics and strain leads to unusual physical properties beyond the bulk properties [1]. Recently, ferroelectric domain walls have attracted attention as conducting and spatially mobile interfaces. In order to make use of the domain wall properties and ultimately design domain-wall-based devices and circuitry for nanotechnology, however, additional functionality beyond just conduction is required that allows the behaviour of classical electrical components to be emulated at the nanoscale.

Figure 1. A moderate gate voltage controls the electronic output at domain walls in

hexagonal manganites. The graph presents the normalized domain-wall current measured over 20 switching cycles between resistive and conducting behaviour [3].

In my talk, I will present unique features that occur at ferroelectric domain walls in

multiferroic oxides and discuss how they may be used to eventually emulate electronic components. In the first part, I will address geometrically driven charged domain walls in hexagonal manganites. For our studies, we choose the p-type semiconductor ErMnO3 as it naturally develops all fundamental types of ferroelectric domain wall at room temperature, including neutral (side-by-side) as well as negatively (tail-to-tail) and positively charged (head-to-head) wall configurations [2]. The walls are explicitly robust and, hence, represent an ideal template onto which the desired electronic behavior can be imposed. I will show how the electronic properties can be optimized and controlled, and discuss the possibility to use such walls for designing, e.g., 2D digital switches and half-wave rectifiers [3]. In the second part, I will consider domain walls in spin-spiral multiferroics with strong magnetoelectric couplings and additional functionality that arises from the interplay of charge and spin degrees of freedom. Because of the coupling, it is possible to reversibly control the configuration at ferroelectric domain walls by magnetic fields, switching between nominally charged state and neutral domain wall states [4,5]. The goal of this research is to emulate electrical components based on domain walls, bringing us one step closer to reconfigurable all-domain-wall circuits for next-generation nanotechnology.

References [1] D. Meier, J. Phys.: Condens. Matter 27, 463003 (2015) [2] D. Meier, et al., Nature Mater. 11, 284–288 (2012) [3] J. Mundy, et al., Nature Mater. 16, 622–629 (2017) [4] N. Leo, et al., Nature Commun. 6, 6661 (2015) [5] M. Matsubara, et al., Science 348, 1112–1115 (2015)

Anomalous Domain Wall Motion in Cu-Cl Boracite: Negative permittivity in an improper ferroelectric?

J. G. M. Guy1, C. Cochard1, R. W. Whatmore2, A. Kumar1, R. G. P McQuaid1, and J. M. Gregg1

1Centre for Nanostructured Media, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK

2Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ, UK

Classical ferroelectric switching is characterised by hysteresis, whereby we observe the nucleation and growth of ferroelectric domains with polarisation aligned parallel to the applied biasing field and vice versa. The boundaries delineating adjacent domains – domain walls – are now under the spotlight, in view of their enhanced functionality and reduced dimensionality; traits now firmly grounded in a decade’s worth of intense global research. If such walls, however, are to play a crucial role in future nanodevices, then we must be able to exert complete control over their spatial distribution and density.

In this work, we characterise the electric-field-induced anomalous motion of charged domain walls injected into a Cu-Cl boracite (Cu3B7O13Cl).single crystal. Unlike conventional ferroelectric switching, specific domain walls are seen to move in a manner that facilitates the growth of domains possessing polarisation components anti-aligned with the applied electric field. These walls in particular have a head-to-head polar configuration as previously reported by McQuaid et al. [1]. Such anomalous motion indicates a negative d /d and, if observed for all applied fields, points towards the development of a negative capacitance. This phenomenon was recently observed for ferroelectric materials adopting unstable configurations as part of a ferroelectric/dielectric heterostructure device format, but single-phase ferroelectric materials exhibiting negative capacitance have yet to be reported. [1] R.G.P. McQuaid, et al. Injection and controlled motion of conducting domain walls in improper ferroelectric Cu-Cl boracite. Nat. Commun. 8, 15105 (2017)

Electric-field poling of an improper ferroelectric

A. Ruff, J. Schaab, M. Fiebig, D. Meier, S. Krohns1

1Experimental Physics V, Center for Electronic Correlations and Magnetism, University of Augsburg, 86159 Augsburg, Germany

Manipulation of domains within ferroelectric semiconductors has attracted attention in recent years for potentially allowing domains and domain walls to be used as functional elements in nanoelectronics. One type of multiferroic material, hexagonal manganites, has shown particular potential because of their unusual, improper ferroelectric properties. In this contribution I show an electric-field poling study of the geometric-driven improper ferroelectric h-ErMnO3. From a detailed dielectric analysis [1], we deduce the temperature- and frequency-dependent range for which single-crystalline h-ErMnO3 exhibits purely intrinsic dielectric behavior [2]. This provides a comprehensive experimental study addressing the switching dynamics in hexagonal manganites. Controlling the domain walls via electric fields brings us an important step closer to their utilization in resource-efficient domain-wall-based electronics. [1] E. Ruff, S. Krohns, M. Lilienblum, D. Meier, M. Fiebig, P. Lunkenheimer, A. Loidl, Conductivity contrast and tunneling charge transport in the vortex-like ferroelectric domain patterns of multiferroic hexagonal YMnO3, Phys. Rev. Lett. 118, 036803 (2017). [2] A. Ruff, Z. Li, A. Loidl, J. Schaab, M. Fiebig, A. Cano, Z. Yan, E. Boureet, J. Glaum, D. Meier, S. Krohns, Frequency dependent polarisation switching in h-ErMnO3, Appl. Phys. Lett. 112, 182908 (2018).

Roles of oxygen vacancies at multiferroic interfaces in magnetic tunnel junctions.

J. Santamaria.1 1GFMC, Departamento de Física de Materiales, Universidad Complutense de Madrid, 28040

Madrid, Spain jacsan@ucm.es

Oxygen vacancies are the most common defect in perovskite oxides, yet they are difficult to detect and usually also to avoid. In nanostructures oxygen vacancies can accumulate under the action of external electric fields and yield deep changes through their ensuing strain and doping fields. Here, we exploit the dynamic control of the vacancy profile in the nanometer thick barrier of a ferroelectric tunnel junction to demonstrate the interplay between resistive (oxygen vacancy) and ferroelectric switching. Oxygen vacancies modify the stability of the ferroelectric polarization and modify the ferroelectric switching fields. I will further show that oxygen vacancies stabilize unexpected domain structures in the ferroelectric barrier which control the tunneling transport providing a major step forward towards “The Wall is the Device” concept [1]. The strong interplay between resistive and ferroelectric switching in found in multiferroic tunnel junctions highlights the coupling between electrochemical and electronic degrees of freedom at oxide interfaces triggered by the known coexistence of multiple valence states in transition metal oxides. Work done in collaboration with J. Tornos1, G. Sanchez-Santolino2, D. Hernandez-Martin1, F. Gallego1, G. Orfila1, M. Cabero1, A. Perez-Muñoz1, J. I. Beltran1, C. Munuera2, A. Rivera-Calzada1, Z. Sefrioui1, F. Mompean2, M. Garcia-Hernandez2, S. J. Pennycook3 , M. Varela1, M. C. Muñoz2, S. Valencia4, Y. H. Liu5,6, V. Lauter5, R. Abrudan4, C. Luo4, R. Hanjo4, F. Radu4 Q. Wang6, S. G. E. te Velthuis6, C. Leon1, 2 Instituto de Ciencia de Materiales de Madrid ICMM-CSIC 3 Department of Mat. Sci.& Engineering, Natnl. Univ.of Singapore, Singapore 117575. 4Hemholtz-Zentrum Berlin für Materialen und Energie, Albert-Einstein-Str.15, Berlin 5 Quantum Condensed Matter Division., Oak Ridge National Laboratory, Oak Ridge, TN37831, USA 6 Materials Science Division, Argonne Natnl. Lab. Argonne, Illinois 60439, USA . References [1] Nature Nanotechnology 12, 655 (2017)

Polarisation and Octahedral Tilting at La0.7Sr0.3MnO3-PbTiO3 Interfaces In Ferroelectric Tunnel Junctions

J. J. P. Peters1, N. Bristowe2, G. Apachitei1, R. Beanland1, M. Alexe1, A. M. Sanchez1

1 Department of Physics, University of Warwick, Coventry, United Kingdom 2 School of Physical Sciences, University of Kent, Canterbury, United Kingdom

Growth of high-quality ultrathin ferroelectric films has advanced the use of ferroelectrics in devices such as the ferroelectric tunnel junction (FTJ) which receive interest due to their potential uses as memory devices.1 Such devices are formed from a ferroelectric layer sandwiched between two electrodes where a tunnel current across the ferroelectric is modulated by the polarisation orientation. It has been demonstrated that the electrodes (and their interfaces) will have an effect on the polarisation within the ferroelectric layer due to the differences in charge screening and structure.2 In ultrathin films, this can have a significant influence on the functional properties.

Here we examine the interface effects of an La0.7Sr0.3MnO3-PbTiO3 (LSMO-PTO) interface using atomic resolution scanning transmission electron microscopy (STEM) to measure the polarisation on a unit cell basis.3 In this system, effects such as polarisation and octahedral tilt suppression can be observed locally at the interface. Supported by DFT calculations, we show how the different polarisation orientations interact with the LSMO at the interface. This provides insight into how such devices may be designed and tuned to achieve the desired performance.

Figure (a) Atomic resolution image of the LSMO-PTO layers. Out of plane profiles of the polarisation and octahedral tilt are shown in (b) and (c), respectively.

References

1. Pantel, D., Goetze, S., Hesse, D. & Alexe, M. Reversible electrical switching of spin polarization in multiferroic tunnel junctions. Nat. Mater. 11, 289–293 (2012).

2. Stengel, M., Vanderbilt, D. & Spaldin, N. A. Enhancement of ferroelectricity at metal–oxide interfaces. Nat. Mater. 8, 392–397 (2009).

3. Peters, J. J. P., Apachitei, G., Beanland, R., Alexe, M. & Sanchez, A. M. Polarization curling and flux closures in multiferroic tunnel junctions. Nat. Commun. 7, 13484 (2016).

Nanoscale domain clustering in tetragonal BaTiO3:

Hysteresis loops and domain switching

Jan Očenášek1, Edgar Abarca2, Jorge Alcalá2

1 New Technologies Research Centre, University of West Bohemia in Pilsen, 30614

Plzeň, Czech Republic 2 Department of Materials Science and Metallurgical Engineering, InSup, ETSEIB.

Universitat Politècnica de Catalunya, 08028 Barcelona, Spain

Here we report on a computational investigation of nanoscale clustering developing in

tetragonal barium titanate (BTO). We show that nanodomain development is a general

characteristic underlying the onset of electric polarization, either under externally applied

electric fields or in the absence of such fields. Nanodomain clustering is investigated at

the atomistic scale, where single nanoclusters typically encompass 30 neighboring

crystallographic cells within a 3D arrangement that depends on the polarization state.

This feature is taken to reflect upon the internal structure of the polarized domains

prevailing at greater material scales. The nanodomains are found to exhibit specific

spatiotemporal configurations, were the displacement of the central Ti atom points

towards each of the four {111} orientations of the tetragonal cell. Interestingly, it is found

that nanoclustering occurs at room temperature even though the thermal fluctuations are

greater than the characteristic offset of the Ti atom along the {111} orientations, a feature

that suggests phonon coupling. Domain switching under an externally applied electric

field is subsequently investigated. This feature is governed by nanodomain nucleation

and growth along specific orientations within the computational cell. In this context, the

modelled hysteresis loops are perfectly symmetric, owing to the absence of “frictional

resistance” between Bloch walls. The simulations are performed through a core-shell

molecular dynamics potential, which is shown to accurately reproduce the experimentally

measured piezoelectric coefficients of BTO as well as its phase diagram as a function of

pressure.

The ferroelectric domain wall phonon polarizer

M Royo1, C Escorihuela-Sayalero2, J Iñiguez2 and R Rurali1

1 Institut de Ciència de Materials de Barcelona (ICMAB{CSIC), Campus de Bellterra, 08193

Bellaterra, Barcelona, Spain

2 Materials Research and Technology Department, Luxembourg Institute of Science and

Technology, 41 rue du Brill, L-4422 Belvaux, Luxembourg

E-mail: mroyo@icmab.es

Thermal devices for applications in the fields of phononics and thermoelectrics require the

manipulation of the thermal conductivity in a specfic material. The established approach to

such control in a solid state device -the one that provides the largest technological impact- is

to modulate the amount of phonon scattering during the phonon transport process. Ferroelectric

materials offer an externally tunable and reversible mechanism for that purpose, the scattering

of phonons in ferroelectric and ferroelastic domain walls. These mobile coherent interfaces

have been shown to scatter heat-carrying phonons thus reducing the thermal conductivity more

and more as the density of domain walls is increased, both at low temperatures [1] and, more

recently, over a broad temperature range -including room temperature - thus boosting its

technological impact. [2, 3] The current challenge in order to use these _ndings in future

applications lies in understanding how the phonon transport is a_ected by domain wall

scattering. However, theoretical simulations of phonon transport in multi-domain structures are

hindered by the inherent complexity of ferroelectric lattice dynamics and the nanometer length-

scale of typical domains.

In this contribution we will present the _rst phonon transport calculations in multidomain

ferroelectrics with atomistic precision. [4] Our modeling is based on a second-principles model

potential approach [5] and a non-equilibrium Green's function calculation extended to obtain

the single-mode contribution of individual phonons to the total transmission function. [6] We

will demonstrate that thermal transport across 180 DWs formed in bulk PbTiO3 is sensitive to

the polarization of the heat carrying phonons. The propagation of transverse phonons across

the DWs is strongly suppressed by the lattice distortion in the transverse directions whereas

longitudinal phonons propagate through samples with multiple DWs without feeling them. All

in all, 180_ DWs behave as longitudinal phonon polarizers of high selectivity whose effect is

robust against deviations from the ideal at shape.

References

[1] Weilert M A, Msall M E, Anderson A C and J P Wolfe 1993 Phys. Rev. Lett. 71 735

[2] Hopkins P E, Adamo C , Ye L, Huey B D, Lee S R, Schlom D G and Ihlefeld J F 2013

Appl. Phys. Lett. 102 121903

[3] Ihlefeld J F, Foley B M, Scrymgeour D A, Michael J R, McKenzie B B, MedlinD L, Wallace

M, Trolier- McKinstry S and Hopkins P E 2015 Nano Lett. bf 15 1791

[4] Royo M, Escorihuela-Sayalero C, _I ~niguez J, Rurali R, 2017 Phys. Rev. Mat. 1 051402

[5] Wojdel J C, Hermet P, Ljungberg M P, Ghosez Ph and _I~niguez J 2013 J. Phys. Cond.

Matt. 25 305401

[6] Ong Z Y and Zhang G 2015 Phys. Rev. B 91 174302

1 of 3  

New Domain Studies in Ferroelectrics

J. F. Scott

Schools of Chemistry and Physics, University of St. Andrews, St Andrews, KY16 9ST, UK

In this talk I will describe three things: First, the electrical measurements of the power spectrum of Barkhausen pulses in PZT,[1] giving an exponent of 1.65+/-0.04 (typical data in Fig.1 below), in excellent agreement with avalanche theory (1.66) and significantly greater than the mean field value of 4/3 (the parameter J is slew rate, which is second derivative of energy with respect to time); second, the achievement of domain wall currents in BiFeO3 sufficiently large (300 nA) for commercial devices;[2,3] and third, the observation of domain instabilities that are hydrodynamic in nature and violate Landau theory.[4-6]

Fig. 1. Probability of Barkhausen voltage pulse of slew rate J = dI/dt versus slew rate (log-log) for PZT at T=293K.[1]

 

Fig. 2. Time sequence in establishing a domain wall current in BiFeO3 via 71 and 109-degree domain walls.[3]

- 2 0 - 1 5 - 1 0 - 5 0 5 1 0 1 5 2 0

- 8

- 6

- 4

- 2

0

Vc 2

I (n

A)

V ( V )

Vc 1

109o71o 71o

109o

 321

4

0o71o71o

71o

109o

109o71o

3 of 3  

Fig. 3. Larger 300 mA domain wall currents in PZT.[3]

Fig. 4. (a and c) Globular nanodomains in lead germanate [4], originally from Ph.D. thesis of A. Gruverman (Ekaterinburg 1990); (b) is a photo of similar magnetic domain evolution (this is a Richtmyer-Meshkov instability). [1] C. D. Tan, M.Sc. thesis, St. Andrews (2018); condmat arxiv (May 2018). [2] J. Jiang et al., Nat. Mater. 17, 49 (2018). [3] Z. L. Bai et al., Hierarchical domain structure and extremely large wall current in epitaxial BiFeO3 thin films, Science Advances (June 2018). [4] M. Dawber et al., J. Phys. Cond. Mat. 18, L71 (2006). [5] J. F. Scott et al., Appl. Phys. Lett. 109, 042901 (2016). [6] J. F. Scott et al., J. Phys. Cond. Mat. 29, 304001 (2017).

0 5 10 15 20

0

100

200

300

400

500

600

80 nm

200 nm

140 nm

I (

nA

)

V (V)

Vc3

0 50 100 150 2000

5

10

15

20

Vc3 (

V)

l (nm)

C  = 0o  = 90o

I  I ‐‐

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“One sees qualities at a distance and defects at close range”: local and

macroscale effects of defects on ferroelectric polarisation and domain walls

Patrycja Paruch DQMP, University of Geneva, Switzerland

In ferroelectric thin films, the intrinsic configuration, growth, shape, and stability of domains with different polarisation depend crucially on the presence of defects. Together with the electrostatic boundary conditions, defects determine the screening of depolarising fields [1], and can influence functional properties, such as local electrical conduction, in particular at domain walls [2]. More broadly, the competition between disorder pinning by the defects and the flattening effects of elastic energy of the interface leads to characteristic roughening, critical depinning, and highly non-linear sub-critical dynamics of the domain walls, with universal scaling exponents whose specific values are related to the dimensionality of the system and its elastic and disorder interactions [3]. Here, we explore the effects of defects on polarisation orientation and switching dynamics using piezoresponse force microscopy and structural and composition analysis. In PbTiO3 thin films, we modulate the built-in field and defect distribution through changes in the growth temperature, allowing full control over the intrinsic polarisation state (monodomain up vs. polydomain vs. monodomain down) and imprint, under varying electrostatic screening. Rutherford backscattering spectroscopy and x-ray diffraction measurements allow us to quantify the differences in defect density and distribution across the depth profile of the films and track their temperature dependence through the ferroelectric transition. Assigning the observed effects to gradients of Pb-O divacancy defect dipoles, we show that the resulting internal electric fields modelled in a Ginzburg-Landau-Devonshire approach agree well with the experimental results obtained by the independent real space and reciprocal space techniques. In Pb(Zr0.2Ti0.8)O3 thin films with different defect densities, grown by pulsed laser deposition and radio-frequency magnetron sputtering, we observe strikingly different nucleation-dominated vs. domain-wall-motion dominated switching dynamics. Tracking these over long-duration measurements under both constant and slowly increasing positive and negative bias, using a computer-vision-based distortion correction algorithm [4], we map with single pixel fidelity the individual domain nucleation, motion and merging events which contribute to the macroscale polarisation switching. While locally “jerky”, these individual events lead to an overall average creep-like dynamics. Statistical analysis of the size/power spectrum of the “jerks” resembles the avalanche statistics previously observed in ferromagnetic and ferroelastic switching, although with unusually large values of the characteristic exponent, possibly linked to inhomogeneous distribution of pinning centers. References 1. Lichtensteiger et al, Ferroelectricity in ultrathin-film capacitors, in Oxide Ultrathin Films, Science

and Technology, Wiley (2011); Lichtensteiger et al, NanoLett., 14, 42025 (2014); Lichtensteiger et al, New J. Phys. 18, 043030 (2016)

2. Guyonnet et al., Adv. Mat. 23, 5377 (2011); Gaponenko et al., APL 106, 162902 (2015) 3. Paruch et al., C. R. Acad. Sci. Paris 14, 667 (2013); Paruch in Lecture Notes of the Les Houches

Summer School, vol 99 (2012) 4. Gaponenko et al., Sci. Rep. 7, 669 (2017)

Selective control of ferroelectric polarization in multiferroic BiFeO3 thin

films using a trailing flexoelectric field

S. M. Park1,2, B. Wang3, S. Das1,2, L. Q. Chen3, S. M. Yang4 and T. W. Noh1,2*

1Center for Correlated Electron Systems, Institute for Basic Science (IBS), Republic of Korea. 2Department of Physics and Astronomy, Seoul National University (SNU), Republic of Korea. 3 Department of Materials Science and Engineering, The Pennsylvania State University, USA.

