“New-Concept Spintronics Devices”tomostler.co.uk/wp-content/uploads/2015/06/abstracts.pdf ·...

11 - 13 June 2015 Departments of Physics and Electronics

on “New-Concept Spintronics Devices”

including the Core-to-Core Kick-Off Meeting,

Sir Martin Wood Prize Lecture and

the 3rd HARFIR Open Workshop

HARFIR!

Thursday, 11th June

08 20 Bus from Monkbar Monkbar Hotel (Guests only)

08 35 Bus from York Pavilion Hotel (Guests only)

09 00 Welcome Chair: Atsufumi Hirohata P/X 001

Brian Fulton (Dean of Faculty of Science Univ. of York)

Toshiya Ueki (Executive Vice-President, Tohoku Univ.)

Burkard Hillebrands (Tech. Univ. Kaiserslautern)

09 20 Introduction to the Core-Core Program

Kevin O’Grady ( Dept of Physics, Univ. of York)

Hard Disk Drives Chair: Simon Greaves P/X 001

09:30 HAMR Media Based on Exchange Bias - Kevin O’Grady

10:00 Thermal Stability of Bit-Patterned Media - Hiraoki Muraoka

10:20 The Role of Strain in Ultra-thin FeRh Films for HAMR - Tom Thomson

10:40 HDD Industry and Technology Challenges Review - Gabriel McCafferty

11:00 Channelling of Spin Waves in Chiral Materials - Bob Stamps

11:20 Tea/Coffee P/L 005

Organic Spintronics Chair: Fumihiro Matsukura P/X 001

11:35 Spin Polarization of Surface and Mono-Atomic Layers for Spinterface Characterization - Yasushi Yamauchi

11:55 Exchange Coupling at Organic Semiconductor/Ferromagnetic Interfaces - Andrew Pratt

12:15 Exploring the Potential of Phthalocyanines for Organic Spintronics - Sandrine Heutz

12:35 Lunch/Posters P/L 005

Domain-wall Memories Chair: Bernard Dieny P/X 001

13:50 A Sound Approach: Manipulating Domain Walls with Surface Acoustic Waves - Tom Hayward

14:10 Simulation of Field-Driven Magnetic Domain Wall Motion Under the Dzaloshinskii-Moriya Interaction

- Yoshinobu Nakatani

14:30 Domain Wall Pinning in nanowires by Exchange Bias - Gonzalo Vallejo-Fernandez

14:50 Domain Wall Pinning in Nanowires by Antiferromagnets - Simon Greaves

15:10 Chiral Domain Walls in Pt/Co/Pt and Ta/CoFeB/MgO - Thomas Moore


High-frequency Spintronics Chair: Masafumi Shirai P/X 001

15:45 Magnon Transport Using Macroscopic Quantum States - Burkard Hillebrands

16:15 Tetragonal Mn-based Heusler for High-frequency Spintronics - Shigemi Mizukami

16:35 Controlling Interfaces in Magnetic Tunnel Junctions - Seiji Mitani

16:55 Modulation of Spin Precession Frequency by Spin Relaxation Anisotropy - Makoto Kohda

Symposium Dinner (Invited Speakers Only)

17:15 Bus to York Pavilion Hotel

17:25 Continue to Monkbar Hotel

17:45 Continue to Railway Station

18:15 Depart Monkbar Hotel

18:30 Depart York Pavilion Hotel

19:00 Symposium dinner St. Vincent Arms, Sutton-upon-Derwent

22:00 Bus to Hotels

HARD DISK DRIVES | Thursday, 11th June – 09:30-11:20 1

Hard Disk Drives Thursday 11th June 09:30-11:20

HAMR Media Based on Exchange Bias

K. Elphick1, G. Vallejo-Fernandez1, K. O’Grady1, T. J. Klemmer2, J. U. Thiele2 1University of York, York, UK 2Seagate Media Research, Fremont, California, USA

In a conventional HAMR system a FePt granular layer is used to provide both a temperature dependent magnetocrystalline

anisotropy and to give the required signal for the retrieval of the information. Hence a single material is used with two

functions. With bi-functionality of this type it is difficult to optimise each function and generally a compromise for both

functions is required. In the case of FePt there are also serious issues associated with achieving the necessary phase

transformation to create the high anisotropy required.

We have undertaken a study with the intention of separating the two functions so that the temperature dependent

anisotropy is generated separately from the ferromagnetic signal required for read back. We are proposing to use an

antiferromagnet (AF) material such as IrMn to provide the temperature dependent anisotropy coupled to a conventional

CoPtCr-SiO2 ferromagnetic (F) layer. In such an exchange bias system the information is stored in the AF. This has the

advantage that the data cannot be field erased and can only be erased by the use of thermal energy.

Our initial work has been aimed at providing proof of principle. We have identified a suitable seed layer structure to

orient the IrMn in the perpendicular direction and we have achieved coupling to the conventional CoPtCr layer.

Furthermore we have achieved a shifted hysteresis loop with segregated grains as required for a low noise medium. A full

description of the results obtained to date and prospects for future developments will be presented.

Thermal Stability of Bit-Patterned Media

H. Muraoka RIEC, Tohoku University, Sendai, Japan

Thermal stability of bit-patterned media (BPM) was studied. In granular media dozens of grains represent a bit as the sum.

In contrast, individual bit is represented with a magnetic dot of the BPM, which is essentially different from the granular

media. The thermal instability of random flipping of magnetic dots does not result in the read amplitude decay in BPM, but

a sudden polarity change of the dot magnetization. This corresponds to a time-dependent write-error. The calculation

shows a relatively large deterioration of write-error rate after 5 years. A larger barrier energy is thus required in order to

avoid the write-error rate deterioration than that for conventional read-amplitude decay criterion. The influence of statistical

dot size distribution is revealed. Since smaller dots are significantly influenced by the thermal instability, the fraction of

particle size distribution is crucial. Typical statistics for the distribution are discussed for the normal and log-normal

distribution. The required barrier energy are quantitatively calculated for the grain structure

The Role of Strain in Ultra-thin FeRh Films for HAMR

C. Barton1, G. Hrkac2 and T. Thomson1 1University of Manchester, Manchester, UK 2University of Exeter, Exeter, UK

Heat assisted magnetic recording (HAMR) is a likely candidate for achieving higher areal densities in future magnetic

recording technologies [1] as thermal energy is combined with an applied magnetic field to reverse the medium allowing

the media trilemma to be circumvented [2]. Current media designs for HAMR are focused on L10 ordered FePt, due to its

high perpendicular magnetocrystalline anisotropy which is heated over nanometer lateral dimensions using a laser and

near-field aperture. This approach requires that the entire medium is heated close to the Curie temperature of FePt (~750 K)

which provides significant thermal management challenges. One possible solution, that would allow the thermal load to be

reduced, is to create a heat sensitive magnetic exchange spring based on FePt. Here, heat is used to active (switch-on) the

exchange spring allowing the applied field (from the head) to reverse the medium. The thermal load is much reduced as

now only the heat sensitive layer needs to be at a high temperature. The material of choice for this intermediate layer is

Fe50Rh50 which undergoes a first order phase transition from antiferromagnetic (AF) to ferromagnetic (F) at a technologically

useful temperature (~370 K), thereby creating the thermal switch for an exchange spring structure e.g. FePt/FeRh/Fe.

Research on bulk Fe50Rh50 and thicker films (~ 20- 50nm) shows promising results [3] but in order to be used in exchange

spring structures much thinner layers will be required and a deeper understanding of the properties and physics of thin

films (2 – 10 nm) is needed. One significant problem that has emerged is associated with the interface of FeRh thin films, i.e.

at the substrate or capping material, where the ferromagnetic phase remains stable at room temperature [4, 5]. This is

HARD DISK DRIVES | Thursday, 11th June – 09:30-11:20 2

undesirable as it would leave the thermal switch effectively in the on-state. In this presentation very recent work aimed at

understanding the magnetic and structural properties of ultra-thin FeRh films will be given where we observe increase of

the c parameter as the film thickness is reduced.

[1] Weller, D., et al, IEEE Trans. Magn. 50, (2014) pp. 3100108

[2] Richter, H., J. Phys. D: Appl. Phys. 40 (2007) pp. 149–176

[3] Thiele, J.-U., et al, Appl. Phys. Lett. 82, (2003) pp. 2859-2861

[4] Baldasseroni, C. J. Appl. Phys. 115, (2014) pp. 043919 1-9

[5] Han, G. C., et al, J. Appl. Phys. 113, (2013) pp.17C107

HDD Industry and Technology Challenges Review

G. McCafferty Seagate Technology, Springtown, Northern Ireland

Review of the trends impacting data storage, factors driving growth and the current HDD market. With the ever increasing

areal density demands resulting in shrinking of head dimensions, we review the impact on reader and writer designs and

how HAMR will provide Tbit/in2 areal density.

Channelling of Spin Waves in Chiral Materials

R. L. Stamps SUPA School of Physics and Astronomy, University of Glasgow, Glasgow, UK

Interface induced Dzyaloshinskii interactions can have significant effects on domain wall structure and mobility in thin

ferromagnetic films. Moreover, spin wave propagation can be strongly affected, especially in regards to scattering from

magnetic domain walls. Propagation of travelling spin waves through and along magnetic domain walls in systems with

interface induced Dzyaloshinskii-Moriya interactions will be discussed and a scheme proposed whereby spin waves can be

confined to narrow channels.

