"ALEXANDRU IOAN CUZA" UNIVERSITY

OF IAŞI

FACULTY OF PHYSICS

PhD Thesis Summary

Magnetoelectric systems based on ferroelectric perovskites

PhD student:

Alexandra Guzu (married Maftei)

Scientific coordinator:

Prof. Univ. Dr. Liliana Mitoşeriu

thesis presented

in partial fulfilment of the requirements

for the title of Doctor of Science in Physics

Iasi

September 2020

1

In the attention of

........................................................................................................

“ALEXANDRU IOAN CUZA” UNIVERSITY, IAȘI

FACULTY OF PHYSICS

we would like to inform you that on 03.09.2020, at 11:00, Mrs. GUZU

ALEXANDRA (married to MAFTEI) will defend, in a public online meeting,

the doctoral thesis entitled MAGNETOELECTRIC SYSTEMS BASED ON

FEROELECTRIC PEROVSKITS, in view of obtaining the scientific title of

doctor in the fundamental field: EXACT SCIENCES, field: PHYSICS.

The doctoral commission has the following composition:

President:

• Prof. univ. dr. Diana MARDARE, Director of the Doctoral School, Faculty

of Physics, “Alexandru Ioan Cuza” University of Iași

Scientific coordinator:

• Prof. univ. dr. Liliana MITOȘERIU, Faculty of Physics, “Alexandru Ioan

Cuza” University, Iași

Reviewers:

• Prof. univ. dr. Viorel POP, “Babeș Bolyai” University of Cluj Napoca

• Prof. univ. dr. Daniel VIZMAN, West University of Timișoara

• Prof. univ. dr. hab. Laurențiu STOLERIU, “Alexandru Ioan Cuza”

University of Iași

The thesis can be consulted at the Library of the Faculty of Physics.

2

The content of the thesis

Abstract 4

Thanks 5

I. Introduction 6

I.1 Introductory notions 8

I.1.1 Magnetoelectric systems 8

I.1.2 Ferroelectrics - general properties 10

I.1.3 Magnetic materials. Ferrite 17

I.1.3.1 Classification of ferrites 18

I.1.3.2 Spinel structure 19

I.2 Magnetoelectric multiferroics 20

I.2.1 Magnetoelectric composite materials 22

I.2.2 Applications of magnetoelectric composites 27

I.2.3 Percolation 33

Bibliography I 40

II. Description of the experimental methods used 45

II.1 Structural and phase analysis by X-ray diffraction 45

II.2 Microstructural analysis 46

II.3 Impedance spectroscopy 49

II.4 Determination of the ferroelectric hysteresis cycle P (E) and

nonlinear dielectric properties ("DC tunability") 51

II.5 Determination of magnetic and magnetoelectric properties

52

Bibliography II 55

III. Preparation of composite ceramics 56

Bibliography III 65

IV. Study of the role of the type of interconnectivity on the macroscopic

properties of composites 0.66BT-0.33CF 66

IV.1 Preparation, structural and microstructural characterization

of samples 66

IV.2 Estimation of effective permittivity 76

IV.3 Dielectric and ferroelectric properties 78

IV.3.1 Weak field electrical properties 78

IV.3.2 Ferroelectric properties 83

3

IV.4 Magnetic and magnetoelectric properties 85

IV.4.1 Magnetic and thermomagnetic properties 85

IV.4.2 Magnetoelectric coupling properties 86

IV.5 Conclusions 88

Bibliography IV 91

V. Contributions to the study of ceramic magnetoelectric systems

consisting of barium titanate with cobalt-zinc ferrites 95

V.1 Composite preparation, structural characterization (XRD)

and microstructural (SEM) 96

V.2 Weak field dielectric properties as a function of temperature

and frequency 102

V.3. Magnetic properties, nonlinear dielectric character and

magnetoelectric coupling 107

V.4 Conclusions 112

Bibliography V 115

VI. Study of the laminar composite 0.33BaTiO3 – 0.33Co0.8Zn0.2Fe2O4 –

0.33BaTiO3 119

VI.1 Microstructural characterization 119

VI.2 Weak field electrical properties 121

VI.3 Electrical properties at different temperatures 125

VI.4 Magnetic properties 127

VI.5 Conclusions 129

Bibliography VI 131

VII. General conclusions 132

List of original publications 138

International conferences 138

National conferences 139

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Acknowledgements

I would like to especially thank Mrs. Prof. Univ. Dr. Liliana

Mitoșeriu, my scientific coordinator, who showed a lot of understanding,

patience and guided me throughout my doctoral studies.

I also thank the ladies Dr. CS II Cristina Ciomaga, lect. univ. dr. hab.

Lavinia Petronela Curecheriu and dr. CS III Felicia Gheorghiu, who are part

of the guiding commission, for the support provided during the period of

doctoral studies and thesis elaboration.

Last but not least, I would like to thank the members of the doctoral

committee present at the public presentation and Mrs. Prof. Univ. Dr. Diana

Mardare for the time given and for the honor of reviewing this paper.

I also express my gratitude to Mr. assistant. univ. Dr. Leontin

Pădurariu and Dr. Mirela Airimioaei and Dr. Nadejda Horchidan from the

group of "Dielectrics, Ferroelectrics and Multiferoics" without which this study

would not have taken shape.