4 Department of Physics, Sookmyung Women’s University, Seoul 04310, Korea. While conventionally controlled by the electrical bias of a scanning probe, the polarization domain in ferroelectric thin films can also be reoriented by mechanical force through the tip by virtue of the flexoelectric effect. Although the mechanical approach for polarization switching has been demonstrated in a number of ferroelectric materials, most investigations concern only the 180° reversal of the out-of-plane polarization1. However, a systematic study is still lacking for the domain switching in more complex situations where both non-180° ferroelastic and 180° ferroelectric switching coexist. In ferroelectric materials, more than one polarization switching pathway can be an important ingredient for novel electronic devices, such as multilevel memory storage. Especially, if the ferroelectric materials possess other ferroic ordering, namely like multiferroic BiFeO3 (BFO), the multiple switching pathways can be utilized for magnetoelectric devices. However, for actual realization of those devices, deterministic control of multiple polarization switching pathways is an essential prerequisite. Although there were many efforts to selectively control the ferroelectric switching pathways and several studies have shown its possibility, typically they required additional laborious processes2,3. In our work, we show that as a new non-electrical approach, ‘trailing flexoelectric field’ offers a simple but very effective route for selective control of multiple ferroelectric switching pathways in a BFO thin film4. The simple pressing of the electrically grounded SPM tip and the control of its motion allow us to select particular domain switching only. Phase field simulations revealed the asymmetric and enhanced trailing in-plane flexoelectric field is essential for such nanoscale domain engineering. Our new finding opens a new perspective on material properties of inorganic solids and a new avenue to engineer ferroelectric domains for non-volatile magnetoelectric coupling devices and information storages. References [1] H. Lu et al. Mechanical writing of ferroelectric polarization. Science 336, 59-61 (2012). [2] S. H. Baek et al. Ferroelastic switching for nanoscale non-volatile magnetoelectric devices. Nat. Mater. 9, 309–314 (2010). [3] A. Morelli et al. Deterministic Switching in Bismuth Ferrite Nanoislands. Nano Lett. 16, 5228–5234 (2016) [4] S. M. Park et al. Selective control of multiple ferroelectric switching pathways using trailing flexoelectric field. Nat. Nanotech. 13, 366 (2018).

Mechanically soft domain walls in hard ferroelectrics

Christina Stefani,1 Kumara Cordero-Edwards,1 Gustau Catalán,1 Neus Domingo.1

1 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193

Barcelona, Spain All ferroic materials are divided into domains that are polarized in different polarization directions. The boundaries between domains, known as domain walls may possess functional properties not existent in the host material, such as conductivity in the walls of insulators, ferromagnetism in the walls of antiferromagnets, or polarization in the walls of ferroelastics. This could potentially be used to make new electronic devices at an unprecedented small scale, where the “active ingredient” are not the domains but the domain walls. Among the many properties of domain walls, mechanical response appears to have been largely neglected, and there are very few, if any, studies specifically aimed at determining the local mechanical properties of domain walls. In this presentation, we will show our first experimental measurements of the stiffness of domain walls in ferroelectric single crystal of BaTiO3 and LiNbO3 as measured by atomic force microscopy. Initially, Piezoresponse Force Microscopy (PFM) was used to image the polarized domains of the materials and consequently to identify different types of domain walls appearing in the materials. PFM is based on the detection of bias-induced surface deformation. The application of an ac voltage to the tip results in alternate expansion or contraction of the sample, depending the polarization direction of the domain, and information on the electromechanical response is extracted by imaging the amplitude and phase difference of the mechanical oscillation of the tip. While the amplitude gives information about the magnitude of the electromechanical response, the phase φ yields information about the polarization direction. Thereafter, Contact Resonance Frequency (CRF) mode was used to identify the stiffness of the domain walls. In CRF, the stiffness of the material is determined from the measured resonance frequencies of the cantilever when the tip is in contact with the sample, as these contact resonance frequencies depend on the tip sample mechanical coupling and undergo distinct shifts when the tip is scanned over areas with different stiffness. This unique mode allowed us to clearly determine the difference between domain´s and domain walls´ stiffness. The key result is that even purely ferroelectric (non-ferroelastic) 180 degree domain walls in uniaxial ferroelectrics are considerably softer than the domains they separate.  

Bloch lines, vortices and fast kinks: elements of functional domain boundaries

Ekhard Salje

University of Cambridge, Downing Street CB2 3EQ, UK

Domain boundary engineering endeavors to develop materials that contain localized functionalities inside domain walls, which do not exist in the bulk [1]. The dominant structural element is, so far, the twin boundary that generates local polarity, superconductivity, and fast ionic transport, to name just a few phenomena. Polarity is generated via the flexoelectric effect and via direct coupling between polar and non-polar order parameters (e.g. biquadratic coupling) [2]. The former determines the orientation of the polarity while the latter allows for polarity inversions in the domain boundary. As a consequence of the inversion, Bloch lines of perpendicular polarity can be created and decorate twin boundaries [3,4]. In addition, vortex structures occur next to twin walls and, in particular, between two parallel walls. Their appearance leads to wall-wall interactions, which have previously been obscure, and play a major part in the pattern formation of twinned nano-structures. These vortices are strongly dependent on external electric fields and constitute an internal instability of the polarity of complex twin patterns [5]. Vortices can induce kinks inside the walls. Computer simulations of such kinks under strain fields have shown that they are extremely mobile. Stress induced movements large exceeded the speed of sound in these materials [6]. Ref. [1]Salje, E. K. H. ChemPhysChem 11, 940-950 (2010) [2] Salje, E. K. H. et al Physical Rev. B 94, 024114 (2016) [3] Salje, E. K. H. and Scott, J. F. APL 105, 252904 (2014) [4] Zykova-Timan, T et al. APL 104, 082907 (2014) [5] Zhao, Z. et al. APL 105, 112906 (2014) [6] Salje, E. K. H. et al. Adv. Func. Mat. 27, 1700367 (2017)

Macroscopic Polarization from Antiferrodistortive Cycloids in Ferroelastic SrTiO3

 

Massimiliano Stengel

ICREA and ICMAB, Barcelona   

Ferroelastic twin walls have received considerable attention in the past few years, as they are characterized by a net dipole moment even if the parent material is nonpolar. Several models have been proposed to rationalize this observation, ranging from flexoelectricity to improper ferroelectricity, but a fundamental theory of the effect is still missing. In this talk I will first give a brief overview of the technical and conceptual challenges that one has to face to describe gradient-mediated couplings from the perspective of microscopic electronic-structure theory. Next, by using ferroelastic twins in SrTiO3 as a testcase, I will show how these challenges can be successfully overcome, leading to a physically consistent, quantitatively predictive description of domain wall-induced polarity. In particular, I will discuss two new mechanisms that crucially contribute to P: a direct "rotopolar" coupling to the gradients of the antiferrodistortive oxygen tilts, and a trilinear coupling that is mediated by the antiferroelectric displacement of the Ti atoms. Remarkably, the rotopolar coupling presents a strong analogy to the mechanism that generates a spontaneous polarization in cycloidal magnets, thereby allowing for a breakdown of macroscopic inversion symmetry (and therefore a macroscopic polarization) in a periodic sequence of parallel twins. These results open new avenues towards engineering pyroelectricity or piezoelectricity in nominally nonpolar ferroic materials.

The Role of Flexoelectricity in Ferroelectric Domain Wall Fracture 

Amir Abdollahi1 and Irene Arias1

1 LaCàN, DECA, Universitat Polit`ecnica de Catalunya irene.arias@upc.edu

Ferroelectric domains are separated by domain walls which can be seen as topological defects in the parent crystal structure of the material. Domain walls have a vital role in ferroelectrics [1], and, in particular, it has been reported that domain walls are highly susceptible to fracture, acting as a preferred path for crack propagation [2]. It is believed that structural mismatch between ferroelectric domains induces a diminishing effect on the fracture toughness. However, the origin and mechanisms of this effect is not well-understood and other possible effects have been neglected so far. One of these effects is flexoelectricity, describing the appearance of polarization in response to a strain gradient, or conversely stress induced by a polarization gradient. We show theoretically that flexoelectricity is partly responsible for the reduced domain wall fracture toughness due to the presence of large polarization gradients [3]. References [1] Y. Cao, L. Chen, and S.V. Kalinin. Appl. Phys. Lett, 110 (2017) 202903. [2] D.N. Fang, Y.J. Jiang, S. Li, and C.T. Sun. Acta Mater, 55 (2007) 5758. [3] A. Abdollahi, and I. Arias. under review, (2018) .1

Modeling Low-Frequency Modes, Electrostriction and Flexoelectricity

at Ferroelectric Domain Walls

Sergey Artyukhin1, Louis Ponet1, Peng Chen1, Gustau Catalan2, Keji Lai3, Sang Cheong4

1Italian Institute of Technology, Genova

2Institut Catala de Nanociencia i Nanotecnologia, Barcelona

3Department of Physics, University of Texas at Austin, TX 78712, USA

4Department of Physics and Astronomy, Rutgers University, NJ 08854, USA

Domain walls (DW) are emerging as a center of a new device engineering paradigm due to

their profound role in switching of ferroic orders, local chemistry, electrical conductivity. Soft

domain wall-localized modes have been recently identified as a source of loss in ferroelectrics.

I will discuss a model for impedance microscopy measurements in hexagonal manganites, that

allows to assign GHz frequency DW-localized impedance signal to a domain wall vibration.

The model calculations are supplemented by first-principles calculations, leading to a realistic

estimate of the frequency and selection rules of the DW vibration.

Dielectric loss at 71-degree DWs in BiFeO3, where a soft DW-localized mode activates due to

electrostriction even though the polarization components along the electric field in the

neighboring domains are the same. Bulk phonons coupled to domain-wall localized modes will

be discussed.

Finally, I will discuss the modification of elastic properties of a material due to the presence of

domain walls.

Discretization-originated Peierls–Nabarro Barriers in Phase-field Simulations of Ferroelectric Domain Walls

P. Marton

Institute of Physics, Czech Academy of Sciences, Na Slovance 2, Praha, Czech Republic, CZ-

182 21 Institute of Mechatronics and Computer Engineering, Technical University of Liberec, Studentská

2, Liberec, Czech Republic, CZ-461 17

(marton@fzu.c) Phase-field simulations within the framework of the Ginzburg–Landau–Devonshire (GLD) model have proven a useful tool for the investigations of domain structures in ferroelectric materials. They allow to deal with large enough spatial regions to host quite large and complicated domain configurations of interest, which are far from being accessible for more accurate atomistic or first-principles approaches. The continuous GLD model always needs to be discretized for simulations. The choice of a discretization mesh – which is usually a compromise between requirement of large supercell and decent sampling of smallest (usually domain-wall related) features appearing in simulations at the same tim – has an impact on the simulation outcomes. In particular, the chosen mesh may influence properties like local permittivities, domain wall mobilities, and critical electric fields for initiation of domain wall motion, to mention just some of them. In this contribution we analyze domain-wall-motion-related energy barriers, which originate from the regular Carthesian discretization mesh. We develop three approaches to determining the barrier height, and we test these on a specific model of BaTiO3. Finally, we show that the barrier properties for both 180 degree and 90 degree domain walls do indeed depend (and how) on the size of the discretization step and on the distance between the domain walls. The presented results aid the understanding of the responses of domain walls in simulations and can be utilized to design a proper mesh for particular purpose, allowing to avoid unwanted effects and helping to distinguish real physics in the phase-field simulations of ferroelectrics from artefacts. [1] P. Marton: Discretisation mesh-originated Peierls–Nabarro barriers in simulations of ferroelectrics, (submitted for publication in Phase Tansitions, 2018)

Bulk- and Flexo-Photovoltaic effects

Marin Alexe, Ming-Min Yang, Dong Jik Kim

University of Warwick, Department of Physics, CV4 7AL Coventry, UK Two years after the invention of modern prototype solar cells, it was found that a ferroelectric material, BaTiO3, exhibits a photovoltaic effect distinct from that of p-n junctions, later called the bulk photovoltaic (BPV) effect. Under uniform illumination, a homogeneous ferroelectric material gives rise to a current under zero bias, i.e. short-circuit current (ISC), that depends on the polarization state of the incident light, and produces an anomalously large photo-voltage well exceeding the bandgap energy. The microscopic origins of this effect are still under debate. It supposed to originate from the asymmetric distribution of photoexcited non-equilibrium carriers in k-space, caused by absence of centrosymmetry in the material. In the recent past, the entire field of photo-ferroelectrics has been revitalized by the reports of photovoltaic effect in BiFeO3 (BFO), which is a ferroelectric/multiferroic material with one of the lowest band gap and significant semiconducting properties. The present talk will firstly present a short history and the basics of the bulk photovoltaic effect, tip enhancement, as well as the electronic origin of the anomalous BPV in some materials such as BiFeO3. Later, potential applications such as energy harvesting or light-induced reversible switching of ferroelectric polarization at room temperature. I will show how the tip-enhanced effect, i.e. enhancement of the short-circuit photocurrent density at an AFM tip contact area, may be at the basis of harvesting devices with efficiency exceeding the Schokely-Quesser limit. Finally, we will discuss a new photovoltaic effect which turns the BPV effect into a universal effect allowed in all semiconductors by mediation of the flexoelectric effect. [1] References [1] M.-M. Yang D. J. Kim, & M. Alexe, Flexo-Photovoltaic Effect, Science (2018) DOI: 10.1126/science.aan3256

Optically-Induced Polarization Switching in MoS2/BaTiO3

heterostructures

A. Gruverman1, T. Li1, A. Lipatov2, H. Lu1, H. Lee3, J. Woo Lee3, Engin Torun4, L. Wirtz4,

C. B. Eom3, J. Íñiguez5, and A. Sinitskii2

1Department of Physics and Astronomy, University of Nebraska, Lincoln, NE 68588 2Department of Chemistry, University of Nebraska, Lincoln, NE 68588

3Department of Materials Science and Engineering, University of Wisconsin, Madison, WI

53706 4Physics and Materials Science Research Unit, University of Luxembourg, L-1511

Luxembourg 5Department of Materials Research and Technology, Luxembourg Institute of Science and

Technology, 5 Avenue des Hauts-Fourneaux, L-4362 Esch/Alzette, Luxembourg

Switchability of ferroelectric polarization enables control of a number of polarization-

dependent electronic, mechanical, optical, and other functional properties, and

provides a basis for development of advanced ferroelectric-based devices with

enhanced functionality. Although polarization reversal is typically realized via

application of an electric field, recently it has been shown that mechanical stress and

chemical environment can also be used as external stimuli for polarization control.

Here, we report a breakthrough finding of optically-induced switching of

polarization in the hybrid electronic structures composed of ultrathin ferroelectric

BaTiO3 and 2D molybdenum disulfide (MoS2) accompanied by a strong

electroresistance effect. This scientifically exciting and potentially technologically

important effect is attributed to the interplay between the photo-generated carriers and

screening charges at the MoS2/BaTiO3 interface, which leads to destabilization of the

existing polarization state and facilitates ferroelectric switching. Specifically, a two-

step process, which involves formation of intra-layer excitons during light absorption

followed by their decay into inter-layer excitons, results in the accumulation of the

positive charges at the interface forcing the reversal of polarization from the upward

to the downward direction. Theoretical modeling of the MoS2 optical absorption

spectra with and without the applied electric field provides quantitative support for the

proposed mechanism.

What is especially important is that the observed optical electroresistance

effect is not contingent on any specific property of the ferroelectric barrier. This

means that it should be present in any hybrid structure composed of a ferroelectric and

photo-absorbing narrow-band semiconductor. The obtained results are critical for

fundamental understanding of the mechanism of the cross-interface coupling between

the intrinsic properties of constitutive materials in hybrid ferroelectric/semiconductor

structures. These results also open a possibility for optical control of the electronic

transport in memory and logic devices composed of 2D materials and ultra-thin

ferroelectrics allowing reduced operation power.

Light-Induced Reversible Control of Ferroelectric Domains

Ming-Min Yang, Zheng-Dong Luo, Marin Alexe

Department of Physics, the University of Warwick, Coventry, CV4 7AL, UK

Email: Mingmin.Yang.1@warwick.ac.uk

Manipulation of ferroic order parameters, i.e., (anti-)ferromagnetic, ferroelectric and ferroelastic, by light at room temperature is a fascinating topic in modern solid state physics due to potential cross-fertilisation in research fields that are largely decoupled. Here we demonstrate full optical control, i.e. reversible switching, of the ferroelectric/ferroelastic domains in BiFeO3 thin films at room temperature by the mediation of the tip-enhanced photovoltaic effect. The enhanced short-circuit photocurrent density at the tip contact area generates a local electric field well exceeding the coercive field, enabling ferroelectric polarization switching. Interestingly, tailoring the photocurrent direction, via either tuning illumination geometry or simply rotating the light polarization, full control of the ferroelectric polarization is achieved. Similarly, the polarization direction in the ferroelectric tunnel junction devices can be reversibly switched by the tip-enhanced photovoltaic effect, enabling optical control of tunneling resistance. Our finding offers a new insight into the interactions between light and ferroic orders, enabling fully optical control of all the ferroic orders at room temperature and providing guidance to design novel opto-ferroic devices for data storage and sensing. Reference: *M.M. Yang, M. Alexe, Adv. Mater. 30, 1704908(2018).

Low-Energy Polar Domain Walls in Halide Perovskites

Andrew R. Warwick1, Peter D. Haynes1, Nicholas C. Bristowe2

1Faculty of Engineering, Department of Materials Science, Imperial College London, London SW7 2AZ, UK

2School of Physical Sciences, University of Kent, Canterbury CT2 7NH, UK

The perovskite methyl ammonium lead iodide (MAPbI3) has recently attracted significant interest [1] in the photovoltaic community owing to its promising power conversion efficiencies, rapidly growing upwards of 22% [2], surpassing that of some existing silicon-based devices. Despite this interest, there are still some open questions regarding the physical properties of MAPbI3, such as the role of domain walls, which appear in abundance [3]. In domain walls emergent functionalities can appear due to the change of structure at the interface. Here, we present density functional theory simulations of domains walls in the inorganic analogue caesium lead iodide, CsPbI3, which serves as a starting point towards simulating the more structurally complex MAPbI3. We find that tetragonal twins in CsPbI3 are likely to have low formation energies, in agreement with their experimental observation [3], and are strongly polar. Our simulations calculate the walls to be very thin and that caesium off-centering is strongly activated at the wall. [1] Li, W., Wang, Z., Deschler, F., Gao, S., Friend, R. and Cheetham, A. (2017). Chemically diverse and multifunctional hybrid organic–inorganic perovskites. Nature Reviews Materials, 2(3). [2] National Renewable Energy Laboratory. Chart of research cell efficiencies, 2017. Online; Accessed May 29, 2018 [3] Rothmann, M., Li, W., Zhu, Y., Bach, U., Spiccia, L., Etheridge, J. and Cheng, Y. (2017). Direct observation of intrinsic twin domains in tetragonal CH3NH3PbI3. Nature Communications, 8, p.14547.

Imaging current distributions near the metal to insulator transition in LaAlO3/SrTiO3

or

SQUID microscopy of interfaces

Eylon Persky, Naor Vardi, Beena Kalisky (Bar-Ilan)

Collaborations: Hwang group (Stanford), Caviglia group (Delft), Jonathan Ruchman (MIT)

Department of Physics and Institute of Nanotechnology and Advanced MaterialsBar-Ilan UniversityRamat Gan 5290002, Israel

Electronic properties at the interface between LaAlO3 and SrTiO3 are gate tunable. By tuning the carrier density using the gate, the interface undergoes a superconductor to insulator transition, and a metal to insulator transition.

We use scanning superconducting quantum interference device (SQUID) to track local changes to the conduction, near the metal to insulator transition. I will show that conduction landscape changes dramatically near the transition, and that the changes are governed by the crystal structure of the underlying substrate.

In-situ imaging of electric field-induced ferroelastic domain motion in

SrTiO3.