[1] L. Heyderman, R. L. Stamps, J. Phys. : Cond. Matt. 25, (2013), 363201

[2] L.R. Silva et al. Phys. Rev. B 89, 5 (2014) 054434

[3] F. Garcia-Sanchez et al. Phys. Rev. B 89, 22, (2014 ), 224408

[4] F. Garcia-Sanchez et al. Phys. Rev. Lett. (2014), To Appear

ORGANIC SPINTRONICS | Thursday, 11th June – 11:35-12:35 3

Organic Spintronics Thursday 11th June 11:35-12:35

Spin Polarization Analysis of Surface and Mono-Atomic Layers for Spinterface Characterization

Y. Yamauchi1, M. Ohtomo1, X. Sun2, A. Pratt3, M. Kurahashi1, M. Yoshitake1 1National Institute for Materials Science, Tsukuba, Japan 2University of Science and Technology of China, Hefei, China 3University of York, York, UK

The remarkable progress of organic displays and other organic devices, which has led to new industries, is extending the

field of spintronics due to the long spin relaxation time of carrier spins expected from the small spin orbit coupling of light

elements. Organic spintronic devices usually include an organic charge/spin-transport channel and inorganic ferromagnetic

spin-injection electrodes. Among the variety of spin-injection methods into inorganic materials, electrical spin injection from

ferromagnetic materials has mainly been investigated also for organic materials. Electrical spin injection, however, has a

problem caused by impedance mismatch, which prevents efficient spin injection from a ferromagnetic metal electrode

having low resistance into a nonmagnetic channel with high resistance such as organic materials. Therefore, pure spin

current approaches, [1] that use ferromagnetic electron resonance without charge current, have also been attempted for

organic devices in addition to electrical injection through tunneling barriers. [2] The performance of such devices primarily

depends on the properties of the materials used as the nonmagnetic channel and spin-injecting electrodes, but is critically

influenced by the spinterfaces. Electrons at interfaces are not directly accessible but, instead, at surfaces or mono-atomic

layers, electrons can be detected by using surface sensitive techniques. In order to investigate the spin polarization of

surface or mono-atomic layers, we use spin-polarized metastable deexcitation spectroscopy (SPMDS) in which polarized

helium atoms interact so delicately with a surface that the spin polarization of only the electrons in the outermost layers is

detected. Our previous studies clarify that mono atomic layers of organic molecules including graphene exhibit relatively

strong interaction with ferromagnetic transition metal surfaces. [3] Since the interaction is expected to be mediated by

inserting an insulating mono atomic layer, we have analyzed the surface spin polarization of a h-BN monolayer on a Ni

(111) surface and observed the contact-induced spin polarization. [4] SPMDS has been developed further to spin-polarized

metastable-atom emission electron microscopy (SPMEEM) with the combination of a hexapole magnet for generating

intense spin-polarized metastable helium atom beams and a photoemission electron microscope (Staib, PEEM350). Spin

images have been successfully obtained for the topmost surface of halfmetallic Fe3O4.

[1] Watanabe, et al., Nature Phys. 10 (2014) 308

[2] Dlubak, et al., Nature Phys. 8 (2012) 557

[3] Carbon 61 (2013) 134

[4] Appl. Phys. Lett. 104 (2014) 051604-1

ORGANIC SPINTRONICS | Thursday, 11th June – 11:35-12:35 4

Exchange Coupling at Organic Semiconductor/Ferromagnetic Interfaces

A. Pratt1, 2, X. Sun3, Y. Bae4, M. Kurahashi2, P. Graziosi5, J. Zhang1, I. Bergenti5, M. Prezioso6, N. Lee4, T. Kim4, A. Dediu5, Y. Yamauchi2 1University of York, York, U.K. 2National Institute for Materials Science, Tsukuba, Japan 3University of Science and Technology of China, Hefei, China 4EWHA Womans University, South Korea 5CNR – ISMN, Bologna, Italy 6University of California, Santa Barbara, California, USA

Following the recent realization that organic semiconductor (OSC)/ferromagnetic (FM) interface states require a much

deeper understanding in order to make progress in the field of molecular spintronics [1], much attention has been paid to

the magnetic exchange coupling that exists at these ‘spinterfaces’ [2]. Early work has shown that stable magnetic order may

be induced in the paramagnetic molecular layer although the alignment direction and origin of this effect are the topic of

much current discussion [3]. Clarifying mechanisms of exchange coupling, suggested to take place either directly or

indirectly across chemical bonds at the interface, is essential to be able to engineer novel organic devices. To this end, we

have conducted a systematic study of a variety of OSC/FM interfaces utilising the perfect surface sensitivity associated with

the technique of spin-polarized metastable (23S) helium de-excitation spectroscopy [4,5]. As will be shown in this talk, the

results obtained reveal a general tendency towards antiferromagnetic coupling between the organic molecule and the FM

substrate helping to elucidate such phenomena as spin injection into an OSC [1] and interfacial magnetoresistance [2].

[1] S. Sanvito et al., Nature Phys. 6, 562 (2010)

[2] K. V. Raman et al., Nature 493, 509 (2013)

[3] J. Girovsky et al., Phys. Rev. B 90, 220404(R) (2014)

[4] A. Pratt et al., Phys. Rev. B 85, 180409(R) (2012)

[5] X. Sun et al., Phys. Chem. Chem Phys. 16, 95 (2014)

Exploring the Potential of Phthalocyanines for Organic Spintronics

S. Heutz Imperial College London, London, UK

Phthalocyanines (Pcs) are multifunctional materials with uses spanning industrial pigments to applications in organic solar

cells. Of particular interest is their ability to incorporate a spin-bearing transition metal at their centre, which may lead to a

combination of semiconducting and magnetic properties.

In this talk, the potential of phthalocyanine materials for organic spintronics will be presented. The spins can be exploited

either in the strong or low coupling regimes. The coupling depends strongly on the crystal phase, which can be tuned by the

growth conditions [1, 2], or by functionalization of the molecular ring [3]. While the transition temperatures have

traditionally been below 10K, limiting the practical relevance of the materials, it will be shown that the cobalt analogues can

be prepared as a one-dimensional antiferromagnet with a magnetic exchange surviving above the boiling point of liquid

nitrogen [4]. At the other extreme, the ability to isolate the spins can be harnessed to obtain ultra-long spin relaxation times

[5].

[1] Heutz et al. Adv. Mater. 19 (2007) 3618.

[2] Wang et al. ACS Nano 4 (2010) 3921.

[3] Wu et al., J. Appl. Phys. 113 (2013) 013914.

[4] Serri et al. Nat. Commun. 5 (2014) 3079.

[5] Warner et al., Nature 503 (2013) 504.

DOMAIN-WALL MEMORIES | Thursday, 11th June – 13:20-15:30 5

Domain-wall Memories Thursday 11th June 13:20-15:30

A Sound Approach: Manipulating Domain Walls with Surface Acoustic Waves

J. S. Dean1, J. D. Cooper2, A. Virbule1, M. T. Bryan1, J. E. Cunningham2 and T. J. Hayward1 1University of Sheffield, Sheffield, UK 2University of Leeds, Leeds, UK

Domain walls (DWs) in magnetic nanowires have great technological potential through the development of memory [1] and

logic devices [2,3]. In these devices highly mobile DWs separate magnetically bi-stable domains, the orientations of which

represents binary data. Coherently moving the DWs transports data along the nanowires, thus facilitating reading, writing

and processing operations. A major challenge in the development of these technologies is finding efficient methods of

transporting DWs. Attempts have been made to exploit a variety of interactions including applied magnetic fields [4],

spin-torque effects [5,6,7] and stresses in composite multiferroic systems [8].

In this talk we will use finite-element micromagnetic simulations to propose a new method of pinning and propagating

DWs. In this approach, counter-propagating surface acoustic waves (SAWs) are electrically launched into a piezoelectric

substrate using a pair of interdigitated transducers (IDTs), thus forming a standing stress/strain wave (SW) along a

Fe70Ga18B12 nanowire placed between them. We will show that this SW creates an array of DW pinning sites within the

nanowire, and go on to explain the nature of the SAW-DW interactions using a semi-analytical 1D model. We will then

demonstrate how applying small shifts to the frequencies of the transducers allows multiple DWs to be synchronously

transported along the nanowire at velocities up to 50 ms-1. Our results indicate the feasibility of a new class of multiferroic

technologies where electrically-induced acoustic waves are used to remotely manipulate DWs.

[1] S. S. P. Parkin et al. Science, 320, (2008) 190-194

[2] D. A. Allwood et al. Science, 309, (2005) 1688

[3] K. Omari and T.J. Hayward, Phys. Rev. Appl. 2, (2014) 044001

[4] G.S.D. Beach et al. Nature Materials 4, (2005) 741

[5] M. Hayashi, L. Thomas, C. Rettner, R. Moriya, Y. B. Bazaliy, S. S. P. Parkin, Phys. Rev. Lett. 98, 037204 (2007).

[6] I. M. Miron, T. Moore, H. Szambo, L. D. Buda-Prejbeanu, S. Auffret, B. Rodmacq, S. Pizzini, J. Vogel, M. Bonfim, A.

Schuhl and G. Gaudin, Nature Mater. 10, 423 (2011).

[7] K.-S. Ryu, L. Thomas, S.-H. Yang and S. S. P. Parkin, Nature Nanotech. 8, 527 (2013).

[8] J. Dean, M. T. Bryan, T. Schrefl and D. A. Allwood, J. Appl. Phys. 109, 023915 (2011).

Simulation of the Field-Driven Magnetic Domain Wall Motion Under the Dzyaloshinskii-Moriya Interaction

K. Yamada, Y. Nakatani University of Electro-Communications, Tokyo, Japan

The research and development of non–volatile memories and logic devices utilizing the magnetic domain wall (DW) motion

in the nanowire have been recently progressed [1,2]. In particular, the studies of the DW motion using a ferromagnetic

ultrathin film with perpendicular anisotropy sandwiched between two non-magnetic layers are hot topics, because it is

possible to reveal various physical properties that are appeared in the interface between a ferromagnetic layer and the

non-magnetic layer. Among of them, the Dzyaloshinskii-Moriya interaction (DMI) has been a great attention [3-6]. It was

reported by the theoretical analysis that the DMI produces an effective chiral field that stabilizes Néel type walls, leading to

efficient coherent motion (the steady motion) much faster than the DW velocity of several hundred at the Walker field [3,4].

Fig. 1. Field-induced domain wall velocity for different value of the DMI parameter D. The results of

the 2D calculation for the wire width (a) w = 50 nm, (b) w = 100 nm, and (c) w = 200 nm.


In these reports, they studied the DW motion around the Walker field, however they have not discussed the DW motion in

a high field region in which it is moved with the precessional motion. In the region, the DW velocity is affected by

fluctuations of DW shape [7], and the vertical Bloch lines (VBLs) are nucleated and moved on the DW. From these reasons,

it is necessary to examine the DW motion in the high field region in detail. In this report, we perform micromagnetic

simulations and investigate the field-driven DW motion under the DMI, in particularly, in the high field region where is

much higher than the Walker field. Simulation results show that the number of breakdowns increases as the wire width and

DMI are increased.