The financial costs were borne by the projects PN-II-PT-PCCA-2013-

4-1119 (MECOMAP) and UEFISCDI PN-III-P4-ID-PCCF-2016-0175

(HighKDevice).

5

I. Introduction

Magnetoelectric materials are those materials in which the ferroelectric

and magnetic order coexist simultaneously, these being a field of current interest

both from a theoretical point of view and for technological applications such as

sensors, actuators, transducers, data storage devices (memories, in which

writing data could be done with an electric field, and reading them with a

magnetic field), etc. It is known that the electrical polarization of a material

changes following the application of an electric field, and magnetization by the

action of a magnetic field. In the case of magnetoelectric materials, by applying

an electric field (respectively a magnetic field) a variation of the magnetization

(respectively of the electric polarization) is observed.

The studies performed on this type of systems aim at obtaining

magnetoelectric materials having simultaneously dipolar and magnetic order

(ferroelectric and ferro, feri- or antiferomagnetic properties) in the same

structure and high magnetoelectric coefficient, in the field of temperatures of

interest for applications, with resistance to high corrosion and mechanical

hardness, and can be made at relatively low prices. There are very few single-

phase magnetoelectric materials, and existing ones usually have these properties

at cryogenic temperatures and as such, there is a permanent interest in finding

new single-phase or composite materials that sum up these properties at ambient

temperature.

The research in this paper focused on the study of magnetoelectric

composites consisting of a magnetostrictive oxide material and a piezo /

ferroelectric material, having two types of arrangements of the constituent

phases: (i) structure multilayered and (ii) structure with randomly mixed phases.

In these composites, none of the component materials taken separately possess

magnetoelectric properties, but together, the magnetoelectric effect may occur

as a product property, through the mechanical coupling between them.

The analyzed composites are composed of oxide materials, namely,

BaTiO3 (BT) the best known ferroelectric oxide with the perovskite structure

ABO3, in combination with spinel ferrites type CoFe2O4 (CF) and

Co0.8Zn0.2Fe2O4 (CZF). It is known that ferroelectric has electrical insulating

properties, is characterized by high permittivity and low losses, while ferrites

are usually semiconductor materials with low permittivity and high dielectric

losses. Usually, compositions are chosen in which the dielectric phase is

predominant, in order to limit the conduction and dielectric losses and to obtain

a better magnetoelectric coupling, but too little ferrite leads to the weakening of

6

the magnetic characteristics (decreased saturation magnetization and remnants

of composite, due to a “dilution” effect due to mixing with a material without

magnetic ordering, BaTiO3).

In this paper, combinations were chosen between the two materials in

which the ferrite concentration is at the limit of the percolation region, ie 33%

to maintain a strong magnetic response and also trying to maintain the dielectric

character in the composite. In order to understand the role of microstructure on

their dielectric and magnetic properties, combinations were made with the same

composition of the two phases (66% ferroelectric - 33% ferrite), but distributed

differently in the volume of the composite. Thus, the magnetoelectric materials

studied in this paper are mixed composites with the formula 0.33CF - 0.66BT

and 0.33CZF - 0.66BT respectively with random phase mixing, as well as

laminated composites (triple-layer) type 0.33BT - 0.33CZF - 0.33BT and

0.33BT - 0.33CF - 0.33BT, respectively. The properties of these composite

ceramics, having the same composition but with the constituent phases placed

in different ways (random mixing or in laminar structures) were analyzed

comparatively and described by finite element modeling. It was also

investigated for the same composition and how the sintering method modifies

the microstructural characteristics (porosity, granulation) and functional

properties when using two different sintering methods: traditional method

sintering and plasma arc sintering, having phases mixed randomly.

I.1 Introductory notions

I.1.1 Magnetoelectric systems

The magnetoelectric effect was first observed by Röntgen in 1888 and

by Pierre Curie in 1894 [2] in two independent studies. Pierre Curie identified

the magnetoelectric effect by analyzing crystalline symmetry criteria. The term

"magnetoelectric" was first used by Debye in 1926 [3], and the first single-phase

material with magnetoelectric switching (having hysteresis M(E) and P(H),

respectively), discovered was Cr2O3, but which had low values of polarization

and field-induced magnetization. Subsequently, the research was extended to a

large number of materials and it was established that more than 80 categories of

single-phase materials (including Ti2O3, GaFeO3, phosphates, boracites) as well

as a large number of their combinations have a magnetoelectric effect.

For a given material, it is important to describe and understand the

relationships between complex electrical, mechanical and magnetic properties.

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These cause-effect (stimulus-response) relationships, in which experimentally

determinable material constants are involved, are schematically illustrated in

the Heckmann diagram [1] (Figure I.1).

From a mechanical point of view, it is interesting the interdependence

between the deforming forces X and the deformations x, which describe the

elasticity as the principal effect. The electrical properties describe the response

of the polarization P to the application of an electric field E, and the magnetic

ones represent the response of the magnetization M to the application of the

magnetic field H. Practically, in the case of simple relations, each property is

independent, an electric field E can determine the polarization P, and the

deforming force X can control the deformation x of the environment.

Figure I.1: The Heckmann diagram shows the relationship between the

electrical, mechanical and magnetic properties of the material [1].

When we can control the polarization by the action of a magnetic field

or reciprocal, the magnetization by the action of an electric field, we are talking

about the existence of a magnetoelectric effect (ME) in the material.