Blai Casals1*, Andrea Schiaffino1, Arianna Casiraghi2, Sampo J. Hämäläinen2, Diego

López González2, Sebastiaan van Dijken2, Massimiliano Stengel1, Gervasi Herranz1

1Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193,

Bellaterra, Catalonia, Spain 2NanoSpin, Department of Applied Physics, Aalto University School of Science, P.O.

Box 15100, FI-00076 Aalto, Finland 3ICREA - Institució Catalana de Recerca i Estudis Avançats, Pg. Lluís Companys, 23,

08010 Barcelona, Catalonia, Spain

*E-mail: blaicasals@gmail.com

Strontium titanate (SrTiO3) is the quintessential material for oxide electronics. One of

its hallmark features is the transition from cubic to tetragonal symmetry, below which

antiferrodistortive (AFD) instabilities dominate and ferroelastic twins emerge. Using

optical imaging we have investigated the evolution of the ferroelastic twin walls upon

application of an electric field. For this purpose we have exploited the magnetoelastic

coupling caused by the imprinting of ferroelastic domains into magnetostrictive films on

SrTiO3. Remarkably, we find that the dielectric anisotropy of tetragonal SrTiO3, rather

than the intrinsic domain wall polarity, is the main driving force for the motion of the

twins [1]. Based on a combined first-principles and Landau-theory analysis, we show

that such anisotropy is dominated by a trilinear coupling between the polarization, the

AFD lattice tilts, and a previously overlooked antiferroelectric (AFE) mode. We

identify the latter AFE phonon with the so-called “R mode” at ∼440 cm−1, which was

previously detected in IR experiments, but whose microscopic nature was unknown.

References

[1] B. Casals et al. Phys. Rev. Lett. 120, 217601 (2018).

Enhanced ferroelectric domain wall motion in thin film hafnia-zirconia (HZO) on

inorganic flexible substrates

Alex Hsain,1,2*, Pankaj Sharma,2 Hyeonggeun Yu,1

Franky So,1 Jan Seidel2, Jacob L. Jones1

1 Materials Science and Engineering Department, North Carolina State University, 911 Partners

Way, Raleigh, North Carolina USA 2 Materials Science and Engineering Department, University of New South Wales, E10 Gate 2

Avenue, Kensington NSW 2033, Australia *Corresponding Author: hahsain@ncsu.edu

Domain walls of ferroelectric (Hf,Zr)O2 (HZO) films are probed to elucidate the effect of film-

substrate coupling on the magnitude of piezoelectric response. HZO films are deposited by non-

rapid thermal annealing on both mechanically cyclable, flexible polyimide and rigid glass

substrates and studied using high-resolution Piezoresponse Force Microscopy (PFM).1 PFM

contact mode reveals mechanical clamping effects of film-substrates through electrically written

ferroelectric domains and domain walls, polarization electric (P-E) loops, and DC biasing. The

polarization electric field (P-E) measurements reveal that the ferroelectric characteristics of these

thin films agree with the observed switchable piezoresponse hysteresis loops as well as electrically

written, oppositely oriented domains (as seen in Figure 1 left). Our investigation of HZO

ferroelectric domain walls suggests that the piezoelectric response is heightened on HZO flexible

substrates and may originate from the mechanical releasing effects of the flexible substrate (Figure

1 right).2 The hysteresis behavior on both samples suggests that the positive piezoelectric response

of HZO is heightened by at least 30% on the flexible substrate when compared to the rigid

substrate. The enhanced magnitude of piezoresponse on HZO is likely due to the decreased biaxial

stress experienced between the polyimide substrate and HZO film which significantly enhances

domain wall motion. Our findings suggest that improving domain wall motion by depositing HZO

on mechanically released substrates such as polyimide may provide opportunities to improve

functionality of future piezoelectric elements.

Figure 1. Domains can be written reversibly onto both flexible and rigid substrates (left). The

piezoelectric response of hafnia-zirconia is heightened on flexible substrates (right).

References:

[1] Yu, H. et al. Flexible inorganic ferroelectric thin films for nonvolatile memory devices. Adv.

Funct. Mater. 27, 21, 1700461 (2017).

[2] Hsain, A.H. et al. Enhanced piezoelectricity of thin film hafnia-zirconia (HZO) by inorganic

flexible substrates. Appl. Phys. Lett. 113, 022905 (2018).

0 2 4 6 8

0

2

4

6

Response (

pm

)

DC Bias (V)

0 2 4 6 8

0

2

4

6

8

HZO_Flex

HZO_Flex

HZO_Glass

Fit

Pie

zo

resp

onse

(pm

)

DC Bias (V)

HZO_Glass

dHZO_Flexeff = 0.45 pm/V

dHZO_Glasseff = 0.25 pm/V

DFT study of point defects at domain walls in YMnO3

Sverre M. Selbach1,* Didrik R. Småbråten1, Sandra H. Skjærvø1,2,3, Thomas Tybell4 and

Dennis Meier1 1Department of Materials Science and Engineering, NTNU Norwegian University of

Science and Technology, NO-7491 Trondheim, Norway. 2ETH, Materials Theory, Wolfgang Pauli Str. 27, CH-8093 Zurich, Switzerland. 3Laboratory for Multiscale Materials Experiments (LMX), Paul Scherrer Institute, CH-

5232 Villigen PSI, Switzerland. 4Department of Electronic Systems, NTNU Norwegian University of Science and

Technology, NO-7491 Trondheim, Norway. *E-mail: selbach@ntnu.no

Understanding the domain wall (DW) mobility in ferroelectrics is key to controlling and

fine-tune the domain structure and hence the ferroelectric properties. Similarly,

controlling the DW conductivity is enabling knowledge for developing DW-based

nanoscale circuitry. The DW mobility and conductivity strongly couples to the defect

chemistry of the material. Aliovalent dopants and oxygen defects can modify the charge

carrier concentration and act as pinning centers. The overall aim of this study is to obtain

chemical guidelines from first principles calculations for how to control the DW mobility

and conductivity through defect chemistry. Improper ferroelectric YMnO3 has a complex

and exotic DW structure, including both neutral and charged head-to-head and tail-to-tail

DWs as well as topologically protected vortex cores. YMnO3 displays great chemical

flexibility, where donor and acceptor doping of both cation sublattices as well as both

oxygen deficiency and excess is possible. This makes the hexagonal manganites an ideal

model system for studying the interplay between point defects and DW properties. Using

DFT calculations, we investigate differences in DW mobility in the presence and absence

of aliovalent dopants on both cation sublattices. We extend this investigation to include

mobile oxygen vacancies and interstitials, before comparing with available experimental

data on hexagonal manganites.

2D Magnetic Domain Wall Ratchet: the Limit of Submicrometric Holes

J. Herrero-Albillosa,b,c, C. Castán-Guerrerob,c, F. Valdés-Bangod,e, J. Bartoloméb,c, F. Bartoloméb,c, F. Kronastf, A. Hierro-Rodriguezg, L. M. Álvarez Pradod,e, J. I. Martínd,e, M.

Vélezd,e, J. M. Alamedad,e, J. Seséh,c,L. M. Garcíab,c

aCentro Universitario de la Defensa, Zaragoza, (Spain) bInstituto de Ciencia de Materiales de Aragón, CSIC – Universidad de Zaragoza (Spain)

cDpto. de Física de la Materia Condensada, Universidad de Zaragoza, (Spain) dDepartamento de Física, Universidad de Oviedo ( Spain)

eCentro de Investigación en Nanomateriales y Nanotecnología, CINN (CSIC – Universidad de Oviedo), El Entrego (Spain)

fHelmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin (Germany) gIN-IFIMUP, Faculdade de Ciências, Universidade do Porto (Portugal)

hInstituto de Nanociencia de Aragón and Laboratorio de Microscopías Avanzadas, Universidad de Zaragoza, (Spain)

In the last decade, magnetic ratchet effect of domain walls has been proposed by several groups as a working principle for spintronic devices like magnetic diodes or shift registers. Indeed, some of the proposed strategies are promising candidates to be seen in the near future in working devices. As an alternative route, we have succeeded in producing and observing magnetic ratchet effects, as well as crossed-ratchet effects, in a ferromagnetic thin film where we have nanopatterned 2D arrays of asymmetric holes in the submicrometric limit for hole sizes. The combination of Kerr microscopy, X-Ray PhotoEmission Electron Microscopy and micromagnetic simulations has allowed a full magnetic characterisation of the domain wall (DW) propagation processes over the whole array and the local DW morphology and pinning at the holes. It is found that the DW dynamics in the submicrometric size limit are governed by the interplay between DW elasticity and half vortex propagation along hole edges: as hole size becomes comparable to DW width, flat DW propagation modes are favoured over kinked DW propagation due to an enhancement of DW stiffness, and pinned DW segments adopt asymmetric configurations related with Nèel DW chirality. Both ratchet and crossed-ratchet effects have been experimentally found, and we propose a new ratchet/inverted-ratchet effect in the submicrometric range driven by magnetic fields and electrical currents [1].

Figure.1 XPEEM, Kerr microscopy and micromagnetic simulations on arrays of asymmetric holes (see text and Ref. [1]). References [1] J. Herrero-Albillos et. Al. Accepted Manuscript. Materials & Design (2017)

A rhombohedral ferroelectric phase in epitaxially-strained Hf0.5Zr0.5O2 thin films

Beatriz Noheda1, Yingfen Wei1, Pavan Nukala1,2, Mart Salverda1, Sylvia Matzen3, Hong Jian Zhao4, Jamo Momand1, Arnoud S. Everhardt1, Graeme R. Blake1, Philippe

Lecoeur3, Bart J. Kooi1, Jorge Íñiguez4, Brahim Dkhil2.

1Zernike Institute for Advanced Materials, University of Groningen, 9747 AG

Groningen,The Netherlands 2Laboratoire Structures, Propriétés et Modélisation des Solides, CentraleSupélec,

CNRS-UMR8580, Université Paris-Saclay, 92295 Châtenay-Malabry, France. 3Center for Nanoscience and Nanotechnology, CNRS-UMR 9001, Université Paris-

Saclay, 91405 Orsay, France 4 Materials Research and Technology Department, Luxembourg Institute of Science

and Technology (LIST), 5 avenue des Hauts-Fourneaux, L-4362 Esch/Alzette, Luxembourg

After decades of searching for robust nanoscale ferroelectricity that could enable integration into the next generation memory and logic devices, hafnia-based thin films have appeared as the ultimate candidate because their ferroelectric (FE) polarization becomes more robust as the size is reduced. This exposes a new kind of ferroelectricity, whose mechanism still needs to be understood. Towards this end, thin films with increased crystal quality are needed. We report the epitaxial growth of Hf0.5Zr0.5O2 (HZO) thin films on (001)-oriented La0.7Sr0.3MnO3/SrTiO3 substrates. The films, which are under epitaxial compressive strain and are (111)-oriented, display large FE polarization values up to 34 μC/cm2 and do not need wake-up cycling. Structural characterization reveals a rhombohedral phase, different from the commonly reported polar orthorhombic phase. This unexpected finding allows us to propose a compelling model for the formation of the FE phase. In addition, these results point towards nanoparticles of simple oxides as a vastly unexplored class of nanoscale ferroelectrics.

The work is submitted for publication by the present authors (as Yingfen Wei et al.) and the full manuscript can be found at https://arxiv.org/abs/1801.09008

Designer Defect Stabilization of the Super Tetragonal Phase in >70-nm-

Thick BiFeO3 Films on LaAlO3 Substrates

Ralph Bulanadi1, Daniel Sando1, Thomas Young1, Xuan Cheng1,2, Yanyu Zhou1, Matthew Weyland2,3, Paul Munroe1, and Valanoor Nagarajan1

1School of Materials Science and Engineering, University of New South Wales (UNSW

Sydney), Kensington, NSW 2052, Australia 2Department of Materials Science and Engineering, Monash University, VIC 3800, Australia

3Monash Centre for Electron Microscopy, Monash University, VIC 3800, Australia Bismuth ferrite (BiFeO3 - BFO) is one of the few multiferroics (with coexisting magnetic and ferroelectric orders) that order above room temperature. In the bulk, BFO is rhombohedral R) [1], and in thin films its properties are sensitive to strain [2, 3]. The discovery of the epitaxially-stabilized "super tetragonal phase" of BFO ('T-like' BFO) [4] incited investigation of the phase transition and its possible functionalities [5]. T-BFO is also multiferroic, with large ferroelectric polarization and antiferromagnetic order, and the strain relaxation-induced T/R phase mixtures and their exceptional piezoelectric responses continue to intrigue and motivate researchers. Here, the stability of the T-like phase of BFO films grown on LaAlO3 (001) has been investigated. Two pulsed laser deposition (PLD) systems, PLD A (Pascal) and PLD B (Neocera) were used to grow films with thicknesses between 11 and 94 nm. Atomic force microscopy and x-ray diffraction (XRD) reciprocal space mapping (Fig. 1) show typical mixed-phase structures in films grown in PLD A, but no mixed-phase regions in films grown in the low incident flux conditions of PLD B. Transmission electron microscopy shows defect "pockets" throughout the films grown in PLD B, however, and further XRD analyses suggest that these defect regions apply a local compressive strain of ~1.8% that stabilizes the T-like phase beyond typical thicknesses. This hypothesis was verified by intentionally introducing amorphous BFO on the substrate surface to impose a strain gradient, which appeared to counteract the compressive strain of the defect regions and produce a mixed-phase film in PLD B. Thus, this work shows the potential to stabilize and grow >70 nm thick T-like phase BFO using these "designer defect pockets". This stabilization of this multiferroic phase may be useful in manipulating domains and domain walls and developing domain wall devices.

Fig. 1. XRD reciprocal space mapping near the 001 reflection for BFO//LaAlO3 films grown

in two growth chambers

1. Sando, D., A. Barthélémy, and M. Bibes, BiFeO3 epitaxial thin films and devices: past, present and future. Journal of Physics: Condensed Matter, 2014. 26(47): p. 473201. 2. Infante, I.C., et al., Bridging multiferroic phase transitions by epitaxial strain in BiFeO 3. Physical review letters, 2010. 105(5): p. 057601. 3. Sando, D., et al., Crafting the magnonic and spintronic response of BiFeO 3 films by epitaxial strain. Nature materials, 2013. 12(7): p. 641. 4. Béa, H., et al., Evidence for room-temperature multiferroicity in a compound with a giant axial ratio. Physical review letters, 2009. 102(21): p. 217603. 5. Sando, D., et al., A multiferroic on the brink: Uncovering the nuances of strain-induced transitions in BiFeO3. Applied Physics Reviews, 2016. 3(1): p. 011106.

The Effect of Mg-Doping Towards the Enhancement of the Ferroelectric Properties of Epitaxially Stabilized Thin Films of the Hard Magnetic Ε-

Fe2O3.

L. Corbellini1, C. Harnagea1, C. Lacroix2, D. Menard2 and A. Pignolet1

1Institut National de la Recherche Scientifique, Université du Québec Varennes, Québec, Canada J3X 1S2

2Department of Engineering Physics, Polytechnique Montréal, Montréal, Québec, Canada H3C 3A7

ε-Fe2O3 is a metastable intermediate phase of iron (III) oxide, between maghemite (γ-Fe2O3) and hematite (α-Fe2O3). Epsilon ferrite has been investigated mainly because of its ferrimagnetic ordering and, in particular, of its strong magnetocrystalline anisotropy, resulting in a coercive field as high as 2T for ε-Fe2O3 nanoparticles, the highest amongst oxides [1]. Moreover, given its orthorhombic crystal structure that belongs to the non-centrosymmetric and polar space group Pna21, it should exhibit ferroelectric behavior, as demonstrated in the isostructural gallium and aluminum ferrite (GaFeO3 and AlFeO3) [2,3]. However, unlike GaFeO3 and AlFeO3, which are characterized by magnetic Curie temperatures below room temperature, ε-Fe2O3 is characterized by a magnetic Curie temperature of circa 500 K, potentially making it one of the few room temperature multiferroic materials. Due to its metastable nature, ε-Fe2O3 needs to be stabilized at room temperature: to date such feature has been obtained mainly by synthesizing it by sol-gel as nanoparticles embedded inside a SiO2 matrix, with the stabilization mechanism being either pressure or size confinement (or both) [1]. Recently however, deposition of epitaxial thin films of ε-Fe2O3 on different single crystal substrates was demonstrated [4,5]; in this case the stabilization is thought to be due to both epitaxial strain and interface interaction between the substrate and the film. Most interestingly, both the growth on single crystal perovskites, such as SrTiO3, LaAlO3 and LSAT, and on cubic YSZ, has evidenced the formation of multiple in-plane single crystalline growth domains, with their orientations depending on the symmetry of the termination of the substrates. Measurements of macroscopic switchable polarization were reported for epitaxial thin film of epsilon ferrite grown on Nb:SrTiO3 [6]; nevertheless, as was the case for thin films of GaFeO3 and AlFeO3, the films were characterized by high leakage currents [2,3]. We report the growth by Pulsed Laser Deposition of epitaxial thin films of ε-Fe2O3 on different single crystal oxide substrates, such as SrTiO3, and YSZ, and discuss the influence of magnesium doping towards the reduction of leakage currents. As was shown for thin films of Ga0.6Fe1.4O3, leakage is due to the presence of non-completely oxidized Fe2+ ions, which results in a population of free electrons, which can then hop between the Fe3+ and the Fe2+ sites, creating low resistance paths. Magnesium doping in Ga0.6Fe1.4O3 thin films, as low as 2%, has proved to be enough to suppress the Fe3+/Fe2+ hopping mechanism and decreasing leakage by 3 order of magnitude [7,8]. Mg doping is therefore expected to similarly reduce the leakage current in the ε-Fe2O3 thin films enabling macroscopic ferroelectric measurements at room temperature. Imaging the polarization in the different growth orientation domains on the microscopic scale allows to characterize the ferroelectric domain

structure within the different growth domains and to study in which conditions they constitute single ferroelectric domains. Finally, magnesium substitution should also affect magnetism, avoiding the formation of the secondary magnetic phase magnetite (Fe3O4) that was evidenced in epitaxial thin films of epsilon ferrite [5]. References [1] S. Ohkoshi et al., Bull Chem Soc Jpn 86, 897 (2013); [2] M. Trassin et al., Appl. Phys. Lett. 91, 202504 (2007); [3] Hamasaki et al., Appl. Phys. Lett. 104, 082906 (2014); [4] M. Gich et al., Appl. Phys. Lett. 96, 112508 (2012); [5] L. Corbellini, et al., Scripta Materialia 140, 63-66 (2017); [6] M. Gich et al., Adv. Mater. (2014); [7] C. Lefevre et al., Appl. Phys. Lett. 100, 1 (2012); [8] A. Thomasson et al J. Appl. Phys. 113, 214101 (2013).

Engineering domain and superdomain architectures in PbTiO3 thin films

Eric Langenberg1, Megan Holtz1,2, Hanjong Paik1, Eva H. Smith1, Hari P. Nair1, David A.

Muller2, Gustau Catalan3, Neus Domingo3, Darrell G. Schlom1,4

1Department of Materials Science and Engineering, Cornell University, Ithaca, New York

14853, USA. 2School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853,

USA. 3Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, Barcelona Institute of

Science and Technology, Campus Universitat Autònoma de Barcelona, Bellaterra, 08193

Barcelona, Spain. 4Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA.

The engineering of nanoscale ferroelastic structures has attracted significant attention in the

last few years. These nanostructures are reconfigurable and non-volatile, making them

attractive for applications that harness the changes in electronic properties that arise at the

ferroelectric-ferroelastic domain walls or novel (nano-)electromechanical devices based on

ferroelastic switching. Here, we study the interplay between epitaxial strain, film thickness,

and electric field in the creation, modification, and design of distinct ferroelectric-ferroelastic

domain and superdomain architectures in the archetype ferroelectric PbTiO3.

PbTiO3 thin films, with thicknesses between 20 and 75 nm, were grown on SrTiO3, DyScO3,

TbScO3, and GdScO3, SmScO3 and PrScO3 substrates by reactive molecular-beam epitaxy,

spanning strains from -1.36% compressive to +1.54% tensile. X-ray diffraction and

piezoresponse force microscopy (PFM) were used to probe the evolution of the ferroelectric

domains in PbTiO3 as functions of both epitaxial strain and film thickness. In addition, the

conducting PFM tip was used to apply a dc bias voltage to assess the reconfigurability of the

ferroelastic structures and study their stability over time.