[1] S. S. P. Parkin, U.S. Patent 6 834 005 (2004)

[2] D. A. Allwood, et al., Science 309, 1688 (2005)

[3] A. Thiaville, et al., Europhys. Lett. 100, 57002 (2012)

[4] O. Boulle, et al., Phys. Rev. Lett. 111, 217203 (2013)

[5] S. Emori, et al., Nat. Mater. 12, 611 (2013)

[6] K.-S. Ryu, et al., Nat. Nanotechnol. 8, 527 (2013)

[7] K.Yamada, et al., Appl. Phys. Express. 4, 113001 (2011)

Domain Wall Pinning in Nanowires by Exchange Bias

I Polenciuc1, A Hirohata2,3, A Vick1, G Vallejo-Fernandez1, T. J. Hayward4, D Allwood4 and K O’Grady1

1Department of Physics, University of York, York, UK 2Department of Electronics, University of York, York, UK 3 PRESTO, Japan Science and Technology Agency, Kawaguchi, Japan 4University of Sheffield, Sheffield, UK

The pinning of domain walls in ferromagnetic (F) wires is one possible technique for the creation of a solid state magnetic

memory [1]. Such a system has been under consideration for some time but one of the main limitations is to achieve

uniformity of domain wall pinning. Techniques such as the lithographic definition of notches and steps in the substrate

have had some success in creating uniform pins but have the disadvantage of being expensive to fabricate and the

reproducibility of the domain wall pinning strength is limited [2]. In this work we examine the viability of a radical

alternative strategy to create the pins using crossed F and antiferromagnetic (AF) wires such that exchange bias can be

introduced at the crossing points. Such a system has the advantage of relative ease of fabrication and the possibility of

creating domain wall pins of controlled strength by varying either the degree of exchange bias and the angle of the

exchange bias relative to the direction of the F wire such that the pinning would align with the angle at which the domain

wall forms. [3]

To examine the feasibility of such a system standard optical lithography has been used. In the structure examined IrMn

AF wires were deposited onto a NiCr seed layer with a CoFe F wire deposited on top of the structure. The magnetic

behaviour of the wire was evaluated using a focussed scanning MOKE magnetometer at various points along the wire.

The variation of coercivity with position was then determined. The measurements show that for this system, the initial

coercivity in the region of the injection pad is of the order of 58 Oe. To push the DW from the pad to the wire a larger field

of around 70 Oe is required. As the first domain wall pin is encountered the coercivity rises to the order of 109 Oe and then

remains constant. This larger coercivity is indicative of the strength of the domain wall pins.

One aspect of the data that is unexpected is that once the domains cross the pinning sites created by the AF layer the

coercivity remains essentially constant. This indicates that increasing the width of the AF wire from 1μm to 2μm does not

affect the domain wall pinning strength which suggests that a much narrower AF wire could be used. Additionally in this

case we have used a relatively wide AF wire spacing of 10μm. Given that the AF wire width can be less than 1μm this may

allow for AF pinning wires to be placed much closer together thereby increasing the linear data density along the wire.

[1] S. S. P. Parkin Science 320 (2008), 190

[2]K. O’Grady et al. Exchange Bias RacetrackMemory GB Patent 1218010.5/2013

[3] A. J. Annunziata et al. IEEE Int. Electron Devices Meet. 539 (2011)


Domain Wall Pinning in Nanowires by Antiferromagnets

S. Greaves RIEC, Tohoku University, Sendai, Japan

Magnetic nanowires have been proposed as a data storage device with data bits represented by domain walls. Domain wall

pinning sites are necessary to maintain a regular spacing between domain walls and notches in the nanowires are one

possible way to achieve this. An alternative approach is to use antiferromagnetic coupling to pin the domain walls at

predetermined locations. In this work magnetic nanowires with antiferromagnetic pinning sites were modelled to

investigate the conditions for pinning and the effect on domain walls.

In the case of wires with in-plane magnetisation transverse domain walls were repelled from the pinning sites when the

pinning field direction was along the length of the nanowire due to magnetostatic interactions. Changing the pinning field

direction to lie along the width of the nanowire resulted in domain wall pinning.

For wires with perpendicular anisotropy and Néel domain walls pinning was also realised when the pinning field direction

was along the width of the nanowire. The Néel wall was transformed to a Bloch wall on reaching the pinning site, but the

Néel wall was restored once the wall exited the pinning site.

The results also show a pulsed current can be used to depin a wall from one pinning site and move it to the next.

Chiral Domain Walls in Pt/Co/Pt and Ta/CoFeB/MgO

T. A. Moore University of Leeds, Leeds, UK

Magnetic domain walls driven by external field and electric current underpin device concepts such as magnetic logic and

domain wall memory [1]. Spin-orbit and interface effects such as the Dzyaloshinskii-Moriya interaction (DMI) and

spin-orbit torque have recently been found to have an important influence on the chirality and dynamics of domain walls in

multilayers with perpendicular magnetic anisotropy (PMA) [2]. However, despite reports of high domain wall velocities

in nanowires with PMA, the critical current density required for domain wall motion remains large, of the order of 1011

A/m2 [3]. Here we examine domain wall creep in Pt/Co/Pt films, allowing us to access the energy barriers that give rise

to these critical current densities. We study the effect of microstructure and strain on domain wall creep and on the DMI.

We also investigate the current-driven motion of chiral domain walls in Ta/CoFeB/MgO, a multilayer with typically lower

coercivity than Pt/Co/Pt.

We have grown epitaxial Pt/Co(0.7nm)/Pt films by sputtering onto heated C-plane sapphire substrates, and found that the

intermixing of the chemical species at the Co/Pt interface may be tuned via the substrate temperature during deposition.

The increase in intermixing observed by X-ray reflectometry for substrate temperatures in the range 100-250C is correlated

with an increase in DMI. We have found that the microstructure affects field-driven creep seen by Kerr microscopy, giving

enhanced domain wall velocities and low pinning energies in the best ordered films [4].

In room-temperature sputtered Pt/Co(t = 0.85-1.0 nm)/Pt grown on glass and bonded to piezoelectric transducers we have

shown that PMA can be reduced by 10 kJ/m3 by a tensile out-of-plane strain of 9 x 10-4, independently of the Co thickness

[5]. We find that the strain increases field-induced domain wall velocities in the creep regime by 30-100%, depending on

the Co thickness, but does not affect the DMI.

For Ta/CoFeB/MgO a DMI that is opposite to similar stacks with a CoFe layer is found, showing that the B at the interface

is responsible for the DMI sign. We observe current-induced domain wall motion against the electron flow. A field

applied along the nanowires strongly affects the domain wall velocity in agreement with the spin-orbit torque model [6].

Using 1D model simulations that include pinning we reproduce the experimental data and extract a spin Hall angle of -0.11

for Ta.

[1] D.A. Allwood et al., Science 309, 1688 (2005); S.S.P. Parkin et al., Science 320, 190 (2008); S. Fukami et al., Symp. on VLSI

Tech. 230 (2009)

[2] I.M. Miron et al., Nat. Mater. 10, 419 (2011); S. Emori et al., Nat. Mater. 12, 611 (2013); K.S. Ryu et al., Nat. Nanotech. 8,

527 (2013)

[3] S.-H. Yang et al., Nat. Nanotech. 10, 221 (2015)

[4] A. Mihai et al., Appl. Phys. Lett. 103, 262401 (2013)

[5] P.M. Shepley et al., Sci. Rep. 5, 7921 (2015)

[6] R. Lo Conte et al., Appl. Phys. Lett. 105, 122404 (2014); R. Lo Conte et al., Phys. Rev. B 91, 014433 (2015)

HIGH-FREQUENCY SPINTRONICS | Thursday, 11th June – 15:45-16:55 8

High-frequency Spintronics Thursday 11th June 15:45-16:55

Magnon Transport Using Macroscopic Quantum States

B. Hillebrands1, P. Clausen1, D. A. Bozhko1, 2, V. I. Vasyuchka3, G. A. Melkov3, A. A. Serga1 1Technische Universität Kaiserslautern, Kaiserslautern, Germany 2Graduate School Materials Science in Mainz, Kaiserslautern, Germany 3Taras Shevchenko National University of Kyiv, Kiev, Ukraine

The field of magnonics addresses the transfer and processing of information by spin waves and their quanta, magnons.

Magnons are bosons and, thus, they can condense into a macroscopic quantum state. Condensates of magnons relate to

Bose-Einstein condensates (BEC), and they spontaneously form a spatially extended, coherent ground state, which can be

established independently of the magnon excitation mechanism even at room temperature. I will discuss that magnon

condensates may support a supercurrent, which represents a novel type of macroscopic quantum transport phenomenon

analogous to the low-temperature effects of superconductivity, superfluidity, and superfluid spin current in 3He. Magnon

supercurrents constitute the transport of angular momentum, which is driven by a phase gradient in the

magnon-condensate wave function. In this presentation, first experimental evidence of a magnonic supercurrent will be

presented and its further implications will be discussed. First experimental evidence of the magnon ac Josephson effect will

be shown by oscillations in the local BEC population due to a phase-induced change in the direction of the magnon

supercurrent.

Tetragonal Mn-based Heusler for High-frequency Spintronics

S. Mizukami Tohoku University, Sendai, Japan

Tetragonal Mn-based Heusler compounds, Mn-Ga binary [1] and Mn-Co-Ga ternary [2], have attracted attentions for

nano-spintronics applications such as memory, since those have a small magnetization [1,2], large magnetic anisotropy [3],

low-loss at high-frequency [4], and high-electronic spin polarization [5].

In this talk, we will focus on alternative tetragonal Mn-based Heusler compound, Mn3Ge (Mn2MnGe). The epitaxial films of

Mn3+xGe grown on single crystalline oxide substrate exhibit small magnetic moment and high magnetic anisotropy [6,7].

Recently we succeeded to grow the high-quality epitaxial films of Mn3Ge that exhibited the well-squared hysteresis loop

and large perpendicular magnetic anisotropy field of about 20 T, owing to a nearly-compensated ferrimagnetism and large

uniaxial magnetic anisotropy [8]. This large effective field in Mn3Ge pushes low-energy magnon frequency into THz wave

range, which implies a possibility to realize spintronics devices working in THz wave range.

This work has been partially supported by Grant-in-Aid for Scientific Research (No. 24686001) and ASPIMATT (JST).

1. B. Balke et al., Appl. Phys. Lett. 90, 152504 (2007).

2. S. Chadov et al., Adv. Funct. Mater. 23, 832 (2013).

3. F. Wu et al., Appl. Phys. Lett. 94, 122503 (2009).

4. S. Mizukami et al., Phys. Rev. Lett. 106, 117201 (2011).

5. H. Kurt et al., Phys. Rev. B 83, 020405 (2011).

6. Q. L. Ma et al., Phys. Rev. Lett. 112, 157202 (2014).

7. H. Kurt et al., Appl. Phys. Lett. 101, 132410 (2012).

8. S. Mizukami et al., Appl. Phys. Express 6, 123002 (2013).

9. A. Sugihara et al., Appl. Phys. Lett. 104, 132404 (2014).


Controlling Interfaces in Magnetic Tunnel Junctions

S. Mitani National Institute for Material Science, Tsukuba, Japan

Controlling interfaces is a key method to obtain and improve desired properties in spintronic devices such as magnetic

tunnel junctions (MTJs). In particular, it is likely to be effectively implemented in monocrystalline devices, and recently

monocrystalline MTJs are revisited for the application to magnetic random access memories (MRAMs) [1]. In this talk, the

following topics in controlling interfaces of layered heterostructures are discussed.