The magnetoelectric effect (ME) represents the variation of an electric

quantity when applying a magnetic field and vice versa; it can be primary or

8

secondary. The primary magnetoelectric effect consists in the appearance of an

electric polarization under the action of a magnetic field P(H):

𝑀𝐸 =𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐

𝑚𝑒𝑐𝑎𝑛𝑖𝑐×

𝑚𝑒𝑐𝑎𝑛𝑖𝑐

𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 (I.1)

or in the occurrence of a magnetization when applying the electric field M(E)

(electromagnetic effect) described schematically as follows:

𝐸𝑀 =𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐

𝑚𝑒𝑐𝑎𝑛𝑖𝑐×

𝑚𝑒𝑐𝑎𝑛𝑖𝑐

𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐 (I.2)

The secondary magnetoelectric effect consists in the variation of the permittivity

under the action of a magnetic field ɛ(H) or the variation of the permeability

to the application of a magnetic field µ(E).

I.1.2 Ferroelectrics - general properties

Ferroelectric materials represent a special class of polar dielectrics,

which have the property of reversing their polarization P(E) or electrical

induction D(E) in the presence of an external electric field. This property of

ferroelectrics underlies many applications based on the controlled and

reversible change in the electrical state of the material.

These substances, unlike linear dielectric media, in which there is a

linear variation of the electric polarization/induction with the applied field (and

whose permittivity is a constant with respect to the electric field), are nonlinear

dielectric media for which the permittivity is a function of the applied field ɛ(E).

Ferroelectric media have hysteresis and electrical remanence properties. Unlike

linear dielectric media, ferroelectric substances also have special mechanical,

thermal and optical properties and have a coupling between them. Ferroelectric

materials can vary their electrical polarization under the action of temperature

variations and therefore have a pyroelectric character and also can vary their

electrical polarization following the application of mechanical actions, so they

are piezoelectric. Consequently, ferroelectrics are multifunctional materials

with memory (hysteresis), being at the same time pyro- and piezoelectric.

The main characteristics of ferroelectric materials are [4-8]:

(1) Spontaneous polarization (PS) is defined as the maximum value of

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the polarization of a single ferroelectric domain in the absence of

an external electric field and an external mechanical deformation.

(2) Ferroelectric hysteresis The main property of ferroelectric

materials is the hysteretic character of the polarization depending

on the applied external electric field.

(3) Dependence of the permeability on the applied electric field

("tunability") Ferroelectric materials have the property of changing

the value of the permittivity according to the value of the intensity

of the applied electric field ("tunability"), a very important property

in various applications.

I.1.3 Magnetic materials. Ferrites

Ferrites are complex oxides that usually contain M2+ divalent metals and

have the general chemical formula 𝑀2+𝐹𝑒23+𝑂4

2−. Ferrites are a class of

materials characterized by weak ferrimagnetism and/or ferromagnetism, having

electrical, dielectric or semiconductor properties and are widely used in technical

applications especially for their combined properties.

Ferromagnetism is a property specific to certain materials that consists

in the presence of a spontaneous magnetization in the absence of the external

magnetic field. Any ferromagnetic material has a Curie magnetic temperature

above which the material loses its ferromagnetic properties, becoming

paramagnetic. Examples of such ferromagnetic materials are Fe, Co, Ni, Mg, Zn

and combinations thereof in alloys or oxide compounds.

I.1.3.1 Classification of ferrites

Ferrites were classified according to the shape of the hysteresis curve

M(H) and according to the values of the main magnetic characteristics (coercive

field Hc and residual induction Br) into two broad categories:

→ Soft ferrites - are characterized by a high saturation magnetization and

a low coercive field, small cycle area;

→ Hard ferrites - usually have a hexagonal crystal structure and have the

following magnetic properties: very high coercive field and high residual

induction, large M(H) cycle area.

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I.1.3.2 Spinel structure

The spinel-type crystalline structure is characteristic of ferrites, and the

specific chemical formula is of the form: 𝐴2+𝐵23+𝑂4

2−, where A is a divalent ion

and B is a trivalent ion. The spinel structure has ions placed in a cubic grid with

compact packing, and depending on the number of neighboring oxygen ions,

cations have two types of interstices: tetrahedral and octahedral. The spinel

elementary cell contains 96 ion-filled interstices or vacancies.

I.2 Magnetoelectric multiferroics

Multiferoic materials are those systems that have in the same phase two

or more types of ferric order (at least two parameters of different order that are

switchable). Magnetoelectric multiferroics (ME) are simultaneously ferro-/feri-

or antiferromagnetic and ferro-/feri- or antiferoelectric in the same phase, and

between the magnetic and ferroelectric order parameter there is a magnetoelectric

coupling [9-12].

The necessary condition for a material to be magnetoelectric is the

coexistence of magnetic and electric dipoles in the same phase, and if they are

also switchable, it is a multiferoic.

I.2.1 Magnetoelectric composite materials

Magnetoelectric composites are made of at least two different materials,

which separately do not possess magnetoelectric properties, but when combined

in the composite, magnetoelectric properties result. One of the materials that

make up the magnetoelectric composite is piezoelectric, and the other is

magnetostrictive. When a magnetic field is applied, the magnetostrictive

component changes its physical dimensions, this deformation being transmitted

to the piezoelectric phase, having as effect the appearance of induced electric

charges. The phenomenon can also occur in reverse: when applying an electric

field there is a change in the physical dimensions of the piezoelectric component,

the effect being the change in the magnetization of the magnetostrictive phase.