Our results show that for large compressive strain pure c-domains (polarization, P, along the

[001] direction) PbTiO3 thin films are obtained. On reducing the compressive strain, a

gradual increase in the population of a-domains (P along [100] and [010] directions, called a1

and a2 domains, respectively) embedded in a matrix of c-domains takes place, giving rise to

a/c domain architectures; the density of domain walls increases on reducing compressive

strain. For tensile strains a competing scenario of a/c and a1/a2 superdomains (ferroelastic

structures formed by individual domains) is found; the ratio between both populations can be

tuned by varying strain and thickness, enabling the superdomain architecture to be engineered

at will. Furthermore, superdomains behave as independent entities; their size dramatically

increases with thickness (reducing the superdomain wall density), in a similar fashion as

individual ferroic domains behave in ferromagnetic, ferroelectric, and ferroelastic materials.

Applying an out-of-plane dc biased voltage to the domain and superdomain patterns reveals

that the ferroelastic structures in PbTiO3 are electrically very malleable, especially when a/c

and a1/a2 superdomains coexist. For example, a vertical electric field fully converts the as-

grown superdomain architecture into a/c superdomains. Moreover, depending on the

ferroelectric switching of the individual c-domains, ordered a/c superdomains can be

achieved. The stability, however, of the electrically written a/c superdomain structures

strongly depends on strain: under low tensile strain they are stable for days, whereas at

moderate tensile strains they rapidly convert into a1/a2 superdomains—the same equilibrium

state as the as-grown films.

Probing domain wall orientations in ferroelectric superlattices

M. Hadjimichael1, E. Zatterin1,2, S. Fernandez-Pena3, S. J. Leake2, P. Zubko1

1. London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, 17-19 Gordon Street, London WC1H 0AH, UK

2. ESRF - The European Synchrotron, 71 Avenue des Martyrs, 38000, Grenoble, France

3. Department of Quantum Matter Physics, University of Geneva, CH-1211 Geneva, Switzerland

Over the last decade, nanoscale domain structures have emerged as a rich playground for discovering unexpected new behaviour and engineering new functionality in ferroic materials. In nanoscale ferroelectrics, the structure and properties of domains and domain walls can differ dramatically from their bulk counterparts, giving rise to unusual polarisation arrangements and endowing the host material with enhanced functional properties. For example, in ferroelectric-dielectric superlattices, strong depolarising fields lead to the formation of nanoscale stripe domains where the polarisation can rotate almost continuously from one domain to the next. Domain wall motion in such artificially layered materials gives rise to enhanced dielectric response with the ferroelectric layers effectively behaving as a negative capacitance. There are, however, many difficulties associated with the non-destructive characterization of such domain structures at the nanoscale, particularly at high temperatures and in buried layers, where scanning probe microscopy becomes very challenging. Techniques that extend our capability to characterise ferroelectric domain structures are therefore highly sought after.

We have employed synchrotron X-ray nanodiffraction to study ferroelectric stripe domains in PbTiO3-SrTiO3 superlattices. We have found that these stripe domains have a preferred domain wall orientation that rotates from {100} walls at low temperature to {110} walls at higher temperatures. Local measurements performed with a nanofocused beam were used to map the spatial variation of domain wall orientations across the sample, revealing a strong preferential alignment of domain walls along features associated with structural defects and ion-milled edges in the sample. Local 3D reciprocal space maps were further used to map out the spatial distribution of the out-of-plane strain and strain gradients associated with the domain wall alignment.

M. Hadjimichael, E. Zatterin, S. J. Leake, S. Fernandez-Peña, P. Zubko, Phys. Rev. Lett. 120, 037602 (2018)

Nonlinear polarization dynamics of relaxor-PT single crystals at cryogenic

temperatures L. M. Riemer1,* and D. Damjanovic1

1Group of Ferroelectrics and Functional Oxides, École polytechnique fédérale de Lausanne,

Lausanne, Switzerland *Corresponding Author: lukas.riemer@epfl.ch

Relaxor ferroelectrics are the material class of choice for high-performance electro-mechanical coupling devices today [1]. While much research has been done to clarify the mechanistic origin of their properties, the fundamental understanding of electro-mechanical coupling in this material class has remained limited. The dynamic polarization response and its contribution to electro-mechanical coupling in these materials remains one of the most discussed subjects in this research area. In the past 20 years, the higher harmonic analysis of the non-linear polarization response has been successfully used to advance the understanding of electro-mechanical coupling in ferroelectrics and relaxor ferroelectrics [2]. Recently, 50 – 80 % of the piezoelectric properties magnitude at room temperature were attributed to the presence of polar entities using cryogenic experiments [3]. Both approaches were combined in this work. Non-linear polarization dynamics of unpoled and poled relaxor-PT single crystals were analyzed along different crystallographic directions in the temperature range between 10 and 290 K. Amplitude and phase angle of the first and third harmonics of the dynamic polarization response were analyzed with respect to the driving field amplitude, the frequency and temperature. The obtained results clearly reveal different types of polarization dynamics along different crystallographic directions in poled relaxor-PT materials and elucidate their evolution with respect to driving field, frequency and temperature, contributing to the fundamental understanding of electro-mechanical coupling.

Figure 1: Phase angle of the third order harmonic of the dynamic polarization response measured along different crystallographic directions for various poled realxor-PT single crystals. a) 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 (PMN-28PT), b) 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN-33PT) and c) 0.93Pb(Zn1/3Nb2/3)O3-0.07PbTiO3 (PZN-7PT). References [1] S. Zhang, F. Li, Journal of Applied Physics, 111, 031301 (2012) [2] S. Hashemizadeh, D. Damjanovic, Applied Physics Letters, 110, 192905 (2017) [3] F. Li, S. Zhang, T. Yang, Z. Xu, N. Zhang, G. Liu. J. Wang, J. Wang, Z. Cheng, Z.G. Ye, J. Luo, T.R. Shrout, L.Q. Chen, Nature communications, Nature Communications, 7:13807 (2016)

Electromechanical Response of Relaxor-Ferroelectric Solid Solutions

Across the Phase Diagram

N. Bassiri-Gharb1,2, L.A. Griffin3, I. Gaponenko1,4, S. Brewer1, K. Williams1, S. Zhang5

1 G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta,

GA, 30332, USA 2 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA,

30332, USA 3 School of Electrical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USA 4 Department of Condensed Matter Physics, University of Geneva, CH-1211, Geneva,

Switzerland 5 Institute for Superconducting & Electronic Materials, Australian Institute of Innovative

Materials, University of Wollongong, Wollongong, NSW 2522, Australia,

Relaxor-ferroelectric (relaxor-FE) solid solution single crystals at the morphotropic phase

boundary (MPB) – when cut and poled along specific crystallographic directions – have

substantially higher piezoelectric response than ferroelectric ceramics. The origin of this high

response has been ascribed to several contributors, including the presence of a relaxor end-

member, large shear piezoelectric response, structural heterogeneities that can contribute to the

electrical and mechanical field induced phase transition, and ergodic and non-ergodic polar and

chemical nano regions (PNRs, CNRs). Due to the solid solution nature of these materials, the

cause of this large electromechanical response should be investigated at the micron and sub-

micron scale.

Here, we report a systematic probing of the local electromechanical response in lead

magnesium niobate-lead titanate solid solution, (1-x)Pb(Mg1/3Mn2/3)O3-xPbTiO3, (1-x)PMN-

xPT, single crystals for x = 0, 36 and 45%. All samples were grown with the Bridgeman method

and all measurements were performed on virgin (001) faces. Specifically, piezoresponse force

microscopy (PFM) was used in band excitation (BE) mode for local characterization of the

electromechanical response and relaxation processes. Machine learning techniques (MLT) –

principle component analysis (PCA), independent component analysis (ICA), and non-

negative matrix factorization (NMF) – were applied to determine the fundamental behaviors

and generate spatial intensity maps.

Each sample was analyzed independently to determine components and maps unique to the

composition. The samples were also analyzed as a single, aggregated dataset to determine

components present across all compositions and generate spatial intensity maps per

composition. Whether sample pulses were analyzed as a single time series or individually was

found to affect the results of these techniques, highlighting not only the necessity for human

supervision, but also the importance of physical constraint application to the otherwise physics-

agnostic MLTs. While, the resulting spatial maps correlated in part with the virgin domain

structure, areas of divergence were clearly observed. Such locations highlight non-polarization-

related contributors to the response. The domain structure, electromechanical response and

relaxation processes were further investigated through Big Data analytic techniques to identify

phenomena related to the presence of both the relaxor (PMN) and the ferroelectric (PbTiO3)

end members. It was found that the MPB composition, specifically, showed features

predominant in both other compositions, highlighting the importance of correlative studies

across multiple compositions within a solid solution series.

PinPoint Piezo Force Microscopy- frictionless imaging technique

Ludger Weisser, Wenqing Shi, Cathy Lee, John Paul Pineda, Byong Kim

Park Systems Europe GmbH, Mannheim, Germany Electromechanical coupling in materials is a key property that provides functionality to a variety of applications including: sensors, actuators, IR detectors, energy harvesting, and biology. Most materials exhibit electromechanical coupling in nanometer-sized domains. Therefore, to understand the relationships between structure and function of these materials, characterization on the nanoscale is required. This electromechanical coupling property can be directly measured in a non-destructive manner using piezoelectric force microscopy (PFM), a well-established method in atomic force microscopy (AFM). The conventional PFM is usually performed in contact mode. The topographic imaging and piezo-response measurements are obtained simultaneously. Contact mode, however, has challenges for a great variety of samples, e.g. as shown in this presentation, the characterization of an annealed phenanthrene thin film on top of an ITO surface. The main difficulty in imaging these samples is due to the phenanthrene thin film forming rod-shaped nanostructures that are very susceptible to displacement by a scanning AFM probe. This is the reason for the newly developed PinPointTM PFM mode by Park Systems. As opposed to standard contact mode, in PinPointTM mode the AFM probe monitors its feedback signal, approaches towards the sample surface until a predefined force threshold is reached, and measures the Z scanner’s height. The AFM probe is then rapidly retracted away from the surface to a user-defined height. During the data acquisition the XY scanner stops movement completely and therefore no AFM probe movement takes place on the sample surface at any time. This allows for a more accurate representation of the surface as the nanorods are not moved from their original position. When comparing conventional PFM versus the PinPointTM PFM, PinPointTM PFM shows higher detail and fewer measuring artifacts as seen in Fig. 1. Furthermore, the differences in electrical polarization are expressed as differences in PFM contrast, with the brighter areas showing a positive polarization and darker areas a negative polarization. The PinPointTM PFM shows no image distortions compared to the conventional measurement and therefore proves its superior attributes for sample characterization. In addition to That PinPointTM PFM also allows the user to access the piezo-response curves at each pixel. A representative example is shown in the inset of Fig. 1.

Figure 1a) Topography image taken with conventional PFM mode; b) PFM quad image taken with conventional PFM mode; c) Topography image taken with PinPoint PFM mode; d) PFM quad image taken with PinPointTM PFM mode. Scan size: 20μm x 20μm. Inset: A representative piezo-response curve that is recorded during PinPointTM PFM; Sample bias sweep: 0V →+1.5V → -1.5V → 0V.

First-Principles Predictions of Domain Walls with Tailored Functional and Topological Properties

Jorge Íñiguez

Materials Research and Technology Department Luxembourg Institute of Science and Technology (LIST),

5 Avenue des Hauts-Fourneaux, L-4362 Esch/Alzette, Luxembourg E-mail: jorge.iniguez@list.lu

I will present our latest theoretical predictions on how to control the properties of functional oxides, and even induce completely new behaviors, by appropriately engineering their domains or domain walls. For example, I will discuss ways to dope domain walls in ferroelastic perovskite oxides so they acquire specific properties (conductive, magnetic, ferroelectric) not present in the surrounding domains. Also, I will describe strategies to stabilize dipole arrangements with the topological features of a skyrmion. In particular, we predict that this can be done by taking advantage of the Bloch character of ferroelectric domain walls in prototype compound PbTiO3, the resulting skyrmions displaying novel features not present in their magnetic counterparts. Works done in collaboration with many colleagues, in particular H.J. Zhao, M.A.P. Gonçalves and C. Escorihuela-Sayalero (LIST); J. Junquera and P. García-Fernández (U. Cantabria). Work at LIST funded by the Luxembourg National Reserach Fund through the CORE (Grant C15/MS/10458889 NEWALLS) and AFR (Grant No. 9934186) programs.

Nanoscale Bubble Domains and Topological Transitions in Ultrathin Ferroelectric Films

Qi Zhang1, Lin Xie2,3, Guangqing Liu1, Sergei Prokhorenko4, Yousra Nahas4, Xiaoqing Pan3,

Laurent Bellaiche4, Alexei Gruverman5 and Nagarajan Valanoor1*

1. School of Materials Science and Engineering, The University of New South Wales, Sydney, New South Wales 2052, Australia

2.National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, Nanjing University, Nanjing, 210093, P. R. China

3. Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697, USA

4. Physics Department and Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, AR 72701, USA

5. Department of Physics and Astronomy, University of Nebraska, Lincoln, NE 68588, USA

Observation of nanoscale ferroelectric “bubble domains”, - laterally confined spheroids of sub-10 nm size with local dipoles self-aligned in a direction opposite to the macroscopic polarization of a surrounding ferroelectric matrix will be discussed. The bubble domains appear in ultrathin epitaxial PbZr0.2Ti0.8O3/SrTiO3/PbZr0.2Ti0.8O3 ferroelectric sandwich structures due to the interplay between charge and lattice degrees of freedom. The existence of the bubble domains is revealed by high-resolution piezoresponse force microscopy (PFM), and is corroborated by aberration-corrected atomic-resolution scanning transmission electron microscopy mapping of the polarization displacements. An incommensurate phase and symmetry breaking is found within these domains resulting in local polarization rotation , which imparts a mixed Néel-Bloch-like character to the bubble domain walls. PFM hysteresis loops for the bubble domains reveal that they undergo an irreversible phase transition to cylindrical domains under the electric field, accompanied by a transient rise in the electromechanical response. Our observations are in agreement with ab-initio-based calculations, which reveal a very narrow window of electrical and elastic parameters that allow the existence of bubble domains. The findings highlight the richness of polar topologies possible in ultrathin ferroelectric structures and bring forward the prospect of emergent functionalities due to topological transitions.

Probing the Atomic Structure and Dynamic Behavior of Polar Vortex

by Advanced Electron Microscopy

Pan Chen1,2, Adeel YounasAbid2, Yuanwei Sun2, Xiaomei Li1, Peng Gao2, Xuedong

Bai1

1Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.

2Electron Microscopy Laboratory, and International Center for Quantum Materials,

School of Physics, Peking University, Beijing 100871, China.

p-gao@pku.edu.cn; xdbai@iphy.ac.cn.

With reduced size/dimension of ferroelectrics such as quantum dots, nanowires,

thin films and supperlattices, the pronounced effects of boundary (surface and

interface) drive emergence of novel polar topological configurations of electric dipole

moments, to name a few of them, are flux-closure domain pattern, dipole wave and

vortex anti vortex structure. Here, we use advanced electron microscopy to study the

static structure and dynamic behavior of vortex structure in PbTiO3/SrTiO3

superlattice.

The intergrated differential phase contrast imaging (iDPC), which enables both

cations and oxygen be visible simutanously a single image, is used to precisely

measure the polarization distribution at subunit-cell level for individual polar vortex.

We quantitatively measure the structural parameters such as bond lengths ,atomic

displacement, in plane /out of plane lattice and subsequently the magnitude of

resultant polarization at both Pb-O and Ti-O plane in PbTiO3/SrTiO3 super lattice.

The switching dynamics of vortex is probed under electric and mechanical

excitations by in situ TEM. We find that both of stimuli can cause a transition from

vortex to a regular ferroelectric phase. Domain nucleation from interface, boundary

propagation behavior is observed and the underlying mechanism is discussed in this

talk.

Fig.1 (a) Image taken by iDPC for PbTiO3/SrTiO3 superlattice overlapped with the

polarization. (b) Schematic of in-situ setup.

Universality of Topological Vortex Domains

Sang-Wook Cheong

Rutgers Center for Emergent Materials

Rutgers, the State University of New Jersey, USA

Engineering of domains and domain boundaries is quintessential for technological

exploitation of numerous functional materials. However, it has only recently realized that the

configuration of these domains/domain boundaries can have non-trivial topology, and these

topological domain configurations are rather universal, unless long-range interaction such as

ferroelastic coupling or dipolar coupling dominates. We will discuss a new topological

classification scheme of domain/domain boundary configurations with one- or two-

dimensional order parameters: Zm×Zn domains (m directional variants and n translational

antiphases) and Zl vortices (where l number of domains and that of domain boundaries

merge). This classification, with the concept of topological protection and topological charge

conservation, has been applied to a wide range of crystallographic, ferroelectric or magnetic

domains and domain walls. We will also discuss the emergent physical properties of domain

boundaries, distinct from those of domains. The presented topological consideration provides

a basis in understanding the formation, kinetics, manipulation and property optimization of

domains/domain boundaries in complex materials.

MFM imaging of skyrmions at room temperature Eider Berganza1, Miriam Jaafar1, Maite Goiriena-Goikoetxea2,3, Javier Pablo-Navarro4,

Alfredo García-Arribas3,5, Konstantin Gusliyenko6,7, Cesar Magén4,8,9, José María de

Teresa4,8,9, Oksana Chubykalo-Fesenko1 and Agustina Asenjo1

1 Instituto de Ciencia de Materiales de Madrid, CSIC, 28049 Madrid, Spain

2 University of California, Berkeley, California, United States. 3 University of the Basque Country (UPV/EHU), Leioa, Spain. 4 LMA-INA, Universidad de Zaragoza, 50018 Zaragoza, Spain.

5 BCMaterials, Parque Tecnológico de Bizkaia, Building 500, Derio, 6 University of the Basque Country (UPV/EHU), 20018 Donostia, Spain

7 IKERBASQUE, The Basque Foundation for Science, 48013 Bilbao, Spain 8 ICMA-CSIC, Universidad de Zaragoza, 50009 Zaragoza, Spain.

9 Universidad de Zaragoza, 50009 Zaragoza, Spain.

aasenjo@icmm.csic.es

Magnetic skyrmions [1] are topologically protected inhomogeneous spin textures on the

nanoscale, currently widely investigated due to their unusual fundamental properties and their

potential applications in memory storage devices or spintronics. With their small sizes and high

mobility under low current densities, skyrmions are envisaged as information carriers in the

racetrack memory type devices. In this sense, control over the formation and manipulation of

skyrmions , particularly in confined geometries [2], becomes of uttermost importance [3].

Most of the works in the literature report on skyrmions in systems with Dzyaloshinskii–Moriya

exchange interaction (DMI) either in ultra-thin multilayers of transition metals and materials

with strong spin-orbit coupling [4]. Moreover, non-chiral Bloch skyrmions (or bubble

domains) can be stabilized in films with out-of-plane magnetic anisotropy by the dipolar

interactions without the need of DMI. Nobody so far assumed that stabilization of the Néel

skyrmions in confined systems without magnetic anisotropy is possible. In this work, we

studied permalloy (Py) sub-100 nm diameter particles (nanodots) with no perpendicular

uniaxial magnetic anisotropy or DMI. Due to the low magnetocrystalline anisotropy, vortex or

a single domain (with in-plane magnetization) state is expected [5,6].

However, Néel (hedgehog) skyrmion spin textures were detected in the present study through

Magnetic Force Microscopy. The magnetization configuration of these Néel (hedgehog)

skyrmions [7] is characterized by spins rotating in radial planes from their cores to the

boundaries, in contrast to Bloch skyrmions, where spins form a close-flux. The evolution of

the magnetic configuration under external applied in-plane [8] fields leaves no doubt of the

existence of hedgehog skyrmions in such Py dots. Analytical calculations showed that these

magnetization textures are metastable high-energetic states and micromagnetic simulations

confirmed this fact.

[1] T. H. R. Skyrme, Nucl. Phys. 31, 556–569 (1962)

[2] F. Büttner, et al., Nat. Phys. 11, 225–228 (2015)

[3] N. Romming et al., Science 80, 341, 636–639 (2013)

[4].J. Sampaio et al. Nat. Nanotechnol. 8, 839–844 (2013)

[5] M. Goiriena-Goikoetxea et al. Nanoscale 9, 11269–11278 (2017)

[6] K. Y. Guslienko & V. Novosad, J. Appl. Phys. 96, 4451–4455 (2004)

[7] I. Kezsmarki et al., Nat. Mater. 14, 1116–1122 (2015).