(1) Perfect lattice matching at the interface of Heusler alloys and spinel oxides

A lattice-matched MTJ is one of the ideal systems to study spin-dependent tunneling transport. By using a Co2FeAl

electrode and a MgAlO tunnel barrier, we have achieved perfectly lattice-matched MTJs, in which almost no misfit

dislocation is observed in cross-sectional TEM images and the TMR ratio reaches ~300% [2]. While the TMR ratio is fairly

large, the result suggests that the chemical state of the interface mainly causes the large discrepancy between the

experimental observation and theoretical predictions.

(2) Perpendicular magnetic anisotropy (PMA) in monocrystalline layered structures

By carefully treating interfaces, a large interface PMA has been achieved in monocrystalline ultrathin Fe/MgO bilayer films,

in which the PMA energy density of 1.4 MJ/m3 is obtained for a 5 monolayer Fe layer [3]. The large interface PMA is

comparable to that in a first principles calculation, and unconventional resonance peaks due to the formation of quantum

well states have also been observed in dI/dV for a MTJ using this Fe layer with PMA [4].

In a Co2FeAl/MgO layered heterostructure, a novel effect of buffer layers on the interface PMA has been observed. By

replacing the Cr buffer layer with Ru, the interface PMA in Co2FeAl/MgO is enhanced, and furthermore we have found

novel interface structure between the Co2FeAl and fourfold-symmetry hcp Ru layers [5].

Acknowledgement:

This work was done in collaboration with T. Scheike, J.W. Koo, Z.C. Wen, H. Sukegawa, S. Kasai, T. Niizeki, K. Inomata, T.

Furubayashi, J. P. Hadorn ,T.T. Sasaki, T. Ohkubo, K. Hono, and was partly supported by ImPACT Program of Council for

Science, Technology and Innovation.

References

[1] http://www.jst.go.jp/impact/program04.html

[2] T. Scheike et al., Appl. Phys. Lett. 105, 242407 (2014).

[3] J. W. Koo et al., Appl. Phys. Lett. 103, 192401 (2013).

[4] J. W. Koo et al., J. Phys. D: Appl. Phys. (Fast Track Commun.) 47, 322001 (2014)

[5] Z. C. Wen et al., Adv. Mater. 26, 6483 (2014).


Modulation of Spin Precession Frequency by Spin Relaxation Anisotropy

A. Aoki1, J. Ishihara2, Y. Ohno3, F. Matsukura2,4, H. Ohno2,4, J. Nitta1, M. Kohda1

1RIEC, Tohoku University, Sendai, Japan 2Tohoku University, Sendai, Japan 3Tsukuba University, Tsukuba, Japan 4WPI-AIMR, Tohoku University, Sendai, Japan

Spin-orbit interaction (SOI) plays an important role to control spin states in semiconductors. Since SOI acts as an effective

magnetic field for electron spins, electrical control of spin precession has been demonstrated without any external magnetic

field [1]. Another possibility of spin precession control is to use anisotropic spin relaxation. In a III-V semiconductor

quantum well (QW), when the spin relaxation time becomes anisotropic for in-plane and perpendicular spin components,

the spin precession frequency depends not only on the external magnetic field, but also on the spin relaxation time. Since

spin relaxation depends on SOI strength, spin diffusion constant and electron scattering time, modulation of spin precession

frequency by anisotropic spin relaxation has potential to control spin states by light intensity and temperature besides an

external gate. In this work, we study the pump power dependence of the precession frequency of the optically injected spins

in a (110) GaAs quantum well. We grow an n-GaAs/AlGaAs QW on a (110) GaAs substrate, where Si δ doping is employed

to hold the Rashba and Dresselhaus SOIs by introducing the asymmetric potential profile. We measure the spin dynamics in

the QW by time resolved Kerr rotation (TRKR) microscopy, and determine the spin precession frequency ω’ under an

constant magnetic field Bext = 0.497 T at 30 K as a function of the pump power P ranging from 2 to 60 mW. Figure 1 shows

TRKR traces at P = 10, 20, and 30 mW, where a small shift in ω’ is observed with the increase of P. Because Bext is constant,

the observed shift is expected to the change of the spin relaxation anisotropy. Figure 2 summarizes the P dependence of ω’,

where ω’' increases gradually with the increase of P, and saturates at ~4.4 GHz above 30 mW. From the results, we derive

the parallel and perpendicular spin relaxation times. They increase with increasing P, indicating that the enhancement of

electron-electron scattering rate suppresses the spin relaxation.

[1] T. Bergsten et al., Phys. Rev. Lett. 97, 196803 (2006).

Fig. 1. TRKR signals as a function of pump power.

Fig. 2. Pump power dependence of precessional frequency ω'. Solid line is ω = gμBBext.


Friday, 12th June

08:20 Bus from York Monkbar Hotel (Guests only)

08:35 Bus from York Pavilion Hotel (Guests only)

08:45 Registration and Coffee/Tea P/L 005

MRAM Chair: Prof. Kevin O’Grady P/X 001

09:00 Nanoscale Magnetic Tunnel Junction - Hideo Ohno

09:30 Ultrafast Deterministic Switching in STT-MRAM with Orthogonal Polarizing Layers - Bernanrd Dieny

10:00 Theoretical Design of Electrode Materials for Magnetic Tunnel Junctions with MgO Barrier: Ferrimagnetic

D022-type Mn-based Heusler Alloys - Masafumi Shirai

10:20 Perpendicularly Magnetized L10-ordered Alloys for Magnetic Tunnel Junctions - Mikihiko Oogane


11:25 Sir Martin Wood Prize Lecture P/X 001

Chair: Atsufumi Hirohata

Koen Lamberts (Vice Chancellor, Univ. of York)

Kanae Kurata (1st Secretary Sci. and Tec. ,Japanese Embassy)

Hideo Ohno (Tohoku Univ. & Millenium Forum)

11:45 Effective Field Measurements and Spin Torque Dynamics in Magnetic Nanostructures

– Spin Orbit Effects in Magnetic Heterostructures - Masamitsu Hayashi

12:45 Oxford Instruments Ltd. Lunch/Reception/Posters P/L 005

Spin-current Devices I Chair: Hiroaki Muraoka P/X 001

13:30 Signature of Spin Sensitivity Amplification in (Ga,Mn)As/GaAs Esaki Diodes - Junsaku Nitta

13:50 Ge Devices Beyond p-MOS: Spintronics in 2-D and 1-D - Stuart Holmes

14:10 Spin Dynamics in Magnetic Nanostructures: Quantum vs. Semiclassical Approach - Tomasz Dietl

14:30 Manipulation of a Spin Current in a Lateral Spin Valve - Atsufumi Hirohata

14:50 DC Voltage in Pt/(Ga,Mn) as Under Ferromagnetic Resonance - Fumihiro Matsukura


Spin-current Devices II Chair: Gonzalo Vallejo-Fernandez P/X 001

15:25 Mechanical Effects in Spintronics - Sadamichi Maekawa

15:45 Helicity Dependent Ballistic Magnetic Domain Wall Motion Driven by Ultra-short Laser Pulses

- Jorg Wunderlich

16:05 Strain Controlled Magnetization Dynamics in Thin Films and Planar Nanostructures - Stuart Cavill

16:25 TBA - Thomas Meyer

16:55 Remarks on Core-Core Program Hideo Ohno & Hiroshi Matsuura

17 15-18 15 Closed Meeting for JSPS-EPSRC-DFG Core-to-Core Project

Informal Dinner (Invited Speakers Only)

17:15 Bus to York Pavilion Hotel

17:25 Continue to Monkbar Hotel

17:45 Continue to Railway Station

18:20 Depart University to Symposium Dinner

18:40 Depart York Pavilion Hotel

18:55 Depart Monkbar Hotel

19:00 Symposium Dinner Deans Court Hotel

22:00 Bus to Hotels

MRAM | Friday, 12th June – 09:00-11:10 12

MRAM

Friday 12th June 09:00-11:10

Nanoscale Magnetic Tunnel Junction

H. Ohno

RIEC, Tohoku University, Sendai, Japan

Power dissipation and interconnection delay are the two main challenges VLSI faces. I review the current state of magnetic

tunnel junction technology developed for magnetoresistive random access memories and nonvolatile VLSIs [1]. I first

discuss the material and device technology related to perpendicular CoFeB-MgO magnetic tunnel junction [2] that is now

passing the 20 nm device dimension; the smallest and well characterized ones reaching 11 nm [3]. The physics involved in

realizing scaling, i.e. high performance nanoscale magnetic tunnel junction in terms of thermal stability and threshold

current for spin-transfer switching, will be addressed. I will then discuss about three terminal magnetic tunnel junction

devices utilizing current-induced domain wall motion [4] and spin-orbit torques [5, 6], and compare the two terminal

magnetic tunnel junction with its three terminal variants. If time allows, electric-field switching of magnetization of

perpendicular CoFeB-MgO MTJs [7], which potentially can reduce the power consumption by orders of magnitude

compared to spin-transfer switching, will be discussed.

[1] H. Ohno, International Electron Device Meeting (IEDM) (invited) 2010.

[2] S. Ikeda, et al. Nature Materials, 9, 721 (2010)

[3] H. Sato, et al. IEDM 2013 and Appl. Phys. Lett. 105, 062403 (2014).

[4] S. Fukami, et al. IEDM 2013 and Nature Comm. 4:2293 doi: 10.1038/ncomms3293 (2013).

[5] M. Yamanouchi et al., Appl. Phys. Lett. 102, 212408 (2013).

[6] C. Zhang et al., J. Appl. Phys. 115, 17C714 (2014).

[7] S. Kanai, et al. Appl. Phys. Lett. 101, 122403 (2012); 103, 072408 (2013); 104, 212406 (2014)

Ultrafast Deterministic Switching in STT-MRAM with Orthogonal Polarizing Layers

B. Lacoste1,2,3, M. Marins de Castro1,2,3, T. Devolder4, R. C. Sousa1,2,3, L. D. Buda-Prejbeanu1,2,3, S. Auffret1,2,3, U. Ebels1,2,3, C. Ducruet5, I. L. Prejbeanu1,2,3, L. Vila6, B. Rodmacq1,2,3, B. Dieny1,2,3 1Univ Grenoble-Alpes, Grenoble, France 2CEA, INAC-SPINTEC, Grenoble, France 3CNRS, SPINTEC, Grenoble, France 4Université Paris-Sud, Orsay, France 5Crocus Technology, Grenoble, France 6CEA, INAC-SP2M/NM, Grenoble, France

STT-MRAM are receiving an increasing interest from microelectronics industry. They are foreseen as the best contender for

DRAM replacement. Thanks to their high switching speed, they can also address some SRAM applications. SRAM require

fast write and read access time, in the range of 2ns to 200ps depending on how close they are from the logic circuits.

However, they do not need to have a very long retention. For some applications, retention of ms can be sufficient.

For this type of applications, in-plane magnetic tunnel junctions (MTJ) with additional perpendicular polarizer are good

candidates. In these memories, the storage layer is sandwiched between two polarizing layers of orthogonal magnetization.