I.2.2 Applications of magnetoelectric composites

Based on the type of magnetoelectric coupling and the mechanisms used

to control various parameters, the variety of applications of ME materials

11

includes: magnetic sensors, high frequency inductors, storage devices and high

frequency signal processing devices.

II. Description of the experimental methods used

In this thesis were studied several oxide magnetoelectric composite

ceramic systems consisting of magnetostrictive material (ferrite) and

ferro/piezoelectric material (barium titanate). In these systems, the arrangement

of the constituent phases was different, they being prepared either in the form of

multilayer (laminar structures) and in the form of ceramics with a random phase

mixing. In these systems, the magnetoelectric effect appears as a product

property, none of the component materials taken separately having distinct

magnetoelectric properties.

The research in this doctoral thesis is dedicated to understanding the

relationship between preparation, microstructural characteristics and electrical

and magnetic properties, their description through theoretical models, and testing

for possible applications. The studied systems contain the same

ferroelectric/ferrite volume ratio (composition at the percolation limit), namely

66% ferroelectric BaTiO3 and 33% cobalt ferrites: pure ferrite CoFe2O4 (CF) and

doped with Zn: Co0.8Zn0.2Fe2O4 (CZF). Composites have two types of constituent

phase arrangements: randomly mixed phase composites (type 0-3 or more

complex connectivity) and triple layer structures in which the ferrite layers are

framed between the two dielectric layers of BaTiO3 (type 2-2 connectivity).

Experimental characterization methods

II.1 Structural and phase analysis by X-ray diffraction

To characterize the systems chosen for the study, diffractograms were

recorded using a Shimadzu LabX 6000 diffractometer from the AMON platform,

Faculty of Physics, with CuKα radiation (λ=1.5405Å), with a scanning increment

of 0.02° and a counting time of 1s/step in the range 2θ = 20-80°.

II.2 Microstructural analysis

The microstructures of the samples from this paper were investigated

with a Hitachi S-3400N II scanning electron microscope from the RAMTECH

center (collaboration with Dr. Sorin Taşcu).

II.3 Impedance spectroscopy

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Complex impedance was measured in the frequency range (20Hz -

2MHz) in the temperature range (20 ÷ 250)°C using an impedance analyzer

precision RLC type Agilent E4980A and for low frequencies Solartron 1260A

Impedance Analyzer (10µHz - 32 MHz) from the endowment of the AMON

platform of the Faculty of Physics. An Agilent E4991ARF impedance analyzer

was used for high frequency dielectric measurements (1MHz-1GHz) performed

at room temperature. Dielectric measurements at low temperatures (-150 ÷

150)°C in the frequency range (1Hz-1MHz) were performed using a dielectric

spectrometer Concept 40 Novocontrol Tehnologies in collaboration with the

Institute of Macromolecular Chemistry of the Romanian Academy „P. Poni ".

II.4 Determination of the ferroelectric hysteresis loop P(E) and

nonlinear dielectric properties ("DC tunability")

The hysteresis cycles of the P(E) polarization were recorded using a

modified Sawyer-Tower circuit, at room temperature using a sinusoidal

waveform of amplitude E0 in the range (1.5-3.5) kV/mm to ensure sample

saturation and different frequencies f = (1-10) Hz. The resistivity of the samples

was checked with a High Resistance Meter (HP 4329A) before the measurements

to check if they are good insulators and if they will withstand cycles of hysteresis

under high voltages.

To determine the nonlinear dielectric properties, ie the dependence ɛ(E)

at high voltages, a circuit designed and built in the Laboratory of Dielectrics,

Ferroelectrics and Multiferoics, of the Faculty of Physics was used.

II.5 Determination of magnetic and magnetoelectric properties

The magnetic properties of the composites were measured at room

temperature under magnetic fields in the (0-14) kOe range using a vibrating

sample magnetometer (VSM, Lake Shore7410, USA) from AMON.

Thermomagnetic analysis (magnetization temperature dependence) was

determined in a magnetic field of 10 kOe, at low temperatures (5÷300 K), using

a QD PPMS-9 system, while measurements above room temperature in the range

(300÷900 ) K were made with the VSM vibrating sample magnetometer model

LakeShore VSM 7410 in collaboration with the National Research Institute for

the Development of Technical Physics Iași, within the research grant in

partnership PN-II-PT-PCCA-2013-4-1119.

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III. Preparation of composite ceramics

The ferroelectric BaTiO3 (BT) powders were prepared by solid phase

reaction using as precursors titanium oxide TiO2 (Sigma Aldrich, 99.5%) and

barium carbonate BaCO3 (Merck, 99%) mixed in stoichiometric proportions.

The magnetic nanoparticles of cobalt ferrite CoFe2O4 (CF) and cobalt

ferrite substituted with zinc Co0.8Zn0.2Fe2O4 (CZF) in this thesis were prepared

by a simple method, which involves relatively low precursor costs and thermal

budget, using as precursors Fe(NO3)3x9H2O (purity > 99.9%, Sigma Aldrich),

Co(NO3)2x6H2O (Merck, purity > 99.5%), ZnO and HNO3. The synthesis

technique used combines the sol-gel method with self-combustion and involves

the use of a combustion and complexing agent, in this case citric acid C6H8O7

(Sigma Aldrich, purity > 99.5%), whose decomposition generates high

temperatures during the combustion process and determines the formation of the

cubic spinel phase [13,14].