[8] M. Jaafar, et al., Ultramicroscopy 109, 693–699 (2009)

Skyrmions and Bloch Walls in Ferroelectrics

J. Hlinka

Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221 Prague

8, Czech Republic

hlinka@fzu.cz

Are there Skyrmions[1] and Bloch walls[1] in the readily studied ferroelectrics or

relaxor-like materials with morphotropic phase boundaries, or do we need to search

elsewhere? Are they topologically protected? How much macroscopic material

properties can be influenced by these nontrivial topological defects and how these

defects can be possibly exploited or addressed individually?

Our understanding to the inner structure of ferroelectric domain walls and

spontaneously forming line defects in triaxial ferroelectrics will be illustrated using

the few relatively well studied systems [1-8 and references therein]. It will be argued

that any convenient definition of a ferroelectric Bloch wall should contain the

requirement of the domain wall symmetry lowering. Recent results of

phenomenological theory, atomistic modeling as well as the state-of-art density

functional theory calculations of ferroelectric domain walls will be summarized. It

will be concluded that there are several cases, where the Bloch walls are energetically

more favorable than their Ising-like counterparts, and first experimental indications of

ferroelectric Bloch walls will be discussed.

Finally, we shall attempt to briefly summarize most recent studies of magnetic

Skyrmions and then to address the question of the current progress and prospects in

investigations of Skyrmions in ferroelectric materials.

References

[1] J. Seidel, Topological Structures in Ferroic Materials. Springer Series in Materials

Science, vol. 228, (2016).

[2] V. Stepkova, P. Marton, J. Hlinka, Phys. Rev. B 92, 094106 (2015) .

[3] V. Stepkova, J. Hlinka, Phase Transit. 90, 11 (2017) .

[4] J. Hlinka, M. Pasciak, S. Körbel, P. Márton, Phys. Rev. Lett. 119, 057604

(2017) .

[5] P. Márton, A. Klíc, M. Pasciak, J. Hlinka, Phys. Rev. B 96 174110 (2017) .

[6] J. Hlinka et al, Phys. Rev. Lett. 116, 177602 (2016) .

[7] B. Hehlen, M. Al-Sabbagh, A. Al-Zein, J. Hlinka, Phys. Rev. Lett. 117, 155501

(2016).

[8] S. Cherifi-Hertel et al, Nat. Commun. 8, 15768 (2017).

Configurable Topological Domain Textures in Strain Graded Ferroelectric Nanoplates

Chan-Ho Yang1,2,3

1Center for Lattice Defectronics, KAIST, Dajeon 34141, South Korea

2Department of Physics, KAIST, Dajeon 34141, South Korea 3KAIST Institute for NanoCentury, KAIST, Dajeon 34141, South Korea

Topological defects in matter are a topic of intense interest in contemporary condensed matter

physics. In particular, vortices and skyrmions in ferroic materials have received considerable

attention as topologically protected quasi-particles that carry energy and information. These

energetically quantized and spatially confined excitations behave collectively to form highly

non-trivial structures called topological textures that are characterized by conserved quantities

such as the winding number. Despite the identification of electric vortex structures, electric

switching of competing vortex textures in dielectrics with deterministic configurability of the

topological number remains experimentally unconfirmed. Here, we show that an epitaxial

ferroelectric square nanoplate of bismuth ferrite enables five discrete levels for the

ferroelectric topological invariant of the entire system. The total winding number of the

topological texture can be configured from −1 to 3 by selective non-local electric switching of

the quadrant domains. By using angle-resolved piezoresponse force microscopy in

conjunction with local winding number analysis, we directly identify the existence of vortices

and anti-vortices, observe pair creation and annihilation, and manipulate the net number of

vortices. Our findings offer a useful concept for the stabilization and control of ferroelectric

vortices for multi-level topological defect memory.

“Seeing is Believing” – Watching Domain Walls @ Work Lukas M. Eng 1,2

1 TU Dresden, Faculty of Physics, Institute of Applied Physics,

Nöthnitzerstr. 61, 01187 Dresden, Germany 2 cfaed – Center for Advancing Electronics Dresden,

TU Dresden, Dresden, Germany

*Corresponding Author: lukas.eng@tu-dresden.de

Topologies are known to play a major role in next-generation nanoscale devices. This talk will

focus on two such areas, addressing on the one hand topologies in ferroelectric domain walls

(DWs), while secondly focusing on DW inspection in multiferroic skyrmion (SkY) materials

that dress with mutually-interacting ferroelectric-ferromagnetic (FE-FM) textures.

Most interestingly, both Bloch- and Néel-type DW textures as familiar to FM systems and also

found here in the SkY-supporting materials FexCo1-xSi [1], GaV4S8 [2,3], and CuOSeO3

[4,5] using magnetic-force microscopy, do equally exist in the standard FE materials such as

PZT [6] and LiNbO3 (LNO) [7]. In fact, we have developed a series of non-invasive optical

methodologies that allow us to address such topologies in FE DWs both with ultra-high

precision and in real time.

In this talk, I will introduce into these techniques, i.e. Optical-Coherence Tomography (OCT)

[8] and Cerenkov Second-Harmonic-Generation (CSHG) Microscopy [9] that have been

applied to a manifold of DW-supporting FE systems, i.e. LNO and LiTaO3, BaTiO3, TGS, and

others more.

We are able to watch the DW dynamics in-situ in real time, for instance upon phase transitions

[10] or when applying electrical fields [11] that induce DW motion and switching. Of central

interest is also correlating this DW dynamic with domain-wall-conductivity (DWC) [12,13]

and its topology [14]: we thus are able to develop clear protocols that then favor DWC to occur

in single crystals.

References:

[1] P. Milde, D. Köhler, LME, et al., Science 340, 1076 (2013).

[2] I. Készmárki, S. Bordács, LME, et al., Nature Materials 14, 1116 (2015).

[3] A. Butykai, S. Bordács, LME, et al., Sci. Rep. 7, 44663 (2017).

[4] S. Zhang, A. Bauer, LME, et al., Nano Lett. 16, 3285 (2016).

[5] P. Milde, E. Neuber, LME, et al., Nano Lett. 16, 5612 (2016).

[6] S. Cherifi-Hertel, H. Bulou, R. Hertel, et al., Nat. Comm. 8, 15768 (2017).

[7] A.-S. Pawlik, T. Kämpfe, LME, et al., Nanoscale 9, 10933 (2017).

[8] L. Kirsten, A. Haußmann, LME, et al., Optics Express 25, 14872 (2017).

[9] T. Kämpfe, P. Reichenbach, LME, et al., Appl. Phys. Lett. 107, 152905 (2015).

[10] L. Wehmeier, Th. Kämpfe, LME, et al., Physica Status Solidi 11, 1700267 (2017).

[11] A. Haußmann, L. Kirsten, LME, et al., Ann. Phys. 529, 1700139 (2017).

[12] Ch. Godau, T. Kämpfe, LME, et al., ACS Nano 11, 4816 (2017).

[13] M. Schröder, A. Haußmann, LME, et al., Adv. Funct. Mater. 22, 3936 (2012).

[14] B. Wolba, J. Seidel, LME, et al., Adv. Electron. Mater. 1700242 (2017).

Digital Holographic Tomograph for Three-Dimensional Observations of Domain Structures in Ferroelectric Single Crystals

P. Mokrý1, P. Psota2, J. Václavík2, K. Steiger2, and V. Lédl2 1Faculty of Mechatronics, Informatics and Interdisciplinary Studies, Technical University of

Liberec, Studentská 1402/2, 46117 Liberec, Czech Republic, 2Regional Centre for Special Optics and Optoelectronic Systems (TOPTEC),

Institute of Plasma Physics, Academy of Sciences of the Czech Republic, Za Slovankou 1782/3, 18200 Prague 8, Czech Republic

pavel.mokry@tul.cz

The principles and construction details of the digital holographic tomograph (DHT) and the implementation of numerical processing methods, which allows the three-dimensional (3D) imaging of domain structures in ferroelectric single crystals, are presented. The constructed DHT allows distinguishing the ferroelectric domains by means of the precise measurements of spatial distribution of the contrast in refractive index, which is produced by the linear electrooptic effect in the ferroelectric single crystal in the external electric field. The 3D imaging process consists of two steps. In the first step, the DHT is used for the measurement of a series of projections of the domain structure in several directions. The scheme of the constructed DHT is shown in Fig. 1(a). In the second step, the 3D image of the ferroelectric domain pattern is computed by means of the tomographic reconstruction methods. Operation of the DHT prototype is demonstrated on the visualization of the domain structure in the whole volume of the periodically poled lithium niobate (PPLN) single crystal, which is presented in Fig. 1(b). Technical parameters of the constructed DHT and implementation details of the used tomographic reconstruction methods are presented and discussed.

(a) (b)

Fig. 1: Scheme of the constructed digital holographic tomograph. It is based on the wavefront-deformation digital holographic microscope (DHM) (a). The measured spatial distribution of ferroelectric domain pattern in the whole volume of the PPLN single crystal (b).

Domain Shapes of Isolated Domains in Bulk Uniaxial Ferroelectrics. from Convex Polygons to Domain Snowflakes

V. Ya. Shur

School of Natural Sciences and Mathematics, Ural Federal University, 620000 Ekaterinburg, Russia

E-mail: vladimir.shur@urfu.ru

The variety of domains shapes appeared in uniaxial ferroelectrics will be presented,

classified and described systematically. The obtained experimental results will be discussed using unified kinetic approach based on the analogy between domain structure evolution and phase growth during first-order phase transformation.

The classical theoretical approach predicted only the regular polygonal shape of isolated domains defined by crystal symmetry [1,2]. Recent systematic investigations of domain shapes allowed revealing wide shape variety which can be divided into: (i) circular shapes, (ii) regular polygons, (iii) irregular polygons, (iv) irregular shapes. The kinetic approach to domain growth based on generation of steps (pairs of kinks) and kink motion has been used for explanation of all obtained shapes [3]. The nucleation probabilities are determined by the local value of the sum of the external field and residual depolarization field at the domain wall.

The key role of the bulk screening retardation in domain growth is demonstrated. The domain shape complication due to screening ineffectiveness was demonstrated experimentally and by computer simulation [3]. Two limiting mechanisms of the step nucleation have been considered: (a) stochastic with equiprobable position of nucleation sites, (b) determined with step generation at fixed points and anisotropic kink motion.

Stochastic nucleation leads to formation of the circular domains, whereas determined nucleation stimulates formation of the polygonal ones. The convex polygons with walls parallel to the main crystallographic axis appeared for effective screening: (a) hexagons for C3v symmetry (lithium niobate, lithium tantalate and lead germanate), (b) squares for C4 symmetry, (strontium-barium niobate), (c) rectangles for C2 symmetry (potassium titanyl phosphate [4]). The domain shape stability effect (fast restoration of the initial concave polygonal shapes after domain merging) was attributed to formation of the short-lived super-mobile walls [5]. It was demonstrated that polygons and stars with concave angles can appear as a result of screening retardation only [6].

The stochastic nucleation at the elevated temperatures leads to lack of the domain shape stability thus opens the way to complicated fractal and dendrite domain shapes [7,8]. The dendrite (snowflakes) domain structures can be created by: (i) discrete switching with subsequent merging, (ii) domain shrinkage under the action of the pyroelectric field or spontaneous backswitching, (iii) domain growth at the elevated temperatures in the plates with artificial dielectric layer [9].

The equipment of the Ural Center for Shared Use “Modern nanotechnology” Ural Federal University was used. The research was made possible by Russian Science Foundation (Grant 14-12-00826).

References [1] V. Gopalan, V. Dierolf, D.A. Scrymgeour, Annu. Rev. Mater. Res., 37, 449-489 (2007) [2] L. Tian, D.A. Scrymgeour, V. Gopalan, J. Appl. Phys., 97, 114111 (2005) [3] V.Ya. Shur. J. Mater. Sci., 41, 199-210 (2006) [4] V.Ya. Shur et. al., Appl. Phys. Lett., 109, 132901 (2016) [5] V.Ya. Shur et. al., Ferroelectrics, 360, 111-119 (2007) [6] A.I. Lobov et. al., Ferroelectrics, 341, 109-116 (2006) [7] V.Ya. Shur et. al., J. Appl. Phys., 112, 104113 (2012) [8] V.Ya. Shur et. al., J. Appl. Phys., 119, 144101 (2016) [9] V.Ya. Shur, A.R. Akhmatkhanov, Phil. Trans. R. Soc. A, 376, 20170204 (2018)

Multi-scale Domain Structure and High Piezoelectricity in PbZr1-xTixO3 (PZT)

Zuo-Guang Ye, Alexei A. Bokov, Nan Zhang, and Yujaun Xie

Department of Chemistry and 4D LABS, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada

Lead zirconate-titanate solid solution, PbZr1-xTixO3 (PZT), has been studied extensively over thepast decades for both industrial applications and fundamental research, but almost exclusively in theforms of ceramics and thin films because of the difficulties encountered in the growth of PZT singlecrystals. On the other hand, the mesoscopic domains, the microstructure and the atomisticmechanisms that cause the outstanding piezoelectric performance of this class of materials near themorphotropic phase boundary (MPB) remain poorly understood. Therefore, it is of particularinterest to grow large single crystals of PZT, which are not only necessary for a thoroughcharacterization of the anisotropic properties of this prototype ferroelectric solid solution system,but are also expected to exhibit superior piezo- and ferroelectric performance over the PZT ceramics,and a higher depoling temperature (Td) and a higher coercive field (Ec) than the relaxor-based PMN-PT and PZN-PT single crystals, which are required for a broader range of advanced applications.The objective of this work is to synthesize the PZT single crystals and to characterize their domainstructures, phase transitions, local structure, and micro polar structures in order to provide a betterunderstanding of the structure-property relations.

Thanks to our capability in growing PZT single crystals with a wide composition range across theMPB (0.20 x 0.65) by a top-seeded solution growth technique and the availability of multi-scalecharacterization and analytical techniques, such as polarized light microscopy, piezoresponse forcemicroscopy (PFM), spherical aberration-corrected transmission electron microscopy, high-resolution neutron total scattering and diffuse scattering, and pair-distribution function analysis, wehave gained new insights into the complex local structure, atomic scale polarization rotation, nano-scale domain structure, intricate phase transition and critical behaviour, and tri-critical points inPZT.

In particular, the atomic structure of PZT crystals with x=0.42 is imaged by means of high-resolutionTEM. The accurate Pb displacements and their directions are successfully determined relative to thecentre of the four B-cations, on the {110} monoclinic mirror plane. The orientation and distributionof local polarizations indicate a mixture of rhombohedral, tetragonal and monoclinic localsymmetry, providing the atomistic evidence for the origin of the monoclinic phase in the PZT ofMPB compositions.

These results provide a better understanding of the relationship between micro-/nano-scopicstructure and macroscopic functional properties for this important material, but also for the piezo-/ferroelectric materials in general.

This work was supported by the United States Office of Naval Research (ONR Grants No. N00014-12-1-1045 and N00014-16-1-3106), the Natural Sciences and Engineering Research Council of Canada (NSERC Grant No. 203773)

Study of the in-Plane Domains Growth and Interaction at the Non-Polar Surfaces of Magnesium Doped Lithium Niobate

D.O. Alikin1,2,*, A.P. Turygin1, Yu.M. Alikin1, M.S. Kosobokov1, A.V. Ievlev3, V.Ya. Shur1

1School of Natural Sciences and Mathematics, Ural Federal University,

620000 Ekaterinburg, Russia, 2Department of Physics & CICECO – Aveiro Institute of Materials, University of Aveiro,

3810-193, Aveiro, Portugal 2 The Center for Nanophase Materials Sciences, Oak Ridge National Laboratory

37831 Oak Ridge, TN *E-mail: denis.alikin @urfu.ru

The controllable formation of the domain and domain wall patterns with determined

geometrical parameters is outstanding opportunity to improve materials properties. From this point both nanoscale control of individual domains and formation of domain ensembles is interesting for the various applications: novel mem-computing concepts [1], nanophotonics devices for the laser frequency conversation [2], elements of nanoelectronics [3] etc. The studies of fundamental processes of the domain nucleation and growth in single crystals are clearest and illustrative due to weak crystal structure deficiency. Nevertheless, it is still a lack of knowledge about in-plane domain formation and growth mostly due to this process in deeply studied uniaxial crystals occurs in the crystal bulk.

Piezoresponse force microscopy method was shown to be unique in the investigations of domain forward growth by the inspection of the tip-induced local switching at the non-polar crystal surfaces with high spatial resolution [4]. Here we shed further light on in-plane domain growth by the example of non-polar cut crystals of lithium niobate doped by MgO. We studied electrostatic interaction of the charged domain walls of the in-plane domains during forward growth and found non-trivial dependence of the domain length on spacing and applied electric field. Complicated self-organized behavior with effect of domain array period multiplication (doubling and quadrupling) and chaotic behavior were observed and explained by correspondent computer simulations [5,6].

The formation of the isolated needle-like domains was studied experimentally in a different external conditions: temperature and humidity. It was shown that humidity and temperature regimes could significantly influence on the domains shape and geometrical parameters through the backswitching appearing after removing of the applied voltage. Obtained results are discussed in terms of kinetic approach, where surface screening of residual depolarization field has principal role. The methods allowing to reduce backswitching and stabilize in-plane domain after switching are proposed.

The equipment of the Ural Center for Shared Use “Modern Nanotechnology” UrFU was used.

The research was made possible by Russian Science Foundation (Grant 14-12-00826).

References [1] A.V. Ievlev et al., Nature Phys., 10, 59–66 (2014) [2] P. Ferraro, S. Grilli, P. De Natale, “Ferroelectric Crystals for Photonic Applications”, Springer-Verlag Berlin Heidelberg (2014) [3] G. Catalan et al., Mod. Phys., 84, 119–156 (2012) [4] D.O. Alikin et al., Appl. Phys. Lett., 106, 182902 (2015) [5] A.P. Turygin et al., Materials, 10, 1143 (2017) [6] A.P. Turygin et al., submitted to Nano Lett.

POSTERABSTRACTS

Ferroelectric Domain Mobility in Bifeo3 Thin Films and Bulk Ceramics: Role of Grain Boundaries and Initial Domain Structure

D.O. Alikin1,2, A.S. Abramov1, A.P. Turygin1, R.O. Rokeah1, A.V. Nikitin4, D.V. Karpinsky4, E.B. Araujo3, V.Ya. Shur1 and A.L. Kholkin1,2

1School of Natural Sciences and Mathematics, Ural Federal University, Ekaterinburg, Russia,

2Department of Physics & CICECO – Aveiro Institute of Materials, University of Aveiro, Aveiro, Portugal

3Scientific-Practical Materials Research Centre of NAS of Belarus, Minsk, Belarus 4São Paulo State University, Ilha Solteira - SP, Brazil

*E-mail: denis.alikin@urfu.ru

The different interfaces exhibiting in BiFeO3 (BFO): neutral and charged domain walls, phase and grain boundaries (GB) have an enhanced conductivity which play a specific role in the macroscopic material properties [1]. The strong relation between dielectric, piezoelectric properties of the material and the microscopic mechanism of the charge transfer across the interfaces was revealed in BFO thin films and ceramics [1-3]. Nevertheless, at the moment there is no complete understanding of the different kind interfaces’ impact onto domain mobility. Due to significant heterogeneity of ferroelectric ceramics and thin films studying of these phenomena independently macroscopically is quite complicated. In this contribution, we used piezoresponse force microscopy to inspect domain wall mobility locally under the action of the electric field of scanning probe microscopy tip in a manner how it is usually done in single crystalline materials [4]. We found that annealing of sol-gel BFO films is followed by self-organized domain arrangement in micro-scale clusters with preferable orientation of spontaneous polarization. The GB confining the clusters, ‘inter-cluster GB’, have significantly higher electrical conductivity in comparison to ‘intra-cluster GB’, where conductivity even smaller the bulk one. The different impact to domain structure kinetics of the high and low conductive GB is shown and discussed in terms of inhomogeneous electric field distribution created by the GB. The local polarization reversal properties explain the difficulties with macroscopic poling of the BFO thin films. In bulk ceramics we focused mostly on properties on initial domain structure influence onto domain nucleation and propagation. The difference of the non-180-degree domain walls and 180-degree domain walls mobility was revealed and discussed in terms of kinetic approach and clamping of domain wall at the GB and pinned charged domain walls. Obtained results are interesting in relation to the local properties of the interfaces in BFO thin films and ceramics and constructing of a comprehensive behavioral model that captures both local and macroscopic properties in the same materials.