One is magnetized in-plane whereas the other is magnetized out-of plane. Both of these polarizing layers exert spin transfer

torques on the storage layer magnetization. The in-plane polarizer tends to induce a bipolar switching towards parallel or

antiparallel state depending on the direction of the current flowing through the stack. In contrast, the perpendicular

polarizer tends to induce a steady-state out-of plane precessional motion of the magnetization which yields an oscillatory

behavior of the storage layer switching probability versus pulse duration. Considering that the associated precessional

frequency is typically in the range of several GHz, this oscillatory behavior means that the write pulse duration must be

controlled with an accuracy of ±50ps to reach the desired final state. This is possible to achieve for a single memory point

but turns out to be very difficult at the scale of a multimillion bits chip.

In this work, we have developed two approaches which allow to realize ultrafast switching of the storage layer

magnetization in such memory with a deterministic control of the final state independently of the pulse duration.

The first approach consists in using cells patterned in elliptical shape with high aspect ratio, typically larger than 3.

Increasing the aspect ratio increases the in-plane anisotropy. It turns out that the precessional dynamics is very sensitive on

the in-plane anisotropy whereas the bipolar switching dynamics due to the in-plane polarizer is not, since it is dominated by

the demagnetizing energy. As a result, increasing the cell aspect ratio provides a way to favor the STT influence from the

in-plane polarizer compared to that of the out-of-plane polarizer resulting in a fast and deterministic switching. Real-time

measurements of the reversal were performed with samples of low and high aspect ratio. For low aspect ratio, a


precessional motion of the magnetization was observed and the effect of temperature on the precession coherence was

studied. For high aspect ratio, deterministic reversal was obtained. The reversal speed increases with the current density

and can get below 200ps for large enough current densities. The final state is controlled by the current direction in the MTJ

cell.

The second approach consists in applying on the cell an in-plane transverse field in a direction orthogonal to the in-plane

polarizing layer i.e. along the short axis of the ellipsoid. This transverse field helps the precessional motion during the first

fourth of the precession period but then hamper the precessional motion during the following half of precession period. As

a result, the storage layer magnetization is stopped in its precessional motion after half a precession making the switching

deterministic. Here also the switching is ultrafast and the final state is controlled by the current direction through the MTJ

stack.

Acknowledgement: This work was partially supported by the ERC grant HYMAGINE.

Theoretical Design of Electrode Materials for Magnetic Tunnel Junctions with MgO Barrier: Ferrimagnetic D022-type Mn-based Heusler Alloys

M. Shirai REIC, Tohoku University, Sendai, Japan

Magnetic materials with high magnetic anisotropy, small magnetization, and low magnetic damping have been desired to

reduce the spin-transfer torque switching current of magnetic random access memories. Recently, D022-type Mn-based

Heusler alloys Mn3-δGa have attracted much attention due to the high magnetic anisotropy (Ku > 1.0 MJ/m3) and the small

magnetization due to the ferrimagnetic structure [1, 2]. It was succeeded to fabricate thin films of D022-MnGe which also

shows high magnetic anisotropy and small magnetization [3].

We studied electronic structures and spin-dependent transport properties of MgO-based magnetic tunnel junctions (MTJ)

with D022-Mn3Ga and Mn3Ge electrodes by using first-principles calculations. First, we calculated the electronic structures of

bulk Mn3Ga and Mn3Ge. We found that Mn3Ga has the totally symmetric Δ1 band crossing the Fermi level both in the

majority- and minority-spin state in contrast to bcc-Fe [2]. On the other hand, the Mn3Ge has the Δ1 band around the Fermi

level only in the majority-spin state. Since the Fermi level of Mn3Ga in the minority-spin state is located at the valence band

maximum of the Δ1 state, additional valence electrons due to substituting Ge for Ga causes the shift of the Fermi level to the

higher energy side and thus half-metallic behavior on the Δ1 state is realized [4]. As a result, a large tunneling

magnetoresistance ratio can be expected in the Mn3Ge/MgO/Mn3Ge MTJ owing to the coherent tunneling through the Δ1

channel. We also discuss the effect of substitution of Fe, Co, Ni for Mn in Mn3Ga on the electronic structure and the

spin-dependent transport properties in MgO-based MTJs.

Acknowledgment:

This work was accomplished in collaboration with Y. Miura and S. Tanibayashi.

[1] B. Balke, et al., Appl. Phys. Lett. 90, (2007) 152504

[2] T. Kubota, et al., Appl. Phys. Express 4, (2011) 043002

[3] H. Kurt, et al., Appl. Phys. Lett. 101, (2012) 132410

[4] Y. Miura and M. Shirai, IEEE Trans. Magn. 50, (2014) 1400501


Perpendicularly Magnetized L10-ordered Alloys for Magnetic Tunnel Junctions

M. Oogane, Y. Kurimoto, H. Saruyama, M. Hosoda, H. Naganuma, Y. Ando Tohoku University, Sendai, Japan

Magnetic tunnel junctions with perpendicularly magnetized ferromagnetic electrodes (p-MTJs) are very promising for the

application to the spintronics devices, such as magnetic random access memory and spin-logic circuit. In order to achieve

high thermal stability and low switching current in p-MTJs, ferromagnetic materials with large perpendicular magnetic

anisotropy (Ku) and low magnetic damping constant (α) are required. L10-ordered alloys such as FePt and CoPt are good

candidates because of their large perpendicular magnetic anisotropy. In this work, we focus on the L10-MnAl and FePd as a

ferromagnetic material with large Ku and small α. [1, 2] large TMR ratio of 90% was observed in MTJs with FePd electrodes

(Fig. 1). The developed MTjs with large Ku, low α and large TMR can be applied to the small magnetoresistive devices. In

addition, TMR effect in MTJs with the MnAl electrode was successfully observed [3]. The developed MTJs with MnAl

electrodes can be applied to the ultra-small (< 20 nm) magnetoresistive devices because of very large Ku of 10 Merg/cc and

low a of 0.006.

Acknowledgement:

This work was supported by the FIRST program, Research and Development Project for ICT key technology to realize

future societies by MEXT and grand-in-aid for scientific research S (No.24226001).

[1] M. Hosoda, et al., J. Appl. Phys., 266, 012104 (2011)

[2] M. I. Khan, et al., J. Appl. Phys. 111, 07C112 (2012)

[3] H. Saruyama, et al., Jpn. J. Appl. Phys., 52 063003 (2013).

Fig. 1：Annealing temperature dependence of TMR effect in MTJs with L10-FePd electrode

SIR MARTIN WOOD PRIZE LECTURE | Friday, 12th June – 11:45-12:45 15

Sir Martin Wood Prize Lecture Friday 12th June 11:45-12:45

Effective Field Measurements and Spin Torque Dynamics in Magnetic Nanostructures -Spin orbit Effects in Magnetic Heterostructures-

M. Hayashi National Institute for Materials Science, Tsukuba, Japan

Strong spin-orbit effects in magnetic heterostructures with broken structural inversion symmetry have opened new

paradigms to control magnetic moments electrically[1, 2]. An ultrathin magnetic layer sandwiched between a heavy metal

layer and an insulating oxide layer form the basis of magnetic heterostructures. In such structures, current applied along the

film plane allow unique control of the magnetic moments. Key to the efficient control of magnetism, e.g. current induced

magnetization switching and domain wall motion[3, 4], is the large spin-orbit coupling within the heterostructure. The

strong spin orbit coupling constant of the heavy metal layer enables generation of large spin current via the spin Hall effect

that can impinge upon the magnetic layer to exert torque on the magnetic moments, now commonly referred to as the

spin-orbit torque. At interfaces, spin orbit coupling can cause the ultrathin magnetic layer to form a chiral magnetic

structure, via the Dzyaloshinskii-Moriya interaction, that can be manipulated by spin current. We have studied the

characteristics of spin-orbit torques and chiral magnetism in ultrathin magnetic heterostructures with magnetization

oriented perpendicular to the film plane[5-8]. Fascinating effects arising from the strong orbit coupling in the system that

encompass the spin Hall effect, spin orbit torque, interface Dzyaloshinskii-Moriya interaction and the spin Hall

magnetoresistance will be discussed.

[1] I. M. Miron et al., Nature 476, 189 (2011).

[2] L. Liu et al., Science 336, 555 (2012).

[3] S. S. P. Parkin et al., Science 320, 190 (2008).

[4] M. Hayashi et al., Science 320, 209 (2008).

[5] J. Sinha et al., Appl. Phys. Lett. 102 242405 (2013).

[6] J. Kim et al., Nat. Mater. 12, 240 (2013).

[7] J. Kim et al., Phys. Rev. B 89, 174424 (2014).

[8] J. Torrejon et al., Nature Comm. 5, 4655 (2014).

SPIN-CURRENT DEVICES I | Friday, 12th June – 13:30-15:10 16

Spin-current Devices I Friday 12th June 13:30-15:10

Signature of Spin Sensitivity Amplification in (Ga,Mn)As/GaAs Esaki Diodes

J. Shiogai1, M. Kohda1, T. Nojima1, J. Nitta1, M. Ciorga2, M. Utz2, D. Schuh2, D. Bougeard2, D. Weiss2

1Tohoku University, Sendai, Japan 2University of Regensburg, Regensburg, Germany

We investigate the correlation between spin signals measured in three-terminal (3T) geometry by the Hanle effect and the

spin accumulation generated in a semiconductor channel in a lateral (Ga,Mn)As/GaAs Esaki diode device. We

systematically compare measurements using a 3T configuration, probing spin accumulation directly beneath the injecting

contact, with results from nonlocal measurements, where solely spin accumulation in the GaAs channel is probed. We find

that the spin signal detected in the 3T configuration is dominated by a bias-dependent spin detection sensitivity, which in

turn is strongly correlated with charge-transport properties of the junction. This results in a particularly strong

enhancement of the detected spin signal in a region of increased differential resistance. We find additionally that two-step

tunneling via localized states in the gap of (Ga,Mn)As does not compromise spin injection into the semiconductor

conduction band [1]. Although our experiments were conducted on spin Esaki diode devices, we find the results are quite

general. Especially, the possibility to amplify the tiny non-local spin signals by engineering a tunnel barrier in the detector

in a way that it shows a high non-linear differential resistance can be of significant importance for the development of future

spintronic devices.