The prepared magnetic and ferroelectric powders were used to obtain

layered (2-2) magnetoelectric composites but also with random phase mixing.

These composites were obtained both by the classical sintering method and by

using plasma arc sintering (SPS). For the realization of dense magnetoelectric

ceramics with fine granulation, in this work was used for densification a plasma

arc sintering system SPS Model FCT- (FAST) HPD5 existing at the National

Research and Development Institute of Technical Physics Iași, in a collaboration

from within the research grant in partnership PN-II-PT-PCCA-2013-4-1119.

IV. Study of the role of the type of interconnectivity on the

macroscopic properties of 0.66BT-0.33CF composites

In this chapter we described the results obtained in the comparative study

of ferroelectric-ferrite-phase composites, having a composition in the vicinity of

percolation: 0.66BaTiO3-0.33CoFe2O3, sintered by SPS, but with different phase

arrangements (randomly mixed phases and structured triple-layer). The aim of

this study was to understand and describe the effect of the type of phase

interconnectivity on the electrical and magnetic macroscopic properties of the

analyzed composites.

IV.1 Preparation, structural and microstructural characterization

of samples

14

Co (CF) ferrite powders were synthesized by the sol-gel method

combined with self-combustion according to the method described in Chapter

III. These, in the case of mixed composite, were mixed in a humid environment

with barium titanate (BT) nanopowders obtained by the hydrothermal method

(Sigma Aldrich), having the characteristics described in Chapter III.

To obtain the layered ceramic composites, a sequence of 0.33BT-

0.33CF-0.33BT was poured into the cylindrical carbon die of the SPS device.

20 30 40 50 60 70 80

0

100

200

300

(311)

(310)

(300)

(220)(211)

(210)(200)

(111)

(110)

(100)

(622)

(533)

(620)

(440)

(511)

(422)

(400)

(222)

(311)

(22

0)

CF

Inte

ns

ity

(u

.a.)

2 (degrees)

BT

0.33BT-0.66CF phase mixture

Figure IV.1: X-ray diffractograms for CoFe2O4 powder, for the BaTiO3 layer

in the laminated ceramic composite and for the ceramic composite with a mixture of

random phases having the composition 0.66BaTiO3 - 0.33CoFe2O4.

The crystalline structure was determined by Rietveld structural

refinement of the entire diffractogram using the GSAS (General Structure

Analysis System) software package developed by Larson and Von Dreele [15].

20 30 40 50 60 70 80

2 (degrees)

Inte

nsit

y (

a.u

)

experimental

calculating

BTO

BTT

BT ceramic

20 30 40 50 60 70 80

2 (degrees)

Inte

ns

ity

(a

.u)

experimental

calculating

BTO

BTT

CF

0.66BT-0.33CF

(phase mixture)

15

Figure IV.2: The results of the Rietveld refinement for the BaTiO3 layer in the

triple-layer ceramic composite and for the mixed composite 0.66BaTiO3 - 0.33CoFe2O4.

SEM microstructure for mixed composite 0.66BaTiO3 - 0.33CoFe2O4:

Figure IV.3: SEM micrograph performed in fracture for the mixed composite

0.66BaTiO3 – 0.33CoFe2O4

In the randomly mixed phase composite, the corresponding BaTiO3 areas

are white and compact, with ultrafine granulation (approximately 150 nm), and

the corresponding dark-colored CoFe2O4 areas are inhomogeneously distributed

in the ceramic, forming large, irregularly shaped areas consisting of large-grained

crystalline agglomerates (~ 1μm). There was a diffusion of Fe and Co ions in

BaTiO3 at the contact interfaces, this doping occurring mainly on the Ti4+

positions, due to the compatibility of their ionic dimensions [16]. This diffusion

of Fe and Co ions in BaTiO3 was also highlighted by the SEM-EDX technique,

confirming the Rietveld structural calculations for this type of composite.

Figure IV.4: SEM micrograph made in fracture for laminated ceramics

16

0.33BaTiO3 – 0.33CoFe2O4 – 0.33BaTiO3

In the case of the laminar structure 0.33BT-0.33CF-0.33BT, sintered in

SPS plasma, SEM micrographs (made in fracture) indicate the obtaining of a

compact, well-densified ceramic, with a clear and regular interface between the

two phases, without pores, obtaining thus a perfect lamination between the two

oxide components through a transition zone with nanometric granules, achieved

by using the plasma sintering technique.

The analysis of the chemical elements performed by SEM - EDX in the

transition region for the laminar structure 0.33BT-0.33CF-0.33BT, indicates the

doping of BaTiO3 with very small amounts of magnetic ions at the interfaces in

the case of the laminar composite.

IV.2 Estimation of the effective permittivity

A numerical estimation of the effective permittivity for the two types of

composites was performed by a technique implemented within the group of

Dielectrics, ferroelectrics and multiferoics, Finite Element Method (FEM). The

simulation results show major differences in the electric field configurations in

the two cases, demonstrating that microstructure and phase connectivity play a

major role in the effective dielectric response. The simulations show that the

composite ceramic with random phase mixture is characterized by an effective

intrinsic permittivity of ~525, almost an order higher than that of the laminated

composite (~58), due to the contribution of the ferroelectric phase which is

subject to a fairly electric field great for such a composition close to the

percolation limit.