The equipment of the Ural Center for Shared Use “Modern nanotechnology” was used. The reported study was funded by RFBR (grant No. 17–52-04074) and BRFFR (grant No. F17RM-036). This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145- FEDER-007679 (FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 778070.

References [1] J. Seidel, “Topological structures in ferroic materials. Domain Walls, vortices and skyrmions”, Springer International Publishing Switzerland (2016). [2] A. Bhatnagar et al. Nat. Commun. 4, 2835 (2013). [3] T. Rojac et al., Adv. Funct. Mater. 25, 2099 (2015). [4] D. O. Alikin et al, Acta Mater. 125, 265-273 (2017).

Domain Compatibility in Polycrystalline Ferroics

M. Arredondo1, K. Holsgrove1, J. Huber2, S. Haigh3, A. Gholinia and E. Prestat3

1 Queen's University Belfast, United Kingdom,

2 University of Oxford, United Kingdom, 3 University of Manchester, United Kingdom

In the last years there has been an increasing interest in investigating domains’ configuration

in polycrystalline materials, where the macroscopic properties are due to collective phenomena

of the individual domains patterns found in each crystal, and which have their origins at the

nanoscale.[1-4] In general it is accepted that polycrystalline materials have a more complex

domain structure when compared to their single crystal counter parts, and it has been shown

that there is a correlation between the resulting domain pattern, grain size and grain geometry

[5-9] which plays a key role in the overall properties and switching mechanism. However, the

complex morphology and dynamics of domains in polycrystalline materials are still up to day

considered not yet fully understood; especially when intergranular compatibility is considered.

In this study, the compatibility and coupling of selected domain pattern across grains is

explored, as well as their behaviour under heating cycles through the Curie Temperature (TC).

Aspects such as pinning, domain nucleation and domain re-orientation are also analysed. This

is done experimentally by cutting lamellae from polycrystalline BaTiO3 (BTO). The grains are

initially characterized by electron backscatter diffraction (EBSD), followed by thin lamellae

fabrication of selected grains with different junctions (containing 1 - 3 grain boundaries) by

focused ion beam (FIB) and characterization by transmission electron microscopy (TEM)

techniques including in-situ heating. An example of a lamellae and local EBSD analysis is

shown in Fig. 1; while Fig. 2 shows an example of a heating the sample up to Tc. The data

obtained is then rationalised in terms of crystallographic theory of martensite in order to

understand and predict the final domain configuration of a grain within a grain junction.

We demonstrate that in the case of a lamellar polycrystalline samples, the relaxation of out-of-

plane constraint gives rise to an underdetermined set of linear equations in the volume fractions

of crystal variants. Additional inequality constraints on the volume fractions lead to (non-

unique) solutions, indicating that groups of twinned grains in lamellae can form stress-free

domain patterns. Since the constraint is much less than that of the bulk, a reorganisation of the

bulk domain structure during heating and cooling of a lamella extracted from the bulk is likely.

Figure 1. EBSD of BaTiO3 lamella (Out-of-plane)

Figure 2. In-situ heat cycle 1 at 1°C/sec.

References

[1] A. Gruverman, O. Auciello, H. Tokumoto, Appl. Phys. Lett. 1996, 69(21), 3191-3193.

[2] J. F. Scott, Ferroelectrics. 1996, 183, 51-63.

[3] A. Gruverman, O. Auciello, H. Tokumoto, Annual Review of Materials Science. 1998, 28,

101-123.

[4] A. Gruverman, H. Tokumoto, A. Prakash, S. Aggarwal, B. Yang, M. Wuttig, R. Ramesh,

O. Auciello, T. Venkatesan, Appl. Phys. Lett. 1997, 71(24), 3492-3494.

[5] M. Demartin, D. Damjanovic, Appl. Phys. Lett. 1996, 68(21), 3046-3048.

[6] G. Arlt, D. Hennings, G. Dewith, J. Appl. Phys. 1985, 58(4), 1619-1625.

[7] F. Griggio, S. Trolier-McKinstry, J. Appl. Phys. 2010, 107(2), 024105.

[8] J. F. Ihlefeld, A. M. Vodnick, S. P. Baker, W. J. Borland, J. Maria, J. Appl. Phys. 2008,

103(7), 074112.

[9] Y. Ivry, D. Chu, J. F. Scott, C. Durkan, Advanced Functional Materials. 2011, 21(10),

1827-1832.

Electron-Beam Domain Patterning in the Uniaxial Relaxor SrxBa1-xNb2O6

Ya.V.Bodnarchuk1*, T.R.Volk1, L.S.Kochanchik2, R.V.Gainutdinov1, and L.I.Ivleva3

1Shubnikov Institute of Crystallography of Federal Scientific Research Centre “Crystallography and

Photonics” of Russian Academy of Sciences, Moscow 119333, Russia 2Institute of Microelectronics Technology and High Purity Materials, Russian Academy of Science,

Chernogolovka, Moscow region, 142432, Russia 3Prokhorov Institute for General Physics of the Russian Academy of Science, 119991, Moscow, Russia

*Corresponding Author: deuten@mail.ru

The fabrication of ferroelectric domain patterns of specified configurations belongs to practically important problems of ferroelectricity owing to the potential of these structures for various applications. Domain fabrication by means of electron-beam (EB) irradiation in SEM offers several advantages over the conventional field method. The elaboration of electron-beam domain writing (EBDW) requires a deeper insight into the mechanism of domain formation under these specific conditions. Recently, we developed the approach to EBDW on the nonpolar surfaces of LiNbO3 crystals (for refs. the review [1]), which permitted us to specify the characteristics of EB written patterns in LiNbO3. To extend this approach to a wider range of materials, it was tested in the relative SrxBa1-xNb2O6 (SBN-x) crystals. Here we present the results of EBDW in SBN-0.61, which was achieved by matching the irradiation conditions, based on the approach [1]. EBDW in SBN was performed for the first time. Fig. 1 exemplifies PFM images of domains written by EB on the non-

polar (X-) and polar (Z-) crystal surfaces. Fig. 1. PFM images of domains written by EB in SBN-0.61 crystal; a) domains written on the X-surface of a ZFC crystal; b) domains written on the –Z surface of FC crystal (the SEM accelerating voltage U = 25 kV)

EB written domain patterns in SBN are completely stable and can be erased by thermal annealing only at T > Tc. It is a crucial difference from domains formed in SBN under external fields (e. g. [2]) or written by dc-AFM-tip voltages [3], which revealed long-term relaxations. The characteristics of EBDW in SBN differ from those in LiNbO3 due to the relaxor origin of SBN. The observed specific features of EBDW in SBN, such as slow kinetics of domain formation, different EBDW characteristics in ZFC and FC crystals, etc., are accounted for by pinning effects characteristic for relaxors. This work was supported by the Russian Foundation for Basic Researches (projects Nos. 16-29-11777ofi-m and 16-0200439a). References [1]. T. R. Volk, L. S. Kokhanchik, e.a. J. Adv. Dielectrics, 8, 1830001, 2018 [2] P. Lehnen, W. Kleemann, e.a., Phys.Rev. B, 64, 224109, 2001 [3] R. V. Gainutdinov,T. R. Volk, e.a. ZhETP Letts, 90, 303, 2009

Real Time Observation of Ferroelectric Switching in BiFeO3

Alan Brunier1, Christelle Kwamen2, Matthias Rössle2, Wolfram Leitenberger3, Matias

Bargheer2,3, and Marin Alexe1

1University of Warwick, United Kingdom 2Helmholtz Zentrum Berlin, Germany

3University of Potsdam, Germany

The mechanisms associated with polarization reversal in ferroelectric materials are still under

investigation because the microscopic dynamics are not yet fully understood. Recently it has

been demonstrated that simultaneous real-time observation of charge flow dynamics and

structural changes associated with ferroelectric switching is possible using synchrotron x-ray

diffraction.

Previous work on Pb(Zr0.2Ti0.8)O3 films [1] has tracked the charge flow in ferroelectric

capacitor devices via the time-resolved change of the lattice constant and the accompanying

peak broadening. It has been shown there that i) the nonlinear piezoelectric response of the

ferroelectric layer develops on a much longer timescale than the RC time constant of the device

and ii) the complex atomic motion during the ferroelectric polarization reversal starts with a

contraction of the lattice, whereas the expansive piezoelectric response sets in after

considerable charge flow due to the applied voltage pulses on the electrodes of the capacitor.

Here we present the results of similar experiments for multiferroic BiFeO3, with a special

emphasis on resolving the domain wall motion during the polarization reversal. This

experiment probes in-situ the structural signatures of dipole reorientation of ferroelectric thin

film capacitors under the simultaneous influence of an electric field and additionally an

ultrafast strain pulse after laser excitation of a metallic transducer.

References:

[1] “Simultaneous dynamic characterization of charge and structural motion during

ferroelectric switching”. C. Kwamen et al., Phys. Rev. B 96, 134105 (2017).

Polarised-Light and Electron Microscopy of the Static Domain

Structure of Ferroic Co3B7O13Cl Boracite at Room Temperature.

A.G. Castellanos-Guzmán1, O. Jiménez1, M.A.González1,2, L.M. Flores-Cova3, J.

Ricardo González3, A. Estrada López3, E.E Velez Barragan3, A.R. Saucedo Corona3&

A. Daniel Muñoz G3.

1Centro de Investigación en Materiales DIP-Cucei Universidad de Guadalajara,

Boulevard Tlaquepaque 1421, 44430 Guadalajara Jalisco. México

2Conacyt. Universidad de Guadalajara.CUCEI. Boulevard Tlaquepaque 1421. 44430

Guadalajara Jalisco. México

3Posgrado en Ciencia de Materiales Cucei. Universidad de Guadalajara, Boulevard

Tlaquepaque 1421, 44430 Guadalajara Jalisco. México

e-mail: gcastel@cucea.udg.mx

The static domain structure of ferroic Co3B7O13Cl, abbreviated in what follows as Co-Cl,

synthetic boracite crystals has been analysed by polarised-light microscopy (PLM) [1.2]

and by field emission scanning electron microscopy (FE-SEM) [3,4

] at room temperature. As for most boracites, Co- Cl possess dielectric, elastic, magnetic

and optical properties of unusual interest.

Upon cooling, CoCl botacite undergoes three successive structural phase transitions: (i)

from the cubic 43m paraelectric phase to an orthorhombic mm2 ferroelectric/ferroelastic

phase at 623 K, (ii) to a monoclinic m ferroelectric/ferroelastic at 538K, and (iii) to a

rhombohedral (3m) ferroelectric/ferroelastic phase at 468K.

Co-Cl boracite single crystals were grown by the chemical-vapour transport technique[6].

At room temperature, the Co-Cl crystals present a beautiful domain structure which

consists of six fully ferroelectric/fully ferroelastic domains are totally coupled[5]

The usefulness and possibilities of combining PLM and FE-SEM for the studying and

visualizing the domain structure of ferroic boracites will be shown

The authors would like to thank Coordinacióbn General Académica and Cucei from

Universidad de Guadalajara for financial support

References

[1] Schmid H., in Ferroelectric Ceramics Ed. By N. Setter. Monte Veritá (1992)

Birkhaussen Verlag.Basel}

[2] Forsbergh P.W. Jr. Phys. Rev, 76 (1949) 1187-1201

[3] Le Bihan R. Ferroelectrics 97 (1989) 67-70

[4] Castellanos-Guzmán A.G:, and O. Jiiméne Ferroelectrics 482 (2015) 46-53

[5] Ye Z-G, A- Janner and H. Schmid. J,. Phys: Conden. Matter 9 (1997) 2607-2621

[6] Schmid H. J. Phys. Chem. Solids 26 (1965) 26 (1965) 973-982

Local Polarization Reversal by Electron and Ion Beam Irradiation in Relaxor SBN Single Crystals

V.A. Shikhova,1* V.V. Fedorovykh,1 D.S. Chezganov,1 E.O. Vlasov,1 P.S. Zelenovskiy,1 E.D. Greshnyakov,1 M.S. Nebogatikov,1 A.L. Kholkin,1,2 L.V. Ivleva,3 V.Ya. Shur1

1 School of Natural Sciences and Mathematics, Ural Federal University, 620000 Ekaterinburg, Russia

2 Department of Physics and CICECO – Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal

3 Prokhorov General Physics Institute, Russian Academy of Sciences, 119991 Moscow, Russia *E-mail: vera@urfu.ru

We have investigated the local polarization reversal under the action of electron and ion

beam irradiation in the single crystals of strontium barium niobate (Sr0.61Ba0.39Nb2O6, SBN). SBN single crystals were grown by modified Stepanov technique, cut normally to the

polar axis and carefully polished. Before irradiation the samples were subjected to different procedures: (1) zero-field cooling (ZFC), (2) field cooling (FC). ZFC represents sample cooling from 200OC to room temperature without application of electric field. FC was a cooling of the sample in constant external field about 600 V/mm. The maze-type nanodomain structure was formed after ZFC, and almost single-domain state with residual nanodomains – after FC.

The scanning electron microscope (Auriga CrossBeam workstation, Carl Zeiss) was used for electron and ion beam irradiation [1]. The dot and stripe exposure regimes with different irradiation doses were used. The created domain structures were visualized by piezoresponse force microscopy (PFM) (at the surface) and by Raman confocal microscopy (in the bulk).

The isolated circular domains were formed as a result of dot exposure in all samples (Fig. 1 a, b). The circular domain shape indicated isotropic domain growth due to wall merging with nanodomains. The domain radius increased and saturated with irradiation dose. Saturation of the domain radius was attributed to saturation of injected charge. Formation of “diffuse domain boundary” and decreasing of domains sizes (as compare with ZFC samples) were obtained in FC samples.

The formation of regular chains of isolated domains (at doses below 100 µC/cm2) and the stripe domains (at doses above 100 µC/cm2) were observed for stripe exposure (Fig. 1 c, d). The increase of stripe domain width with dose was observed. The domain width was independent on the stripe orientation.

High stability of the created domains was revealed. The dependence of the depth of the created domains on the irradiation dose was obtained for dot and stripe irradiation. The domain growth behavior is similar for irradiation by electron and ion beams. The obtained results can be used for the domain engineering in relaxor SBN single crystals.

Fig. 1. PFM image of domain structure in ZFC samples for dot (a, b) and stripe (c, d) irradiation by e-beam (a, c) and i-beam (b, d). Doses: (a) 11 pC, (b) 5 pC, (c) 300 µC/cm2, (d) 350 µC/cm2.

The equipment of the UCSU “Modern nanotechnology” UrFU was used. The research was made possible by Russian Foundation of Basic Research (Grant 16-02-00821-а).

[1] V.Ya. Shur et al., Appl. Phys. Lett., 106, 232902 (2015).

Domain Patterning by Electron Beam Irradiation

in Lithium Niobate and Lithium Tantalate Crystals

D. S. Chezganov1, E. O. Vlasov1, L. V. Gimadeeva1, M. A. Chuvakova1,

V. Yu. Mikhailovskii2, V. Ya. Shur1 1School of Natural Sciences and Mathematics, Ural Federal University,

620000 Ekaterinburg, Russia 2Research Park, IRC Nanotechnology, St. Petersburg State University,

198104, St. Petersburg, Russia

E-mail: chezganov.dmitry@urfu.ru

We have studied the domain formation induced by focused electron beam (e-beam)

irradiation at the room [1] and elevated temperatures in congruent lithium niobate (CLN) and

lithium tantalate (CLT) crystals covered by artificial dielectric layers. The obtained results were

explained in terms of kinetic approach [2].

The samples represented the 0.5-mm-thick Z-cut CLN and CLT plates. The domain

structures have been produced by irradiation of Z– polar surface covered by dielectric layer

using scanning electron microscopes Auriga Crossbeam and Merlin (Carl Zeiss). The irradiation

parameters and beam positioning were controlled by e-beam lithography system Elphy

Multibeam (Raith). The irradiation at the temperatures up to 250°C was carried out using thermal

stage C1003 (Gatan Inc.). Both dot and stripe irradiations were used. Several dielectric materials

were used for surface covering. The static domain structures were visualized: (1) at the surface

by scanning electron microscopy after selective chemical etching and (2) in the bulk by Raman

confocal microscopy and Cherenkov second harmonic generation microscopy.

It was shown by Kelvin probe force microscopy measurements of surface potential

distribution that after irradiation with appropriate electron energy [3] the charge was localized

inside the dielectric layer. The better charge localization with relaxation time about one hour was

revealed in negative e-beam resist layer.

The charge dose and temperature dependences of domain shape and size were measured. It

was shown that the regular polygonal domain appeared at room temperature, whereas the

temperature increase led to lack of the domain shape stability and even to discrete switching at

the temperature above 100ºC due to correlated nucleation [1,2]. The domain area depends

linearly on the dose and domain merging resulted in formation of irregular shaped domains. The

growth of switched area with temperature elevation was attributed to decrease of the threshold

field.

We assume that the irradiation with various doses is an analog of the switching by field

pulses of various amplitudes and durations. Thus the main types of domain structure after stripe

irradiation have been revealed and considered as the stages of domain evolution. The domain

structure formation was characterized by dose dependences of domain density and length, period

of domain rays and stripe domain width. The threshold irradiated charge dose necessary for

formation of solid stripe domain has been revealed. It was shown that the temperature increase

led to drastic change of the domain structure. The revealed temperature dependence of the stripe

domain shapes was discussed.

The obtained effects were attributed to screening retardation by artificial dielectric layer

and to change of domination nucleation mechanism from determined at low temperatures to

stochastic at the elevated ones. The obtained results will be used for development of the domain

engineering methods.

The equipment of the Ural Center for Shared Use “Modern nanotechnology” UrFU was

used. The research was made possible by the Russian Science Foundation (Grant № 17-72-

10152).

References

[1] V.O. Vlasov et al., Scanning, 2018, 7809826 (2018).

[2] V.Ya. Shur, J. Mater. Sci., 41, 199 (2006).

[3] V.Ya. Shur et al., Appl. Phys. Lett., 106, 232902 (2015).

Anomalous Domain Wall Motion in Cu-Cl Boracite:

Study of the Dynamics of the Motion

C. Cochard,1 J. G. M. Guy, R. W. Whatmore, A. Kumar, R. G. P McQuaid, and J. M. Gregg

1 Centre for Nanostructured Media, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK.

The observation of properties at domain walls that are not endogenous to the bulk of the material has fostered research examining the origin of the different properties of these nanoscale objects. Cu-Cl boracite (Cu3B7O13Cl) is one of the examples where domain walls exhibit properties distinct from the domains. McQuaid et al. [1] demonstrated the possibility, for conducting and insulating domain walls, to be injected in a site-specific manner and moved by the application of an electric field. Interestingly, some of the domain walls exhibit an anomalous motion under electric field: the motion leads to the growth of the domain with polarisation anti-aligned to the applied electric field, contrarily to what is commonly observed in other ferroelectrics. In this work, we characterise the dynamics of the movement of the domain walls, comparing walls which exhibit “conventional” motion with that of the “anomalous” case. Special attention is focussed on the bowing of the domain walls and the jerky behaviour associated with their movement. [1] R.G.P. McQuaid, et al. Injection and controlled motion of conducting domain walls in improper ferroelectric Cu-Cl boracite. Nat. Commun. 8, 15105 (2017)

Ferroelectric Surfaces & Adsorbates

ElzbiethaPach1, Kumara Cordero1, Irena Spasojevic1, Ma José Esplandiu1, Carlos Escudero2, Virginia Pérez-Dieste2, Albert Verdaguer1,3 NeusDomingo1

1 Institut Català de Nanociencia i Nanotecnologia, CSIC and The Barcelona Institute of

Science and Technology, Campus UAB, Bellaterra, Barcelona 08193, Spain 2 Laboratori de Càlcul Numèric (LaCàN) Universitat Politècnica de Catalunya (UPC)

Campus Nord UPC-C2, E-08034 Barcelona, Spain 3 Departament de Física Universitat Autònoma de Barcelona (UAB) Edifici Cc, E-0819

Bellaterra, Spain 4 Institució Catalana de Recerca i Estudis Avançats (ICREA) Pg. Lluís Companys 23,

E-08010 Barcelona, Spain neus.domingo@icn2.cat

Ferroelectric Surfaces present strong electric stray fields that need to be screened to stabilize the bulk ferroelectric polarization. Mechanisms for ferroelectric polarization screening are wide and still not well understood, but water is shown to play an important role on the electrochemistry of this surfaces. We have applied AFM in controlled atmosphere and we have been able to determine that water dipoles have a fundamental role on screening of surface electric stray fields from bulk ferroelectric polarization.[1]

On the other hand Near Ambient Pressure XPS has allowed us to determine the chemical species and their dynamics on ferreoelectric surfaces. Polarization dependence of water adsorption and desorption on LiNbO3 surfaces was demonstrated using X-ray photoelectron spectroscopy (XPS) carried out in situ under nearambient conditions. Positive and negative (0001) faces (z-cut) of the same crystal were compared for the same temperature and pressure conditions. Our results indicate a preferential adsorption on the positive face of the crystal with increasing water pressure and also higher desorption temperature of the adsorbed molecular water at the positive face. Adsorption measurements on the (1100) face (y-cut) showed also strong affinity to water, as observed for the zcut positive surface. We found a direct relation between the capacity of the surface to discharge and/or to screen surface charges and the affinity for water of each face. XPS spectra indicate the presence of OH groups at the surface for all the conditions and surfaces measured. [2] We could also demonstrate that the direction of the polarization determines the nature and the dynamics of the adsorbates and that the ionic species have a predominant role on the screening and adsorption of water.