[1] J. Shiogai et al., Phys. Rev. B 89, 081307(R) (2014)

Ge Devices Beyond p-MOS: Spintronics in 2-D and 1-D

S. N. Holmes Toshiba Research Europe Limited, Cambridge Research Laboratory, Cambridge, UK

Ambient temperature mobilities in strained p-Ge quantum wells grown in a commercial chemical vapour deposition system

of > 4x103 cm2/V.s have been demonstrated recently at Warwick University. At low temperature the mobility is ~ 1x106

cm2/V.s matching or even exceeding state-of-the-art values for III-V semiconductor heterostructures. In this talk we outline

the spintronic properties of these quantum wells, in particular for the low carrier density regime ~ 1011 cm-2 where hole

interaction effects start to dominate the transport. The spin-splitting in applied magnetic field is entirely consistent with a

Zeeman effect from the heavy-hole, angular momentum J = 3/2 (mj = ±3/2) state with the spin orientation driven by the

biaxial compressive strain with the relaxed Si0.3Ge0.7 barrier layers and quantised in the growth direction perpendicular to

the conducting channel. There is no k-linear or k-cubic, zero-field Rashba spin-splitting irrespective of the structural

inversion asymmetry of the confining potential in p-doped or undoped Ge quantum wells at 1011 cm-2 without coupling to

the light hole (mj = ±1/2) states. Device structures with polyimide or SiO2 gate dielectric have been fabricated with

patterned gates of widths 350 to 500 nm. In 1-dimension, ballistic quantization of the conductance is observed at 0.5 (2e2/h)

in a 200 nm long channel in addition to a strong 0.7 (2e2/h) feature. DC bias (> 0.6 mV) produces a quarter plateau at 0.25

(2e2/h). This preliminary study confirms the importance of the role that strained Ge will play in beyond p-MOS device

schemes where long spin-lifetimes are expected at the expense of weak spin-orbit coupling effects.

Acknowledgement:

This research was performed in collaboration with Warwick University, the University of Cambridge and University

College London. The work programme at Warwick University, the University of Cambridge and Southampton University is

funded by an EPSRC grant “Spintronic device physics in Si/Ge heterostructures” EP/J003263/1 and platform grant

EP/J001074/1


Spin Dynamics in Magnetic Nanostructures: Quantum vs. Semiclassical Approach

T. Dietl WPI-AIMR, Tohoku University, Sendai, Japan Institute of Physics, Polish Academy of Sciences, Warszawa, Poland Institute of Theoretical Physics, University of Warsaw, Warszawa, Poland

A number of quantum models and corresponding numerical diagonalization procedures has been proposed to describe spin

dephasing of an electron injected to a quantum dot containing typically 103 to 106 nuclear magnetic moments. These

theoretical studies have revealed many unanticipated behaviours such as fast oscillations of the magnetic spin, a shift

in its energy, and lack of the complete dephasing in the abence of an external magnetic field. These findings have been

assigned to the quantum nature of the bath spins, and discuss in terms of the quantum entanglement between the central

and bath spins, the Lamb shift, and the Bose-Einstein condensastion [1]. A question then arises whether it is justified to

describe spin dynamics of spintronic devices, such as domain-wall cells and magnetic tunnel junctions (having now

diameters down to 11 nm [2]), using semiclassical approaches, in which injection of electron spins is treated quantum

mechanically but spin dynamics is described by using essentially classical Landau-Lifshitz-Gilbert equation.

In our work [3], employing a previous semiclassical approach to the central-spin problem [4], we re-examine spin dephasing

of a confined electron at times shorter than intrinsic transverse relaxation time T2 and NI/ωs where NI is the a number of

bath spins in the electron electron confinement region and ωs is the electron precession frequency for saturated bath spins.

The proposed model allows us to consider hitherto unexplored ranges in magnetic fields B, polarizations pI and lengths I of

the bath spins as well as to take into account Berry’s phase, magnetic polaron effects and spin-spin interactions within the

bath. In this way we provide a formalism suitable to describe experimental results in a wide parameter space, allowing us to

benchmark various implementations of quantum theory.

As exemplified in Fig 1., we find quantitative agreement between semiclassical and quantum predictions, which

demonstrates that for the pertinent number of bath spins their quantum nature is irrelevant.

In addition to dephasing of electron spin by a bath of neighboring localized nuclear or magnetic moments in non-magnetic

or magnetic semiconductor structures, our formalism is also suitable for describing dissipation less dynamics of confined

superconductors and ultra-cold gases.

[1] A. Faribault and D. Schuricht, Phys. Rev. Lett. 110, 040405 (2013), and references therein.

[2] H. Sato et al. Appl. Phys. Lett. 105, (2014) 062403

[3] T. Dietl, Phys. Rev. B 91, (2015) 125204

[4] T. Dietl and J. Spałek, Phys. Rev. Lett. 48, 355 (1982); Phys. Rev. B 28, 1548 (1983).

[5] E. Barnes, Ł. Cywi´nski, and S. Das Sarma, Phys. Rev. Lett. 109, 140403 (2012).

Fig 1.Coherence function |ωt| for narrowed spin bath computed within the semiclassical theory (valid for arbitrary bath polarization pI and spin I) as a function of the square root of time (solid line). Predictions of the quantum model [5] for the same value of the magnetic field and number of bath spins (entering via the parameter Δ0/σ) as well as for pI=0.4 and various values of I are shown by dots. The data in the inset demonstrate the equivalence of the semiclassical approach (solid line) and the quantum model for pI=0.4 and I=3/2; dots) at early times.


Manipulation of a Spin Current in a Lateral Spin Valve

A. Hirohata 1, R. M. Abdullah 1,B. A. Murphy 2, M. Samiepour1, A. J. Vick2 1Department of Electronics, University of York, York, UK 2 Department of Physics, University of York, York, UK

Recent advancement in nanofabrication and growth allows the utilisation of spin-polarised electrons in transport and

dynamics, resulting in the development of spintronic devices [1]. In the spintronic devices, the key technologies are injection,

manipulation and detection of spin-polarised electron in a non-magnetic media with high efficiency.

Conventionally such a spin-polarised electron current has been injected into a non-magnetic material by flowing an

electrical current through a ferromagnetic layer. However, its spin polarisation is dependent upon the interfacial properties,

such as conductance matching, junction resistance and interfacial resonant states. We have been investigated a series of

lateral spin-valves to demonstrate their geometrical ratchet effect on spin-current signal amplification and modulation [2].

We calculated diffusive electron transport in a LSV, consisting of 100-nm-wide Ni81Fe19 (Py) wires bridged by a

100-nm-wide Cu wire, using our simple model. Here, the central part of the Cu wire was modified into the following

geometrical ratchet shapes with allowing 50 nm separation between the Py wires and the ratchets; (i) one or two-pairs of

right-angled triangles (base: 100b450 nm and height: 0h100 nm) and (ii) one or two-pairs of obtuse-angled triangles

(base: 100b450 nm, height: 0h100 nm and the distance between the top ends and base ends of the triangular: 50 nm). We

found that the spin amplification takes its maximum for the single-paired right- and obtuse-angled triangles with b=250 nm

and h=75 and 80 nm, respectively. The above LSV devices were fabricated by conventional electron-beam lithography and lift-off processes. Two Py nanowires

were designed to be 30 nm thick and 200 nm wide with different shapes at their ends (square and sharp) to induce a

difference in their magnetisation-reversal fields. These wires were bridged by a Cu nanowire (70 nm thick and 100 nm wide).

We demonstrated spin-current amplification in a lateral spin-valve (LSV) using a geometrical ratchet effect. Two Py

nanowires were designed to be 30 nm thick and 200 nm wide bridged by a Cu nanowire (70 nm thick and 100 nm wide).

Here, the central part of the Cu wire was modified into the following geometrical ratchet shapes with allowing 50 nm

separation between the Py wires and the ratchets. We measured over 700% spin-current amplification for the right-angled

triangles with 100 nm base and 60 nm height. By utilising these fundamental building blocks, we can also develop a large

variety of new devices.

Figure 1: (a) SEM image of the LSV device. (b) Non-local signals for the LSV with triangular ratchets with heights of 0, 20, 37

and 60 nm [2].

Acknowledgment:

This work has been partially funded by JST-PRESTO and EPSRC (EP/I000933/1 and EP/M02458X/1).

[1] A. Hirohata and K. Takanashi, J. Phys. D: Appl. Phys. 47, 193001 (2014).

[2] R. M. Abdullah et al., J. Phys. D: Appl. Phys. 47, 482001(FTC) (2014).


DC Voltage in Pt/(Ga,Mn) as Under Ferromagnetic Resonance

F. Matsukura Tohoku University, Sendai, Japan

The combination of spin pumping and the inverse spin Hall effect (ISHE) is employed to generate and detect pure spin

current in nonmagnet/ferromagnet bilayer structures. In this work, we measure dc voltage Vdc in a Pt/(Ga,Mn)As bilayer

under ferromagnetic resonance (FMR) to investigate possible spin injection from semiconducting to metallic materials.

A 10-nm thick Pt is deposited by sputtering on a 50-nm thick (Ga,Mn)As epitaxially grown on a GaAs (001) semi-insulating

substrate. We place the sample near the center of TE011 microwave cavity with 9.0 GHz excitation, and measure FMR

spectrum of the (Ga,Mn)As layer and dc voltage between two In contacts on the Pt layer are measured simultaneously. All

the measurements are done at 30 K lower than the Curie temperature ~90 K of (Ga,Mn)As.

We observe characteristic dc voltage in the vicinity of the resonant field, which possesses nearly symmetric lineshape,

indicating that Vdc is dominated by the planar Hall effect (PHE) and/or ISHE [1]. We determine the ratio of the symmetric

component originated from the PHE to that from the ISHE to be 31:-69 from the out-of-plane magnetic-field angle

dependence of Vdc. The opposite polarity of the ISHE in the present system to that in Py/Pt may result from the opposite

spin polarization at the Fermi level for Py and (Ga,Mn)As. The spin mixing conductance g↑↓ at Pt/(Ga,Mn)As interface is

determined to be 6.2×1019 m-2, which is 13 times greater than that in (Ga,Mn)As/p-GaAs [1], and is comparable to those in

Pt/ferromagnetic metals. The result indicates that one can inject spin current from a ferromagnetic semiconductor to a metal

with a relatively large efficiency.

Acknowledgement:

This work was done with H. Nakayama, L. Chen, H. W. Chang, and H. Ohno, and was supported in part by JSPS through

FIRST program, R&D for ICT Key Technology of MEXT, Grants-in-Aid for Scientific Research from JSPS (No. 26790037) and

MEXT (No 26103002).

[1] L. Chen, F. Matsukura, and H. Ohno, Nat. Commun. 4, 2055 (2013).

SPIN-CURRENT DEVICES II | Friday, 12th June – 15:25-17:15 20

Spin-current Devices II

Friday 12th June 15:25-17:15

Mechanical Effects in Spintronics

S. Maekawa Japan Atomic Energy Agency, Tokai, Japan

A. Einstein and W.J. de Haas discovered experimentally the equivalence of magnetic moment and mechanical rotation in

1915 [1].

In the same year, S.J. Barnett showed that the mechanical rotation can generate a magnetic field, i.e., the so-called Barnett

field, even in a body with no electric charge [2].