IV.3 Dielectric and ferroelectric properties

IV.3.1 Weak field electrical properties

The dielectric properties measured at room temperature as a function of

frequency for the two types of composites analyzed are presented in comparison

in Figure IV.5. Both types of structures show a monotonous decrease in

permittivity as a function of frequency, with a tendency to saturate at a high

frequency above 10 kHz, from giant values corresponding to the frequency of 1

Hz (15000 and 9000 for mixed composite and laminate ) up to 1000 and 60 at 1

17

MHz, respectively.

100

101

102

103

104

105

106

0

3000

6000

9000

12000

15000

BT-CF (composite with

random mixture)

BT-CF (laminate composite)

Th

e r

ea

l p

art

of

perm

itti

vit

y

Frequency (Hz)

100

101

102

103

104

105

106

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0 BT-CF (composite with

random mixture)

BT-CF (laminate composite)

tg

Frequency (Hz) (a) (b)

Figure IV.5: Dependence of the real part of the permittivity (a) and of the

dielectric losses (tgδ) of frequency at room temperature for the two types of structures

BT-CF analyzed composites

The dielectric losses are quite high, indicating a semiconductor character

of the dielectric, rather than an insulating one, with maximums due to the

relaxation phenomena at low frequencies of 10 Hz, especially in the mixed

structure, but also around 1 kHz for both structures. The losses tend to decrease

and reach values corresponding to a dielectric behavior only above 10 kHz, when

these relaxation phenomena cease (fact observed by linearizing the real part of

the permittivity in Figure IV.5 (a).

The experimental dielectric response contains a strong influence of

extrinsic phenomena that are discussed comparatively.

High losses, DC conductivity and the thermally activated relaxation

mechanism, with an activation energy of less than 0.3 eV compared to 0.5 eV are

present in mixed ceramics, compared to layered ones, in which the ferrite layer

with low conductivity is completely isolated from those of BaTiO3.

IV.3.2 Ferroelectric properties

In order to verify the existence of the ferroelectric character in the two

types of composites, the hysteresis cycles of the P(E) polarization in dynamic

regime were measured.

18

-20 -15 -10 -5 0 5 10 15 20-4,5

-3,0

-1,5

0,0

1,5

3,0

4,5

BT-CF randomly

mixed

P (

C

/cm

2)

E (kV/cm)-20 -15 -10 -5 0 5 10 15 20

-40

-30

-20

-10

0

10

20

30

40

P (

C

/cm

2)

E (kV/cm)

BT-CF layered

(a) (b)

Figure IV.6: Hysteresis cycles of polarization at room temperature measured in

dynamic mode for composition ceramics 0.66BaTiO3-0.33CoFe2O4:

(a) randomly mixed, (b) layered structure.

The composite with random phase mixture has a linear dielectric

character (the permittivity is invariable as the field increases), unlike the

laminated composite which is characterized by a nonlinear hysteresis cycle (the

behavior of a nonlinear capacitor over which a leakage resistive component

overlaps).

IV.4 Magnetic and magnetoelectric properties

IV.4.1 Magnetic and thermomagnetic properties

In a composite made of material with magnetic order (ferrite) together

with one without magnetic order (dielectric), the magnetic properties will be

derived from those of ferrite, ie they should have a typical ferrimagnetic magnetic

order determined by uncompensated antiparallel spines from the pure system

CoFe2O4.

The results of the magnetic characterization of the two types of studied

composites are presented comparatively in Figure IV.7, when applying a parallel

magnetic field, respectively perpendicular to the layers.

19

-20 -15 -10 -5 0 5 10 15 20-30

-20

-10

0

10

20

30

-5.0 -2.5 0.0 2.5 5.0

-20

-10

0

10

20

H (kOe)

M (emu/g)

BT-CF (randomly

mixed)

BT-CF (layered):

H

H⊥

M (

em

u/g

)

H (kOe)300 400 500 600 700 800 900

0

5

10

15

20

25

30

400 500 600 700 800 900-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

632 K

745 K

T (K)

720 K

dM/dT (emu/g/K)

BT-CF (randomly

mixed)

BT-CF (layered)

Temperature (K)

M (

em

u/g

)

Figure IV.7: (a) The M(H) curves corresponding to the studied BT-CF

composite systems (randomly mixed and laminated (triple-layer)) under the action of a

magnetic field applied parallel/perpendicular to the layers (the area of low fields in the

insect is also highlighted); (b) Temperature magnetization dependence for the two

composites (the field is applied perpendicular to the layer); Inset: temperature

dependence of the dM/dT derivative.

The magnetization in both composites is low compared to the known

values for cobalt ferrite [17], as a consequence of the sum property. Both ceramic

composites have saturation magnetizations in the (23-29) emu/g range, values

that fit very well with the expected values of magnetization as an effect of the

sum property (e.g., one-third reduction from typical values for pure phase of

ferrite characterized by a saturation magnetization of 82 emu/g) [17].