[1] JJ. Segura, et al, “Surface screening of written ferroelectric domains in ambient conditions”J. Appl.Phys113 (2013)187213 [2] N. Domingo, et al., Giant reversible nanoscale piezoresistance at room temperature in Sr2IrO4 thin films, Nanoscale 7 (2015) 3453

[2] K. Cordero-Edwards et al., “Water Affinity and Surface Charging at the z-Cut and y-Cut LiNbO3 Surfaces: An Ambient Pressure X-ray Photoelectron Spectroscopy Study”J. Phys. ChemC 120(2016) 24048

Local and correlated studies of humidity-mediated ferroelectric thin film surface charge dynamics

I. Gaponenko (1,2), L. Musy (1), N. Domingo (3), N. Stucki (4), A. Verdaguer (3), N. Bassiri-Gharb (2,5), and P. Paruch (1)

(1) DQMP, University of Geneva, 1211 Geneva, Switzerland (2) G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology,

Atlanta, GA, 30332 (3) Institut Català de Nanociència i Nanotecnologia (ICN2), Campus UAB, 08193,

Bellaterra (Barcelona), Spain (4) University of Applied Sciences Western Switzerland in Geneva (HES-SO/hepia), 1213

Geneva, Switzerland (5) School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta,

GA, 30332 Surface water is present on all materials exposed to ambient environmental conditions, inherently modifying the ground state in fundamental studies as well as affecting the operation of bare-chip devices. By virtue of its polar nature, water strongly interacts with domains and domain walls in ferroelectric materials: it influences polarisation switching dynamics in Pb(Zr0.2Ti0.8)O3 (PZT) thin films [1], and plays a key role (together with redistribution of oxygen vacancies) in the reversible control of electrical transport at 180° domain walls in this material [2].

Such water-polar surface interactions can be probed at the nanoscale by functional scanning probe microscopy (SPM), and analysed within the framework of unsupervised machine learning matrix factorization [3]. Here, we present our studies of the interaction of adsorbed water with written ferroelectric domains in thin films of PZT by Kelvin probe force microscopy imaging. Both the effect of polarization on the adsorbed water behaviour and the influence of surface water on the underlying material are investigated. Correlating the results of domains written with positive and negative tip voltage with the as-grown state of the film, we demonstrate changes in the strength of the electrostatic interactions between the SPM tip and the surface as a function of relative humidity and time. Statistical analysis performed on the acquired data sets suggests that the observed response is a combination of two competing physical behaviours: a fast materials-dependent process and a slower effect of polarization history. More in-depth scrutiny reveals spatiotemporal correlations invisible to the naked eye through the use of unsupervised machine learning techniques such as principal component analysis and dictionary learning.

[1] C. Blaser and P. Paruch, New J. Phys. 17, 013002(2015)

[2] I. Gaponenko, P. Tückmantel, J. Karthik, L. W. Martin and P. Paruch, Appl. Phys. Lett. 106, 162902 (2015)

[3] A. Belianinov, R. Vasudevan, E. Strelcov, C. Steed, S.M. Yang, A. Tselev, S. Jesse, M. Biegalski, G. Shipman, C. Symons, A. Borisevich, R. Archibald and S. Kalinin, Adv. Struct. Chem. Imag. 1, 6 (2015)

Magnetic Phase Competition and Magnetoelectric Coupling in Co-

Doped MnWO4 Studied by Resonant Magnetic X-Ray Scattering

J. Herrero-Martin1, C. Mazzoli2, S. Francoual3, J. Strempfer3, F. Fabrizi4, P. Bencok4,

P. Steadman4, A. N. Dobrynin4, A. Bombardi4, A. A. Mukhin5, V. Skumryev6, and J.

L. Garcia-Muñoz7

1ALBA Synchrotron Light Source, Cerdanyola del Vallès, Spain;

2Brookhaven Natl. Lab., Natl. Synchrotron Light Source 2, Upton, NY 11973 USA; 3Deutsch Elektronen Synchrotron, Notkestr. 85, Hamburg, Germany;

4Diamond Light Source, Didcot, Oxfordshire, United Kingdom; 5Probkhorov General Physics Institute, Russian Academy of Science, Moscow,

Russia; 6Dpt. Fisica, Universitat Autònoma de Barcelona, Bellaterra, Spain;

7Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Bellaterra, Spain

Tuning synthesis towards the induction of magnetic frustration and complex

magnetic orderings breaking the spatial inversion symmetry is regarded as an

effective way of producing multiferroic (MF) and magnetoelectric (ME) materials.

The (Mn,Co)WO4 series has become a reference model for the analysis of the

interaction between spins and polar order. Co-doping favors a strong competition

between its large magnetocrystalline anisotropy (McA) and Mn-Mn exchange

interactions, stabilises a ferroelectric (FE) groundstate and allows the appearance of

new FE phases induced by distinct acentric magnetic structures. Interestingly, the two

different magnetic ions (Mn and Co) occupy the same crystallographic position which

makes this series intrinsically inhomogeneous.

Resonant magnetic x-ray scattering (RMXS) has allowed us to investigate the

magnetic properties in a MF crystal with the Mn0.85Co0.15WO4 critical composition.

Up to four different phases compete in the crystal varying temperature, two of them

being FE. Co moments tend to follow their own strong uniaxial McA, while the nearly

isotropic charge density distribution around Mn ions permits them a large flexibility

to adopt a variety of complex magnetic structures bearing distinct anisotropies. Their

interplay with the macroscopic FE polarization observed in this compound is analyzed

in terms of the inverse D-M term associated to non-collinear Mn spins. The associated

spin-induced ME effect could be in- situ observed by studying variations in the

scattered light polarization by the sample as a function of the E-field applied across

the wolframite crystal.

Experimental realization of atomically flat and AlO2-terminated LaAlO3 (001) substrate surfaces.

J. R. Kim1,2, J. N. Lee3, Y. Kim4, Y. J. Shin1,2, B. Kim1,2, S. Das1,2,

L. F. Wang1,2, M. Kim4, M. Lippmaa3, T. H. Kim5, and T. W. Noh1,2

1Center for Correlated Electron Systems, Institute for Basic Science (IBS), Republic of Korea. 2Department of Physics and Astronomy, Seoul National University (SNU), Republic of Korea.

3Institute for Solid State Physics, University of Tokyo, Japan 4Department of Materials Science and Engineering and Research Institute of Advanced

Materials, Seoul National University, Republic of Korea

5Department of Physics and Energy Harvest Storage Research Center (EHSRC), University of

Ulsan, Republic of Korea

Oxide single crystal substrates with atomically smooth and chemically uniform surfaces are

indispensable for constructing well-defined oxide heterostructures. Here, we report a simple method to realize atomically flat and AlO2-terminated LaAlO3 (001) [LAO(001)] substrate surfaces. The LAO(001) substrate has extensively been used for oxide thin film growth (e.g., strain engineering of ferroelectric thin films)1. However, well-established methods for achieving atomically flat, singly terminated LAO(001) surfaces have rarely been reported. This is mainly due to the unstable charged surfaces of LaO+ or AlO2

-, which hinders simultaneous stabilizations of atomic-scale smoothness and single termination2. To overcome this problem, we combined thermal annealing and subsequent deionized water leaching to treat the LAO(001) surface. We used atomic force microscopy to investigate the evolution of the LAO(001) surface during the water leaching and confirmed the atomically flat surface the water-leached sample. We further demonstrated the single AlO2-termination of the LAO(001) surface via co-axial impact-collision ion scattering spectroscopy3. Using the treated substrates, we are able to grow perovskite oxide films (i.e., SrRuO3) on the LAO(001) substrate with atomically sharp heterointerfaces. Our work provides an effective means for controlling the surface and interface of transition metal oxide heterostructures at the atomic scale.

[1] D. G. Schlom, et al., Annu. Rev. Mater. Res. 37, 589 (2007). [2] A. J. H. van der Torren, et al., Phys. Rev. B 91, 245426 (2015). [3] T. Ohnishi, et al., Appl. Phys. Lett. 74, 2531 (1999).

Visualization of LiNbO3 micro-dimensional domain structures by the method of nonlinear optical microscopy.

S.D. Lavrova, A.S. Elshina, E.D. Mishinaa, T.R. Volkb

a MIREA - Russian Technological University, Moscow, 119454, Russia

b Shubnikov Institute of Crystallography of Federal Scientific Research Centre “Crystallography and Photonics” of Russian Academy of Sciences, Moscow, 119333,

Russia

Lithium niobite (LiNbO3) is one of the most well-known materials for the creation of optoelectronic devices. Domain ordered structures in bulk LiNbO3 crystals are of considerable interest for the creation on their basis of integrated optical patterns and devices. The domain structures considered in this work were obtained with the help of a focused electron beam. This technique allows the creation of domain structures in a LiNbO3 crystal with a specified shape and with high accuracy.

Investigation of the created structures was carried out by the method of nonlinear optical confocal microscopy. This technique allows efficient visualization of planar domain structures with high spatial resolution. It was shown that it allows effective control of lateral parameters of domain structures. A periodic nature of the second optical harmonic generation intensity along the growth direction of the created domains was observed. A theoretical model for estimating the intensity distribution of the second optical harmonic as a function of the thickness of the domain structures is proposed. Using this model, it possible to explain the periodic dependences obtained in this study. Based on the proposed model, the possibility of determining the thickness of domain structures in bulk crystals of LiNbO3 is shown. The etching of domain structures was carried out and the depth of the created domains was determined. It is shown that the proposed theoretical model is in good agreement with the experimental data.

This work was supported by the Ministry of Education and Science of the Russian Federation (State task No. 3.7335.2017/9.10).

Incommensurate phase transition in antiferroelectric materials

Madhura Marathe and Massimiliano Stengel

Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain

Lead zirconate (PZ) is a prototypical antiferroelectric (AFE) material with a high-temperature cubic phase and undergoes a phase transition to an orthorhombic phase at temperature ~ 500 K. The origin of AFE transition has been explained considering the numerous instabilities that exist in the cubic reference phase of the material, such as (anti-)ferroelectric instability (polar displacements dominated by lead), or antiferrodistortive (AFD) modes (rotations and tilts of oxygen octahedra). Two main models have been proposed: thefirst one involves a trilinear coupling [1, 2] between AFD and polar modes, while in the second one it is the unstable polar branch that drives the transition via flexoelectricity [3]. Here, we attempt a unfication of the above two models by considering a more general set of gradient coupling terms, involving not only the strain (as in flexoelectricity), but also the oxygen tilts. This idea has many connections to that proposed recently to explain the emergence of polarity at ferroelastic twin walls in SrTiO3 [4]. References [1] J. Hilnka, et al., Phys. Rev. Lett. 121 (2014) 197601. [2] J. Iniguez, et al., Phys. Rev. B 90 (2014) 220103. [3] A. K. Taganstev, et al., Nat. Commun. 4 (2013) 2229. [4] A. Schiffino and M. Stengel, Phys. Rev. Lett. 119 (2017) 137601.

First-principles-originated Landau-Devonshire-potentials for ferroelectrics

P. Marton1,2, A. Klíč1, M. Paściak1, and J. Hlinka1

1 Institute of Physics, Czech Academy of Sciences, Na Slovance 2, Praha, Czech Republic, CZ-

182 21 2 Institute of Mechatronics and Computer Engineering, Technical University of Liberec, Studentská

2, Liberec, Czech Republic, CZ-461 17 (marton@fzu.cz) Phenomenological description is valuable and indispensable for understanding of properties related to domain structure in ferroelectric materials. Conventional development scheme for parametrization of phenomenological models heavily relies on experimental data, and is therefore limited. This is in particular true for materials like BiFeO3, where reliable experimental intrinsic data – such as dielectric, piezoelectric and elastic tensors – are lacking. Recently, we have developed a first-principles based 0-K Landau-Devonshire parametrization for BiFeO3 with an ambition to properly describe the low-temperature energy landscape in terms of three order parameters: ferroelectric polarization, oxygen-octahedra rotations and mechanical strain [4]. The presented parametrization is supposed to serve a solid starting point for development of a temperature-dependent potential via Monte Carlo or molecular-dynamics simulations. We present details of the methodology and its application to other important ferroelectric perovskites, such as BaTiO3, KNbO3, and PbTiO3. We compare material properties recovered from these material descriptions with available experimental data.

[1] P. Marton, A. Klíč, M. Paściak, and J. Hlinka, Phys. Rev. B 96, 174110 (2017).

Magnetoresistance and Synapse Behaviour Using Conducting

Domain Walls

J.P.V. McConville1*, P.W. Turner1, M. Colbear1, M.P. Campbell1, A. Kumar1, and

J.M. Gregg1

1Centre for Nanostructured Media, Institute of Physics, Queen’s University Belfast

University Road, Belfast, Northern Ireland, UK, BT7 1NN

*James McConville: jmcconville22@qub.ac.uk

Domain walls in ferroic materials are a crucible for exciting physics. These extremely

localised distortions of the lattice have complex dynamical behaviours, phonon

scattering effects, confined electronic states and defect interactions that have been a

rich vein for challenging research. Engineering these properties to fabricate unique

devices is being explored, as these nanoscale tuneable features drastically alter the

measured bulk properties. Progress has been held back by the difficulty in predicting

behaviour in different materials, or even materials with the same nominal composition

but different growth processes. The variety of behaviours of macroscopic domain wall

properties has also created demand for characterisation of the fundamental nanoscale

properties of domain walls.

Our research has focussed on measuring fundamental charged carrier properties such

as the conductivity, carrier mobility and density using highly developed scanning probe

microscopy techniques to detect the Hall Effect on the nanoscale [1]. We have also

developed experimental approaches to measure magnetoresistance in domain walls to

compare quantitative data with our earlier work and between ferroelectric domain walls

in very different materials. We continue to develop new experimental geometries and

approaches to measure these fundamental properties and more, such as phonon

scattering, photoconductivity effects and domain wall junctions with a particular focus

on uniaxial trigonal ferroelectrics such as lithium niobate and lead germanate.

We also demonstrate repeatable, scalable and memristor behaviour using conductive

domain wall density as an n-ary memory state. The insulating properties of the bulk

ferroelectric combined with the high conductivity and density of walls allows currents

to be tuned from the femtoampere to the milliampere scale through the crystal. Our

geometry is comparable to the work done on ferroelectric tunnel junctions where these

memory states were used to develop synapse-like responses [2]. Our experiments are

based on thin single crystals instead of relying on the ultra-thin films necessary for

electronic tunnelling. This constitutes the first device to work with domain wall density

as the controlled parameter, with wide potential applications.

Figure 1: a Amplitude, phase and conductance map of a 35um charged domain wall

written into a y-cut LNO crystal b Schematic of the AFM Hall experimental geometry

References:

[1] M. P. Campbell, J.P.V. McConville, R.G.P. McQuaid, D. Prabhakaran, A. Kumar,

J. M. Gregg. Hall effect in charged conducting ferroelectric domain walls. Nat.

Comms. 2016, 7, 13764. doi:10.1038/ncomms13764

[2] S. Boyn, J. Grollier, G. Lecerf, B. Xu, N. Locatelli, S. Fusil, S. Girod, C.

Carrétéro, K. Garcia, S. Xavier, J. Tomas, L. Bellaiche, M. Bibes, A. Barthélémy, S.

Saïghi, V. Garcia. Learning through ferroelectric domain dynamics in solid-state

synapses. Nat. Comms, 2017, 8, 14736. doi:10.1038/ncomms14736

Coupling of electrostatic field to orbital angular momentum in FE semiconductors

Louis Ponet

1Italian Institute of Technology, Genova It has been shown that ferroelectric semiconductors allow for electric control of spin-polarized states [1]. Moreover, certain ferroelectrics, namely those with a large atomic spin-orbit coupling show a giant spin-splitting. Naively one would assume from the linear splitting and the ferroelectricity, that this is caused by the well-known relativistic Rashba-effect. However, more careful examination of the strength and band specific properties render this explanation void. We have shown that an electrostatic efect, which couples the electric polarization to the orbital-angular momentum[2] lies at the heart of the large effect. When this coupling occurs, a giant spin-splitting will follow, provided that there is a large atomic spin-orbit coupling.

Mud-Crack-Like Strain Relaxation and Domain Patterning in Epitaxial VO2 Thin Films

L. Rodríguez1, E. Del Corro1, J. Santiso1, F. Sandiumenge2, G. Catalan1,3

1Institut Catala de Nanociencia i Nanotecnologia (ICN2), CSIC and The Barcelona Institute of Nanoscience and Technology (BIST), Barcelona, Spain.

2Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Barcelona, Spain. 3ICREA-Institucio Catalana de Recerca i Estudis Avançats, Barcelona, Spain.

E-mail: laura.rodriguez@icn2.cat VO2 is a widely known material by its first order metal-insulator transition (MIT) near room temperature (~ 68ºC in bulk) but on the other hand, widely controversial due to the non-well understood transition mechanisms -nowadays the discussion is still open. This MIT couples an electronic transition to a structural one: the insulating (semiconductor) state is associated to a monoclinic structural phase while the metallic state does it to a rutile (tetragonal) phase. Nevertheless, it may not occur this way under a certain strain and thickness conditions, which makes it even more difficult to interpret[1-3]. Doping, defects, applied voltage, current excitation or photon irradiation among others may also play a role in the MIT behavior. Since strain is the key to control the structural phase transition (SPT) during the MIT in VO2, this work focuses on how the strain state of VO2 can make diverge the correlated MIT-SPT from the one happening in the relaxed state (bulk). More concretely, we want to show how cracks, as a consequence of a high level of strain, affects the SPT. Our studies are based on VO2 thin films with different thicknesses that we have grown by pulsed laser deposition (PLD) on (001)-oriented TiO2 substrates. R-TiO2 has a similar in-plane lattice parameter than R-VO2 (mismatch ~0.85%) that eases strained films. However, this is not the case of M-VO2 (mismatch ~9%) producing an extremely unstable phase boundaries (PB) when both R and M phases coexist during the MIT and, probably due to the tension generated in these PB, microcracks appear as a local-relaxation mechanism. We will show how the cracks induce local structural changes at the microscale by using different characterization techniques. References 1. M. Yang et al., Scientific reports 6, 23119 (2016) 2. H. Qiu et al., New J. Phys. 17, 113016 (2015) 3. T.V. Slusar et al., Scientific reports 7, 16038 (2017)

Pretransitional Charge-Order Tweed Pattern In Strained VO2 Films

Felip Sandiumenge1*, Laura Rodríguez 2, José Santiso 2, Gustau Catalán 2

1Institut de Ciència de Materials de Barcelona (ICMAB-CSIC). Campus de la UAB, 08193

Bellaterra, Catalonia, Spain.

*E-mail: felip@icmab.cat 2Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST. Campus

UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain.