These phenomena are caused by the angular momentum conservation between electron spin and mechanical rotation,

which has been proved in the general relativistic quantum mechanics [3].

We introduce mechanical effects in spintronics and propose a variety of novel spintronics phenomena. In particular, the

coupling between nuclear spin and mechanical rotation is demonstrated [4-6]. Since the Barnett field is enhanced more than

three orders of magnitudes in nuclei than electron spins, the mechanical nuclear-magnetic-resonance (NMR) may provide

new applications of NMR.

The mechanical generation of spin and spin current opens a door from “Spintronics” to "Spin-Mechatronics".

Acknowledgement:

This work has been done in collaboration with H. Chudo, M. Matsuo, J. Ieda, M. Ono, K, Harii, S. Okayasu, H. Yasuoka and

E. Saitoh.

[1] A.Einstein and W.J.de Haas, Verhandl.Deut.Physik.Ges, 17,154 (1915).

[2] S.J.Bernett, Phys. Rev. 6, 239 (1915).

[3] M.Matuo et al., Phys. Rev. Lett. 106, 076601 (2011).

[4] H.Chudoet al., Appl. Phys. Express 7, 063004 (2014).

[5] H. Chudo et al., J. Phys. Soc. Jpn. 84, 043601 (2015).

[6] K. Harii et al., Jpn. J. Appl. Phys. 54, 050302 (2015).

Helicity Dependent Ballistic Magnetic Domain Wall Motion Driven by Ultra-short Laser Pulses

J. Wunderlich Hitachi, Cambridge, UK

We show that a magnetic domain wall in a perpendicular magnetic film of GaMnAsP propagates ballistically by its own

inertia after being exposed to individual ultrashort optical spin transfer torque (oSTT) pulses. By exploiting the elastic

property of a geometrically pinned domain wall with an in-situ controllable pinning potential we show that the magnetic

domain wall still continues to move without excitation ~3 order of magnitudes longer than the duration of the initial spin

excitation.

Our experiments also allow us to isolate the effect of heat from the oSTT mechanism and we show that the transfer of

angular momentum to the domain wall magnetization can occur at time-scales much faster than the precession time of the

responding domain wall magnetisation.

This is a first demonstration of light-induced ballistic DW motion in a perpendicular system.


Strain Controlled Magnetization Dynamics in Thin Films and Planar Nanostructures

S. A. Cavill1,2, D. E. Parkes3, S. Bowe3,2, R. Beardsley3, C. Reardon1, A. W. Rushforth3, T. A. Ostler1, R. W. Chantrell1, I. Isakov4, P. Warburton4 1The University of York, York, UK 2Diamond Light Source, Didcot, UK 3The University of Nottingham, Nottingham, UK 4University College London, London, UK

The interplay between strain-induced, shape-induced, and magnetocrystalline anisotropy energies in (sub)-micron

scaled magnetostrictive devices coupled to an underlying ferroelectric (FE) layer is presented. Varying the voltage applied

to the FE layer not only tunes the shape of the magnetic hysteresis loop and the magnetic reversal processes [1], but also

provides a means of controlling and stimulating magnetization dynamics [2-4]. Using both experimental imaging

techniques and numerical simulations I will present a means of manipulating vortex core dynamics in a planar structure,

displaying the landau flux closure ground state, utilizing both static and time-varying strain induced anisotropy.

[1] S. A. Cavill et al. Appl. Phys. Lett. 102, 032405 (2013).

[2] D. E. Parkes et al., Appl. Phys. Lett 105, 062405 (2014).

[3] D. E. Parkes et al. Scientific Reports 3, 2220 (2013).

[4] J. V. Jäger et al., Appl. Phys. Lett. 103, 032409 (2013).


Saturday, 13th June

HARFIR Open Workshop and Project Committee Meetings

EU/JST/HARFIR

Morning

08 20 Bus from York Monkbar Hotel (Guests only)

08 35 Bus from York Pavilion Hotel (Guests only)

Heusler-alloy Junctions I Chair: Uli Nowak P/T 111

08 45 Exchange Bias Effects in Epitaxially Grown Ni2MnAl/X Bilayers (X: Fe, Co, Co2MnSi) - Koki Takanashi

09 15 Ru-Mn Heuslers and MnN as New Exchange Bias - Gunter Reiss

09 35 Critical Thickness of Antiferromagnets - Chiharu Mitsumata

10 05 Spin-model Parameters for Antiferromagnetic Heusler Alloys and their Interfaces with Fe - Laszlo Szunyaogh

10 25 Tea/Coffee P/T 111

Heusler-alloy Junctions II Chair: Koki Takanashi P/T 111

10 40 X-ray and Neutron Analysis of Heusler Alloys - Kanta Ono

11 00 Exploring Exchange Bias in Heusler Alloy Bilayers - Rocio Yanes-Diaz

11 20 The Role of Structural Defects on the Functional Properties of Fe3O4 Thin Films and Nanoparticles

- Vlado Lazarov

11 40 Topological Insulator and Topological Metal - Binghai Yam

12 00 STT-MRAM for Embedded Memory - Mahendra Pakala

12 30 Remarks on HARFIR Atsufumi Hirohata

12 30 Lunch P/T 111

13 00-14 00 Closed Meeting for EU-JST HARFIR

13 00 Bus to York Railway Station

HEUSLER-ALLOY JUNCTIONS I | Saturday, 13th June – 09:00-10:40 23

Heusler-alloy Junctions I

Saturday 13th June 09:00-10:40

Exchange Bias Effects in Epitaxially Grown Ni2MnAl/X Bilayers (X: Fe, Co, Co2MnSi)

K. Takanashi, T. Kubota, T. Tsuchiya, T. Sugiyama

Institute for Materials Research, Tohoku University, Sendai, Japan.

Mn3Ir is an antiferromagnetic material widely used in spin valve structure. Mn3Ir has attractive properties of large exchange

bias field and high blocking temperature. However, the high price and the scarcity of Ir are crucial problems.

Antiferromagnetic Hesuler alloys are promising materials for the replacement of Mn3Ir. There have been only a few reports

to date about the exchange bias properties of antiferromagnetic Heusler alloys[1]. In this study, we have focused on Ni2MnAl

with a comparatively high Néel temperature, and structural, transport and magnetic properties of epitaxially grown

Ni2MnAl/X bilayers (X: Fe, Co, Co2MnSi) have been investigated systematically.

Ni2MnAl films with a thickness of 100 nm were prepared on MgO (100) single crystal substrates with different growth

temperatures from RT to 600˚C by magnetron sputtering. A ferromagnetic X layer of 3 nm was deposited on the Ni2MnAl

film. The structural, transport and magnetic properties of the films were measured by using a x-ray diffractometer, van der

Pauw technique and a superconducting quantum interference device, respectively.

B2-ordered Ni2MnAl (100) films were successfully grown on MgO (100) substrates. The temperature dependence of the

electrical resistivity showed an anomaly at a temperature, which was varied from 220 to 300 K depending on the growth

temperature of the film. The anomaly is considered to correspond to the Néel temperature. Loop shifts by exchange bias

were observed in the magnetization curves at 10 K for Ni2MnAl/X bilayers. Interfacial exchange coupling energy density

for X = Co2MnSi, Fe was estimated to be about 0.03 erg/cm2 that was higher than those for X = Co (0.02 erg/cm2), which

implies an importance of lattice matching between the ferromagnetic material and Ni2MnAl layer.

Acknowledgement:

This work was supported by HARFIR (SICORP-EU) from the JST, and is a cooperative program (No. 14G0412) of the

CRDAM-IMR, Tohoku University.

[1] X. Y. Dong et al., J. Cryst. Growth 254, 384 (2003).

Ru-Mn Based Heuslers and MnN as New Exchange Bias Systems

G. Reiss, J. Balluf, M. Meinert Bielefeld University, Bielefeld, Germany

We will discuss recent progress in the preparation of antiferromagnetic thin films and the realisation of exchange bias

between these films and ferromagnets.

Ru2MnGe was observed to crystallise at low temperatures (300°C). Rocking curves for the (200) and (400) x-ray diffraction

peaks were found to be extremely narrow with a FWHM of about 0.03°. At temperatures higher than 500°C, Mn was

observed to evaporate from the films. Hence, the deposition temperature was capped at 500°C. We found a vanishing

magnetic moment of this compound by XMCD measurements and confirmed exchange bias to a 2nm thick Fe film.

In parallel, a series MnxZ (1 ≤ x ≤ 3 and Z=Ge, Ga, N) was realised, which was designed to vary between a tetragonally

distorted Heusler structure and a L10 arrangement and to be ferrimagnetic with low saturation magnetisation and complete

magnetic compensation close to room temperature. The compound MnN, however, turned out to be an antiferromagnet that

shows an exchange bias to CoFe of up to 1.3 kOe at room temperature.

HEUSLER-ALLOY JUNCTIONS I | Saturday, 13th June – 09:00-10:40 24

Critical Thickness of Antiferromagnets

C. Mitsumata

National Institute for Materials Science, Tsukuba, Japan

The exchange bias between the ferromagnetic (FM) and antiferromagnetic (AFM) bilayer was investigated within the

framework of the classical Heisenberg model. The dependence of the exchange bias on the AFM layer thickness was also

calculated by using the Landau–Lifshitz–Gilbert equation. The triple-Q (3Q), AF-I and T1 spin structures are obtained in the

-phase, ordered L10-, and L12-type lattices, respectively. The exchange bias is caused by the formation of the

interfacial domain wall in the AFM layer, and the critical thickness dc of AFM layer is dominated by the varied spin

structures. Under the condition where the magnetic anisotropy energy is fixed to equivalent values in different alloys, the

critical thickness d3Q -phase layer with the 3Q spin structure is thinner than that dAF-Ic of the ordered

L10-type layer with the AF-I spin structure. Also, the critical thickness dT1c is thinner than dAF-Ic in ordered L12- and

L10-type alloys. The relation among the critical thicknesses is dominated by the formation of a magnetic domain wall in the

AFM layer. Consequently, the relation of the critical thickness can be represented as √3 d3Q c=√2 dT1c= dAF-Ic.

Spin-model Parameters for Antiferromagnetic Heusler Alloys and their Interfaces with Fe

E. Simon, S. Khmelevskyi, L. Szunyogh Budapest University of Technology and Economics, Budapest, Hungary

The growing technological demand for spintronics applications raised increased interest for searching novel

antiferromagnets (AFM). Promising candidates are the full Heusler compounds like Ni2MnAl in the B2 phase or Ru2MnZ

(Z= Si, Ge) that have Néel temperatures above the room temperature. We use the Korringa-Kohn-Rostoker method to

determine the electronic structure self-consistently and then the magnetic force theorem to derive the exchange interactions.