It turns out that the interfaces and possible doping, as well as the

arrangement of the phases play a minor role in the magnetic properties of these

magnetoelectric composites. Small magnetization is typical of mixed composites

(23 emu/g) for a coercive field of ~ 320 Oe and a saturation field of ~ 4kOe.

In the case of laminated composites, the magnetization shows a weak

anisotropy when the magnetic field has been applied perpendicular or parallel to

the ceramic layers. Figure IV.7 (b) shows the temperature dependence of the

magnetization for mixed and layered composites on a field cooling sequence,

when applying a magnetic field H┴ = 10 kOe. One method of accurately

determining magnetic anomalies is to derive magnetization from temperature.

The curve dM/dT = f(T) is inserted in Figure IV.7 (b) and shows two well-

pronounced minima, one at 720 K for mixed ceramics and another at 746 K for

laminated ceramics, corresponding to the ferromagnetic-paramagnetic phase

transition, and another at 632 K for both ceramic systems.

IV.4.2 Magnetoelectric coupling properties

Next, the magnetoelectric response in composites was determined, in

20

dynamic regime, measuring the electric potential induced by the action of a small

AC variable magnetic field (Hac = 10 Oe), while the ceramic sample is

simultaneously subjected to the action of a large DC continuous magnetic field

of bias (Hdc), in a configuration in which both magnetic fields are applied parallel

to the ceramic electrodes (so perpendicular to the direction of the electric

polarization field). Due to the very high dielectric losses, it was practically not

possible to complete the polarization of the sample with a mixture of random

phases and therefore the magnetoelectric response could be recorded only for

ceramics with laminar structure.

The dependence of the transverse magnetoelectric coefficient depending

on the applied static magnetic field presents a complex but completely

reproducible hysteretic nonlinear variation, with many maxima and minima

depending on the frequency of the applied field. This behavior can be caused by

the complex coupling phenomena of electric and magnetic fields through

mechanical stresses, but also the nonlinear nature of the properties of

magnetostriction, permittivity and conductivity that can distort the shape of

curves describing the field dependence of the ME coefficient [18]

The original results presented in this part of the doctoral thesis were

published in the paper: A. Guzu, C.E. Ciomaga, M. Airimioaei, L. Padurariu,

L.P. Curecheriu, I. Dumitru, F. Gheorghiu, G. Stoian, M. Grigoras, N. Lupu, M.

Asandulesa, L. Mitoseriu, Functional properties of randomly mixed and layered

BaTiO3 - CoFe2O4 ceramic composites close to the percolation limit, J. Alloys

& Compds. 796, 55-64 (2019) [19].

V. Contributions to the study of ceramic magnetoelectric systems

consisting of barium titanate with cobalt-zinc ferrites

This chapter presents the results of a comparative study of feroelectric-

ferrite composite ceramic composite systems, which have the same composition

(close to the percolation limit): 0.66BaTiO3-0.33Co0.8Zn0.2Fe2O4, but they were

sintered differently: (i) by the classical method, respectively (ii) by SPS plasma

arc sintering. The aim was to investigate the effect of the sintering method used

and the resulting microstructures on the macroscopic properties of the

composites.

V.1 Composite preparation, structural (XRD) and microstructural

characterization (SEM)

21

Magnetic powders of Co0.8Zn0.2Fe2O4 (CZF), were prepared using as

precursors: Fe(NO3)3·9H2O, Co(NO3)2·6H2O, ZnO and HNO3. The synthesis

method combines sol-gel and self-combustion procedures, consisting in the use

of a combustion agent (citric acid C6H8O7) [20, 21]. Self-ignition was initiated

by heating in the first stage at 350 °C, and complete formation of the spinel phase

(CZF) took place after a heat treatment at 500 °C for 3 hours.

Commercial BaTiO3 (BT) nanopowders produced by hydrothermal

synthesis (Sigma Aldrich, purity > 99%, average particle size of 60 nm) were

chosen as the ferro/piezoelectric phase. After wet mixing in suitable

compositions, the mixture was either:

(a) pressed into tablets, then sintered by the conventional method at 1200

°C for 2 hours, or

(b) sintered by SPS at 1000 °C for 5 min under a pressure of 50 MPa.

The SPS sintered ceramic was subsequently reoxidized at 800 °C for 72

hours, then slowly cooled to reduce the amount of oxygen vacancy.

Figure V.1 shows the diffractograms of the constituent powders BT and

CZF obtained by the two sintering methods (CM and SPS).

Figure V.1: Diffractograms made for: (a) BT, CZF powders and ceramic

composites sintered by CM and SPS; (b) detailed representation in the field

2θ ~ (44÷58)°

Regardless of the sintering method, only the constant phases are present

in the composite, i.e. a pure di-phase composite was formed after sintering,

without secondary phases. The major difference between the XRD

diffractograms of the two types of ceramics is related to a difference in the crystal

structure. The composite ceramic (CM) shows a separation of the diffraction

maxima corresponding to the planes (200) and (210) of the perovskite phase of

22

BT, which indicates that in the composite composite (CM) the BT phase has a

tetragonal structure (T). In SPS sintered composite ceramics, the maximum (200)

is not split, similar to the initial BT powder (Figure V.1 (b)). This indicates a

pseudo-cubic structure, which is a typical feature of nanocrystalline BT particles

and nanostructured ceramics [22-25].

The relative density measured by the Archimedes method is quite low,

85% for BT-CZF (CM) and much higher, 98% for BT-CZF ceramics (SPS).