Ferroelastic transitions are often accompanied by pretransitional phenomena bridging the initial

high and final low symmetry states. Such effects are essentially attributed to the accommodation

of the spontaneous strain arising from the symmetry breaking at the transition temperature, and

act as a precursor of the final domain structure typically found in the ferroic phase. Such

precursor states exhibit a nanoscale needle like morphology known as tweed, formed by

extremely narrow interpenetrating crystallographic domains with an incipient order parameter.

In this work we report on the development of a pure electronic tweed pattern induced by charge

order prior the stabilization of strained metallic VO2 films. X-ray absorption spectroscopy

across the metal to insulator transition (MIT) indicates a change of the average V oxidation

state from 4+ in the insulating monoclinic M1 phase to 3.7+ in the metallic rutile phase. High

resolution transmission electron microscopy (HRTEM), on the other hand, indicates phase

coexistence at the nanoscale, with full lattice matching among coexisting phases along specific

crystallographic directions. Image fast Fourier transforms (FFTs) show that, superimposed to

the composite pattern, the periodicity is tripled perpendicularly to the rutile {011} planes.

Notably, in the HRTEM images, the superstructure is only manifested by contrast variations

without effects on the position of atomic columns. After analyzing the possible occurrence of

Magnéli phases accommodating variations in the average V valency, we don't find solutions

simultaneously matching the FFT and image features. The image features, however, can be

satisfactorily captured considering a charge ordered insulating monoclinic M2 phase with V

atoms having the same charge alternating every three (011) planes. Our findings provide direct

evidence for the formation of a pure electronic tweed superimposed to an otherwise

untransformed atomic M2 structure, highlighting, in turn, the role of electron-electron

correlations in the MIT of VO2.

Forward Growth of Ferroelectric Domains

V. Ya. Shur1, *, D. O. Alikin,1 A. P. Turygin,1 A. V. Ievlev,2 and S. V. Kalinin2 1School of Natural Sciences and Mathematics, Ural Federal University,

620000 Ekaterinburg, Russia, 2 The Center for Nanophase Materials Sciences, Oak Ridge National Laboratory

37831 Oak Ridge, TN *E-mail: vladimir.shur@urfu.ru

The growth of single domain in polar direction (“forward domain growth”) has been studied experimentally with high spatial resolution by local polarization reversal on non-polar surfaces of uniaxial ferroelectric lithium niobate LiNbO3. The original mechanism of domain growth based on kinetic approach allowed explaining the domain growth in the areas with negligible value of applied external electric field.

The forward growth of the isolated domains with charged domain walls is one of the main stages of the domain structure evolution during polarization reversal from the single domain state in any ferroelectric [1]. It has never been studied systematically with high enough precision due to low spatial resolution of the used experimental methods. In our experiments the forward domain growth has been studied for the first time with high resolution by local polarization reversal at non-polar cuts (Y- and X-cuts) of LiNbO3 single crystals by biased conductive tip of scanning probe microscope [2-4]. The obtained wedge-like shape and extremely large domain length differ drastically from the theoretical prediction [5]. The domain patterns have been visualized by piezoresponse force microscopy and scanning electron microscopy (after selective chemical etching) [6].

Formation of the domains with charged walls contradicts to dielectric discharge model commonly used for explanation of the forward domain growth. It was proposed by M. Molotskii et al. that “the main driving force for ferroelectric domain breakdown is… rather internal force generated owing to the minimization of depolarization field energy when the domain elongates” [7]. Nevertheless the formation of the metastable domains with charged domain walls is demonstrated in many experimental situations.

The experimental results have been discussed in the framework of the kinetic approach based on the analogy of the domain structure evolution with the first order phase transformation (crystal growth) [1]. The domain growth in the area with negligible value of the applied electric field was attributed to the self-maintained domain wall motion by propagation of the interacted charged kinks [8].

The obtained knowledge is very important for understanding of the domain structure evolution during polarization reversal. Moreover it can be used for development of the domain engineering and domain wall engineering [9]. The new information about domain growth mechanism is applied successively for micro- and nano-domain patterning [10].

The equipment of the Ural Center for Shared Use “Modern Nanotechnology” UrFU was used. The research was made possible by Russian Scientific Foundation (grant 14-12-00826).

References [1] V.Ya. Shur, J. Mater. Sci., 41, 199-210 (2006) [2] D.O. Alikin et al, Appl. Phys. Lett., 106, 182902 (2015) [3] A.V. Ievlev et al., ACS Nano, 9, 769-777 (2015) [4] A.N. Morozovska et al., Phys. Rev. B, 93, 165439 (2016) [5] N.A. Pertsev, A.L. Kholkin, Phys. Rev. B, 88, 174109 (2013) [6] V.Ya. Shur, P.S. Zelenovskiy, J. Appl. Phys., 116, 066802 (2014) [7] M. Molotskii et al., Phys. Rev. Lett., 90, 107601 (2003) [8] V.Ya. Shur et al., Appl. Phys. Lett., 109, 132901 (2016) [9] V.Ya. Shur et al., Appl. Phys. Rev., 2, 040604 (2015)

[10] A.P. Turygin et al., Materials, 10, 1143 (2017)

Adsorbates and surface screening at the ferroelectric oxide surfaces

I. Spasojevic,1 E. Pach,1 K. Cordero-Edwards, J.Santiso,1 G. Catalan,1A.Verdaguer1,2 N. Domingo1

1Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193

Barcelona, Spain 2 Institut de Ciència de Materials de Barcelona, 08193 Bellaterra, Spain

*Corresponding Author: irena.spasojevic@icn2.cat

Ferroelectric materials show strong electric fields at the surface. These electric fields can be screened by different mechanisms, namely intrinsic, such as charge carriers or defects, or extrinsic, mainly adsorbates, that play a crucial role in the stabilization of polarization domains. In this sense, when considering a ferroelectric material, the electrostatic interactions between the surface and adsorbates are a critical aspect for the polarization dynamics: screening and atomistic processes at the surface is a key to control low-dimensional ferroelectricity as it has been proven in different Piezoresponse Force Microscopy (PFM) experiments [1]. Among the adsorbates in ambient conditions, water molecules due to its ubiquitous presence and its polar nature play a critical role. Despite its well known influence little is known about the water induced electrochemical reactions at the surface of ferroelectric materials for different environmental conditions. Ambient Pressure X-ray photoelectron spectroscopy (AP-XPS) has proved as a powerful tool to study the interface of oxide surfaces in the past [2] such as SrTiO3 and also ferroelectric surfaces. Using this technique, we studied the composition of the surface of ferroelectric single crystals such as LiNbO3 [3] at the line CIRCE in ALBA. In this contribution we will show our measurements on BaTiO3 thin films and single crystals in different controlled water atmospheres. We were able to identify the presence and measure the thickness of molecularly absorbed water, OH groups and the formation of carbonates for different conditions of pressure (humidity), temperature and polarization state. Results can be used to explain PFM results in the literature when working in ambient conditions. Moreover, we will show how the modification of the surface through the use of different molecules is correlated with ferroelectric response.

References [1] J.J. Segura, N. Domingo, J. Fraxedas, A. Verdaguer J. Appl. Phys. 113, 187213 (2013). [2] N.Domingo, E. Pach,, K. Cordero-Edwards, V. Pérez-Dieste, C. Escudero, A. Verdaguer (manuscript under revisión J.Phys. Chem C) [3] K. Cordero-Edwards, L. Rodríguez, A. Calo, M. J. Esplandiu, V. Peŕez-Dieste, C. Escudero, N. Domingo and A.Verdaguer J. Phys. Chem. C 120, 24048 (2016).

Significant increment of the dielectric permittivity and domain properties

in the (1-x)PbTiO3-x(Na0.5Bi0.5)TiO3 crystals

J. Suchanicz1, P. Czaja1, M. Piasecki2, M. B. Zapart3, K. Konieczny1, J. Michniowski3,

D. Sitko4, G. Stachowski4, K. Kluczewska1

1Institute of Technics, Pedagogical University, ul. Podchorążych 2, 30-084 Kraków, Poland 2Institute of Physics, Jan Dlugosz University, Al. Armii Krajowej 13-15, 42-201 Częstochowa, Poland 3Institute of Physics, Technical University of Częstochowa, Al. Armii Krajowej 19, 42-200

Częstochowa, Poland 4Institute of Physics, Pedagogical University, ul. Podchorążych 2, 30-084 Kraków, Poland

e-mail of the presenting author: jan.suchanicz@up.krakow.pl

Abstract

Crystals of PbTiO3 and 0.9PbTiO3-0.1(Na0.5Bi0.5)TiO3 were obtained by the flux growth

method whereas crystals of (Na0.5Bi0.5)TiO3 were growth by the Czochralski method.

Raman spectroscopy and polarized light microscopy were performed at room

temperature. The Raman spectra of 0.9PbTiO3-0.1(Na0.5Bi0.5)TiO3 shown significant

changes comparing to the base materials PbTiO3 and (Na0.5Bi0.5)TiO3. A domain

structure was investigated by use polarized light microscopy. Dielectric permittivity

measurements were carried out in the temperature range from 20°C to 550°C and

a frequency from 1 kHz to 1 MHz. These showed higher dielectric permittivity for the

crystals 0.9PbTiO3-0.1(Na0.5Bi0.5)TiO3 than the source materials PbTiO3 and

(Na0.5Bi0.5)TiO3.

The high value of dielectric constant makes it possible to applied 0.9PbTiO3-

0.1(Na0.5Bi0.5)TiO3 as efficient dielectric medium in a capacitors. The small size in the

domain structure with the easy possibility of switching by application of an external

electric field, give opportunities to applied this materials for FRAM memory

applications. Moreover, the high sensitivity of these materials to the surrounding gases

e.g. ammonia, chlorine, hydrogen, etc., allows the construction of sensor devices.

Keywords: perovskite, ferroelectric materials, dielectric properties, optical

properties

Nonvolatile ferroelectric domain wall memory

Nagarajan Valanoor1

1 The University of New South Wales Sydney -UNSW Sydney Ferroelectric domain walls are atomically sharp topological defects that separate regions of uniform polarization. The discovery of electrical conductivity in specific types of walls gave rise to “domain wall nanoelectronics,” a technology in which the wall (rather than the domain) stores information. This paradigm shift critically hinges on precise nanoengineering of reconfigurable domain walls. Using specially designed nanofabricated electrodes and scanning probe techniques, we demonstrate a prototype nonvolatile ferroelectric domain wall memory, scal- able to below 100 nm, whose binary state is defined by the existence or absence of conductive walls. The device can be read out nondestructively at moderate voltages (<3 V), exhibits relatively high OFF-ON ratios ( 103) with excellent endurance and retention characteristics, and has multilevel data storage capacity. This achievement is made possible through a combination of electron beam (e-beam) nanolithography, judicious selection of the crystallographic growth direction of high-quality epitaxial bismuth ferrite thin films and custom-designed scanning probe micros- copy (SPM) approaches. In particular, the specially designed in-plane geometry of the electrodes allows for encoding and retrieval of in- formation via moderate electric fields rather than electric currents, thus enabling low-energy operation. Therefore, data storage densities of these FEDW memory devices can be improved markedly compared to what is achievable using traditional binary bits. Our work thus contitutes an important step toward integrated nanoscale ferroelectric domain wall memory devices. This work is a collaboration between the UNSW Sydney, U Washington, Xiangtan University, St. Louis University and Shenzen Institutes of Advanced Technology. It appears as Sharma et al., Sci. Adv. 2017;3:e1700512 References [1] Sharma et al., Sci. Adv. 2017;3:e1700512

Flexoelectricity and Electrocaloric Effect in Antiferroelectrics

P. Vales-Castro1, Romain Faye2, Emmanuel Defay2, Krystian Roleder3, Lei Zhao4, Jing-Feng

Li4, Dariusz Kajewski3, Gustau Catalan1,5

1 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus

Universitat Autonoma de Barcelona, Bellaterra 08193, Spain; email: pablo.vales@icn2.cat

2 Luxembourg Institute of Science and Technology (LIST), Materials Research & Technology

Department (MRT), 41 Rue du Brill, L-4422 Belvaux , Luxembourg

3 Institute of Physics, University of Silesia in Katowice, ul. Uniwersytecka 4, 40-00 Katowice,

Poland.

4 State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science

and Engineering, Tsinghua University, Beijing 100084, China.

5 Institut Català de Recerca i Estudis Avanc¸ats (ICREA), Barcelona 08010, Catalonia;

email: gustau.catalan@icn2.cat

Antiferroelectrics are characterized by a spontaneous zero net polarization that can be switched

into a ferroelectric state with an external electric field. Moreover, their dielectric constant

undergoes a Curie-Weiss anomaly at the phase transition, where it increases sharply. They have

not being as researched as their ferroelectric counterparts, partly due to their centrosymmetric

and non-polar ground state, which make them less obvious for applications. Here we present

experimental results on the anomalous electrocaloric and flexoelectric responses of these

materials.

The electrocaloric effect of antiferroelectrics has been researched due to their anomalous

(negative) response, in which the samples decrease their temperature when a voltage step is

applied. We have examined the response close to and beyond their Curie temperature, where

we measured a giant figure of merit |ΔT|/|ΔE| and the negative-to-positive electrocaloric

transition with an infrared camera. Meanwhile, flexoelectricity- the coupling between

polarization and strain gradient- is a universal property of all materials, but it has never been

experimentally characterized in antiferroelectrics. And, yet, it has been hypothesized that the

antiferroelectric phase might be stabilized by the influence of flexocoupling on the free energy

of the system, thus pointing into a higher flexocoupling value than their ferroelectric or non-

polar counterparts with the same lattice structure. We show that this flexocoupling is not higher

than in simple dielectrics; however, unexpectedly, this flexocoupling is not constant as a

function of temperature but increases sharply at the antiferroelectric-paraelectric phase

transition.

Characterization of Periodical Domain Patterns Created by Electron Beam in MgO-doped Lithium Niobate by Second Harmonic Generation

E. O. Vlasov1, D. S. Chezganov1, M. M. Neradovskiy1,2, C. Montes2, A. R. Akhmatkhanov1, I. S. Baturin1, V. Ya. Shur1

1School of Natural Sciences and Mathematics, Ural Federal University, 620000 Ekaterinburg, Russia

2Université Côte d’Azur, CNRS, InPhyNi, Parc Valrose, 06100 Nice, France E-mail: chezganov.dmitry@urfu.ru

The spatial inhomogeneity of the periodically poled (PP) domain structures created by electron beam (e-beam) irradiation in MgO-doped congruent lithium niobate (MgOLN) covered by a surface dielectric layer was studied experimentally by second harmonic generation (SHG) and by computer simulation.

The studied samples represented the 1-mm-thick Z-cut MgOLN (Crystal Tech.) plates. The PP domain structure creation has been realized by e-beam irradiation of the Z- polar surface covered by resist layer AZ nLOF 2020 (Microchemicals) using Auriga Crossbeam Workstation (Carl Zeiss). The patterning parameters and beam positioning were controlled by e-beam lithography system Elphy Multibeam (Raith) [1]. The SHG experiments were carried out using single-pass scheme in experimental setup, which allowed measuring the spatial distribution of the SHG efficiency in ZY plane. The diode pumped single-mode linearly polarized CW Yb fiber laser (YLR-10-LP, IPG Photonics) operating at 1064 nm was used as a pumping source with a power about one Watt.

It was shown that excessive charging of the irradiated surface leads to degradation of the PP domain structure quality by appearance of the period and duty cycle inhomogeneities. This fact was attributed to electrons deflection from the given trajectories. It was demonstrated experimentally and by finite element computer simulation that the effect can be significantly reduced by increasing of accelerating voltage (U).

The quantitative analysis of the PP domain structure quality was carried out by the measurement of temperature dependencies of SHG power by 2D scanning in ZY plane. The estimation of quasi-phase matching (QPM) temperature dispersion, FWHM of the SHG main peak, SHG power and the efficiency of the SHG process showed that increase of the accelerating voltage from 10 to 14 kV leads to improvement of the spatial homogeneity of the PPLN period. The spatial distribution of the QPM temperature (PPLN period) over PPLN element demonstrated the homogeneity of the structure along the polar direction, while the homogeneity along the Y-direction depends on the U value. The best PPLN homogeneity was obtained at 14 kV. The SHG power about 2.1 mW and the efficiency of 0.3% / W were achieved.

A comparison with PPLN reference element created by the traditional electric field poling showed that the e-beam poled PPLN demonstrates higher period homogeneity, while the SHG efficiency can be enhanced by improving of the PPLN duty cycle.

The numerical simulation based on the model proposed in [2] was used to explain the experimental results. The analysis of simulation results showed that the presence of regions with different periods leads to effects similar to the write field stitching defects [2]: (1) the appearance of several peaks with high intensity, (2) the shift of the main peak relative to the peak position in the PPLN with a constant period, (3) the decrease in the main peak intensity. The magnitude of the effects depends on the difference in the period of regions and the number of such regions. The presence of PPLN regions with different periods and stitching defects leads to a complicated shape of the SHG temperature curve.

The equipment of the Ural Center for Shared Use “Modern nanotechnology” UrFU was used. The research was made possible by the Russian Science Foundation (Grant № 17-72-10152).

References

[1] V.Ya. Shur et al., Appl. Phys. Lett., 106, 232902 (2015). [2] M. Neradovskiy et al., J. Opt. Soc. B., 35, 331 (2018).

Ferroelectrically tunable magnetic skyrmions in ultrathin oxide heterostructures

Lingfei Wang1,2 and Tae Won Noh1,2 1Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul, Republic of

Korea 2 Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of

Korea E-mail: Lingfei.wang@snu.ac.kr

Magnetic skyrmions are topologically protected, whirling spin texture in nanoscale.[1] Its small size, topologically-protected stability, and solitonic characteristics together hold great promises for future spintronics applications.[2] To translate such compelling features into practical spintronic devices, a key challenge lies in achieving effective controls of skyrmion size, density, and thermodynamic stability. Here, we report the discovery of ferroelectrically tunable skyrmions in ultrathin BaTiO3/SrRuO3 bilayer heterostructures.[3,4] The ferroelectric proximity effect at the BaTiO3/SrRuO3 heterointerface can trigger a sizable Dzyaloshinskii-Moriya interaction, thus stabilizing robust high-density skyrmions. The minimum skyrmion size in this system can reach approximately 10 nm. Moreover, by manipulating the ferroelectric polarization of BaTiO3, we achieve local, switchable and nonvolatile tunability of both skyrmion density and thermodynamic stability. Such ferroelectric control of skyrmion properties heralds a novel approach toward improvements in the integratability and addressability of skyrmion-based functional devices. References

[1] Nagaosa, N. & Tokura, Y. Nature Nanotechnology 8, 899 (2013). [2] Fert, A., Reyren, N. & Cros, V. Nature Reviews Materials 2, 17031 (2017). [3] L. Wang, et al. Nano Letters 16, 3911 (2016). [4] Wang, L. et al. Advanced Materials 29, 1702001 (2017).

Flexoelectric Polarizations at Ferroelastic Domain Walls in Non-Ferroelectric WO3 Thin Films

Shinhee Yun1, Kanghyun Chu1, Chang-Su Woo1, Gi-Yeop Kim2, Si-Young Choi2, Kyung Song3 and Chan-Ho Yang1,4

1 Department of Physics, KAIST, Daejeon, 34141, Republic of Korea, 2 Department of Materials Science and Engineering, POSTECH, Pohang, 37673, Republic of

Korea 3 Korea Institute of Materials Science, Changwon, 51508, Republic of Korea,

4 KAIST Institute for the NanoCentury, KAIST, Daejeon, 34141, Republic of Korea It has been known that WO3 has a centrosymmetric monoclinic structure at room temperature with a peculiar hierarchical twin-domain structure [1]. The twin walls are different from the domain areas in terms of electronic structure and ionic potential, and they are stable places where charged particles such as electrons and point defects gather. In addition, the twin walls where two different ferroelastic domains are colliding provide a unique crystal structure due to the existence of strain gradients, whereby leading to the flexoelectric phenomenon which is a universal physical property existing in all crystalline materials. Experimental observations of polarizations purely caused by the flexoelectric effect, however, are few. In this work, we present emergence of the flexoelectric polarizations at the twin walls in the non-ferroelectric material, which is characterized by angle-resolved piezoresponse force microscope [2]. [1] S. Yun et al., Appl. Phys. Lett. 107, 252904 (2015) [2] K. Chu et al., Nat. Nanotech. 10, 972 (2015)