The variation of the Néel temperature and the stability of the AFM phase depending on the atomic disorder are illustrated

[1]. On the way to explore the possibility for exchange bias (EB) effect in bilayers formed by these AFM Heusler alloys and

ferromagnetic (FM) metals like Fe, we present relativistic calculations of the spin-model parameters at the interface. Here we

derive tensorial interactions that, beyond the isotropic interactions, account for the two-site anisotropies and the

Dzyaloshinsky-Moriya (DM) interactions. In case of Ni2MnAl/Fe bilayer we find AFM interface Fe-Mn coupling and

in-plane magnetic anisotropy. In particular, at the interface between the Mn-Si plane of Ru2MnSi and the Fe layer, we

calculated large Mn-Mn and Fe-Mn DM interactions.

Acknowledgements:

This work is supported by the European Commission via the Collaborative Project HARFIR (Project No. 604398).

[1] S. Khmelevskyi et al., Phys. Rev. B 91, 094432 (2015).

HEUSLER-ALLOY JUNCTIONS II | Saturday, 13th June – 10:55-12:20 25

Heusler-alloy Junctions II Saturday 13th June 10:55-12:20

X-ray and Neutron Analysis of Heusler Alloys

K. Ono Institute of Materials Structures Science, High Energy Accelerator Research Organization

Structure and magnetic structure analysis of Hausler alloys are very important to develop novel antiferromagnetic Heusler

alloys that replace Iridium. We have performed structure and magnetic structure analysis of Heusler alloys using x-rays and

neutrons, such as x-ray diffraction (XRD), neutron diffraction, x-ray magnetic circular dichroism (XMCD), and polarized

neutron reflectometry (PNR).

At first, we performed a polarized neutron reflectometry (PNR) experiment on magnetic thin film test samples L10-ordered

FeNi at BL-17 of MLF, J-PARC, Japan. We have established an analysis method to estimate spin asymmetry and depth

profiles from PNR for thin film samples. The structural and magnetic depth profile of the multilayer structure obtained by

PNR is important for fabrication process optimization and magnetic structure analysis of Heusler alloys.

Then we characterize Heusler alloy samples Co2FeAl0.5Si0.5 using XRD at BL-8 of Photon Factory, KEK for structure analysis

and XMCD at BL-16 for magnetic analysis. As we use two-dimensional x-ray detector (imaging plate IP) in our XRD system,

we can obtain both in plane and out of plane diffraction patterns in a short time (typically ~5min.). XMCD measurements

were also performed for Co2FeAl0.5Si0.5 samples.

For the analysis of antiferromagnetic Heusler alloys, we have performed neutron scattering experiments for Ni2MnAl and

Mn2(Co,V)Ga bulk samples.

Exploring Exchange Bias in Heusler Alloy Bilayers

R. Yanes, E. Simon, D. Hinzke, L. Szunyogh and U. Nowak University of Konstanz, Konstanz, Germany

The exchange bias (EB) effect is a unidirectional anisotropy of a magnetic system, which is characterized by a shift in the

hysteresis loops. The EB is related to the coupling between a ferromagnet (FM) and an antiferromagnet (AFM), and its

stiffness depends on the exchange coupling through the interface. The EB effect is used in magnetic devices to stabilized the

magnetization as in GMR sensors, magnetic tunnel junctions etc. This fact has increased the demand of AFM and it has led

to an increased interest for novel AFM materials with N´eel temperature above room temperature, being Heusler alloys

promising candidates for that.

In this work we studied the magnetic properties of a series of Heusler alloys bi-layers using a multiscale modeling, linking

ab initio calculations with dynamical spin model simulations. In terms of the fully relativistic Screened

Korringa-Kohn-Rostoker method [1, 2], we performed self-consistent calculations of a Ni2MnAl(B2)/ FM bi-layers, where

FM=Co, Fe and Ni2MnAl(L21). Based on the spin-cluster expansion technique [3], we defined a classical Hamiltonian,

which will be used later in our spin dynamics simulations, and derived the exchange interactions in the system. This

technique provides us direct knowledge of the exchange interactions at the ferromagnet/antiferromagnet interface.

In order to check the possible existence of EB effect, numerical calculations of the hysteresis loops of the AFM/FM bilayers

were carried out for different values of the thickness of the AFM substrate (tAF ). Also a first attempt of consider chemical

disorder or existence of magnetic vacancies was done. Our preliminary results indicate that for perfect Ni2MnAl/FM

bilayers there is no (in-plane) EB effect. However, when some defects are included in the AFM layer a small EB appears.

[1] L. Szunyogh et. al Phys. Rev. B, 49, 2721 (1994)

[2] R. Zeller et al., Phys. Rev. B 68, 104436 (2003)

[3] L. Szunyogh et. al, Phys. Rev. B, 83,024401 (2011)

HEUSLER-ALLOY JUNCTIONS II | Saturday, 13th June – 10:55-12:20 26

The Role of Structural Defects on the Functional Properties of Fe3O4 Thin Films and Nanoparticles

V. K. Lazarov1, D. Gilks1, Z. Nedelkovski1, L. Lari1, K. McKenna1, M. Weinert2, R. Evans1, T. Susaki3, K. Matsuzaki3, S. Majetich4 1University of York, York, UK 2University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, USA 3Tokyo Institute of Technology, Nagatsuta, Japan 4Carnegie Mellon University, Pittsburgh, Pennsylvania, USA

The calculated band-structure of bulk Fe3O4 shows a band gap at the Fermi Level in the majority states compared with

conductive states crossing the Fermi Level in the minority. This should give rise to 100% spin polarised electrons at the

Fermi level. The high curie temperature ~858 K and lattice match and growth characteristics on technologically relevant

barrier oxides including MgO and MgAl2O4 makes Fe3O4 an ideal candidate for integration into tunnel junctions and spin

injection/detection systems. However, structural defects such as antiphase domain boundaries in Fe3O4 have a significant

effect on the macroscopic properties, and could have detrimental effect on the device performance. In this talk I will focus

on correlation between the structural defects and the anomalous properties of both thin films and nanoparticles. Possible

approaches to overcome naturally occurring defects in magnetite will be presented.

Topological Insulator and Topological Metal

B. Yan Max Planck Institute, Dresden, Germany

Topological states have been refreshing our understanding of the band theory and materials, and also promise interesting

spintroic applications. Although, the Heusler topological insulators were predicted in theory several years ago, their

topological surface states still wait for experiment verification. In this talk, I will update our recent progress on looking for

real topological states in Heusler materials from both theoretical simulations and ARPES measurement. We find the unique

surface states exist inside the bulk valence bands, different from other known topological materials.

POSTER PRESENTATIONS | Thursday 11th June-Friday12th June 27

List of Poster Presentations (P/L 005 and 006)

Spintronic memories and logics (01~04) 01 Domain wall creep driven by adiabatic spin transfer torque in magnetic metals

Samik DuttaGupta (RIEC, Tohoku Univ.) 02 Racetrack memory based on exchange bias

Ioan Polenciuc (Dept. of Phys., Univ. of York) 03 Fabrication and measurement of a lateral spin transfer nano-oscillator

Benedict Murphy (Dept. of Phys., Univ. of York) 04 Development of a nano-spin motor

Atsufumi Hirohata (Dept. of Elec., Univ. of York)

Multilayers and structured materials (05~10) 05 Interface in exchange bias

Helene Labasque (Dept. of Phys., Univ. of York) 06 Temperature and distance dependence of inter-granular exchange coupling

Matthew Ellis (Dept. of Phys., Univ. of York) 07 Exchange bias in nanostructures

Robert Carpenter (Dept. of Phys., Univ. of York) 08 Perpendicular exchange bias in Co/Pt multilayers

Kelvin Elphick (Dept. of Phys., Univ. of York) 09 Growth of Fe(110)/MgO(111)/GaN(0001) heterostructure by molecular beam epitaxy

Jun-Young Kim (Dept. of Phys., Univ. of York) 10 Interfacial structure dependent spin mixing conductance of Co thin-films

Mustafa Tokac (Durham Univ.)

Magnonics and spin dynamics (11~12) 11 Magnetisation dynamics in the presence of thermal gradients

Thomas Bose (Dept. of Phys., Univ. of York) 12 Thermally nucleated reversal processes in CoFeB/MgO nanodots

Andrea Meo (Dept. of Phys., Univ. of York) Half-metallic ferromagnets (13~22) 13 Exchange bias effect in Ni2MnAl/X (X=Fe, Co, Co2MnSi) bilayer samples

Takahide Kubota (IMR, Tohoku Univ.) 14 Characterisation of Polycrystalline Heusler Alloys

Teodor Huminiuc (Dept. of Phys., Univ. of York) 15 Determination of antiferromagnetic order in thin films

John Sinclair (Dept. of Phys., Univ. of York)

POSTER PRESENTATIONS | Thursday 11th June-Friday12th June 28

16 Characterisation of Polycrystalline Heusler Alloys Haokaifeng Wu (Dept. of Phys., Univ. of York)

17 Polarized neutron reflectivity and atomic structure study of a Co2Fe(Al,Si)/Si(111) heterointerface

Balati Kuerbanjiang (Dept. of Phys., Univ. of York) 18 X-ray diffraction and magnetic circular dichroism of Heusler alloy

Nobuhito Inami (Inst. of Mater. Str. Sci., KEK) 19 Direct band-gap measurement on a NiMnSb Heusler alloy at room temperature

Tariq Alhuwaymel (Dept. of Elec., Univ. of York) 20 Crystallographic optimisation of Co2FeSi with perpendicular anisotropy

William Frost (Dept. of Phys., Univ. of York) 21 Exchange bias induced at a Co2FeAl0.5Si0.5/Cr interface

Chris Nga Tung Yu (Dept. of Phys., Univ. of York) 22 Spin-transfer torque induced magnetization switching in Py/Cu/half-Heusler nanopillar

Marjan Samiepour (Dept. of Phys., Univ. of York)

Fundamental properties of magnetic materials (24~27) 23 Superconducting Sn1-xInxTe nanoplates

Satoshi Sasaki (Sch. of Phys. Astro., Univ. of Leeds) 24 III-V-Bismide alloys for spintronic applications

Robert Simmons (Univ. of Surrey) 25 Ultra low energy (spin polarised) SEM and STEM

Christopher Walker (Inst. of Sci. Instruments) 26 Magnetic Micro-Cooling Using Thin Films of Magnetocaloric Gd5Si4

Shane Harstad (Iowa State Univ.)

Organic spintronics (27~29) 27 A single-molecule electrochemical transistor utilizing a Nickel-pyridyl spinterface

Richard Brooke (Univ. of Bristol) 28 Antiferromagnetic coupling at organic semiconductor/ferromagnetic interfaces

Jason Zhang (Dept. of Phys., Univ. of York) 29 Stimulation of the Brain with Time Varying Magnetic Fields

Erik Lee (Iowa State Univ.)

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