Attempts to increase density by increasing the temperature and sintering time in

the case of the CM method have led to the formation of secondary phases [26].

Figure V.2 shows the SEM microstructural image made in fracture of the

sintered ceramic composite by the classical method.

Figure V.2: SEM image in fracture of composite ceramics

0.66BaTiO3-0.33Co0.8Zn0.2Fe2O4 sintered by the classical method (CM)

The microstructures are relatively porous and the co-existence of two

phases with distinct morphologies is observed: BT has a smaller ceramic granule

size (average granulation of ~ 700-800 nm), while CZF ferrite formed larger

ceramic granules, with faceted appearance (about 1-2 μm medium grain), which

are agglomerated in areas extending to tens of μm.

The SEM-EDX elemental chemical analysis performed in different areas

of the pottery indicates a mixture of phases and/or mutual doping.

The microstructure of plasma arc sintered ceramics indicates a lower

degree of homogenization than in the case of sintering by the traditional method,

23

with the presence of distinct areas corresponding to the two phases: dense areas

with ultrafine granulation (~300 nm) corresponding to BaTiO3 containing

elongated clusters corresponding to ferrite, having ceramic granules with

dimensions of approximately 1μm for the CZF phase.

Figure V.3: Overview of SEM in SPS sintered ceramic fracture

It turns out that the SPS sintering method ensures an almost perfect

densification (porosity of only 2%), and the contact between the two phases is

perfect, although overall no ideal homogenization of the two phases has been

achieved, i.e. ferrite is present in the form of clusters elongated with large

granulation inside the ultra-dense and fine BT ferroelectric matrix. The SEM-

EDX elemental analysis shows in each of the analyzed areas a mixture of ions in

relative quantities, one of the phases being predominant.

V.2 Weak field dielectric properties as a function of temperature

and frequency

Weak field dielectric properties measured with LCR (Concept 40

Novocontrol Technologies) in the frequency range (1÷106) Hz and temperature

range (-150 ÷ 200)°C.

20 μm

24

-150 -100 -50 0 50 100 150 2000

250

500

750

1000

1250

TR-O

(-67oC)

TO-T

(17oC)

Re

al p

art

of

pe

rmit

tiv

ity

Temperature (oC)

BT-CZF (CM)

500kHz

100kHz

50kHz

10kHz

TC

(126oC)

(a)

-150 -100 -50 0 50 100 150 2000

250

500

750

1000

1250

(b)

Real p

art

of

perm

itti

vit

y

Temperature (oC)

BT-CZF (SPS)

500kHz

100kHz

50kHz

10kHz

TC

(104oC)

-150 -100 -50 0 50 100 150 2000,0

0,1

0,2

0,3

0,4

0,5

0,6

(c)

Temperature (oC)

BT-CZF (CM)

Die

lec

tric

lo

ss

500kHz

100kHz

50kHz

10kHz

-150 -100 -50 0 50 100 150 2000,0

0,1

0,2

0,3

0,4

0,5

0,6

(d)

Temperature (oC)

Die

lectr

ic lo

ss

BT-CZF (SPS)

500kHz

100kHz

50kHz

10kHz

Figure V.4: Temperature dependence of the real part of the permittivity (a), (b) and

of dielectric losses (c), (d) for composite ceramics sintered by

CM and SPS at several selected frequencies.

In the case of the compound sintered by the classical CM method the

existence of three peaks from the structural phase transitions of the BaTiO3

component at temperatures of 127°C, 17°C and -62°C (Fig.V.4 (a)), while in BT-

CZF (SPS) ceramics, only the Curie temperature could be identified by a flat

maximum around 104°C, the other structural transitions not being localized,

although some anomalies are still observed below 0°C (Figura V.4 (b)).

Permittivity and losses increase at high temperatures, above the Curie

range, mainly at low frequencies, due to slow thermally activated relaxations

generated by space tasks (Maxwell - Wagner relaxation). This phenomenon

overlaps with the decrease of the Curie - Wiess permittivity, which is usually

observed in ferroelectrics, in their paraelectric state, and in the studied ceramics

it can be observed at high frequencies. The permittivity of SPS sintered ceramics

is characterized by a remarkable thermal stability in a wide temperature range.

25

101

102

103

104

105

106

2000

4000

6000

8000

10000

101

102

103

104

105

106

700

800

900

1000

1100

Frequency (Hz)

Re

al

pa

rt o

f p

erm

itti

vit

y

T=230C

Frequency (Hz)

Re

al p

art

of

pe

rmit

tiv

ity BT-CZF (CM)

-1450C

2000C

(a)

101

102

103

104

105

106

0

1000

2000

3000

4000(b)

101

102

103

104

105

106

300

400

500

600

700

800

Frequency (Hz)

Rea

l p

art

of

perm

itti

vit

y

T=230C

-1450C

Frequency (Hz)

Real p

art

of

perm

itti

vit

y BT-CZF (SPS)

2000C

100

101

102

103

104

105

106

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

(c)BT-CZF (CM)

Frequency (Hz)

Co

nd

uc

tiv

ity

(S

/cm

)

-1450C

2000C

100

101

102

103

104

105

106

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

BT-CZF (SPS)

Frequency (Hz)

Co

nd

ucti

vit

y (

S/c

m)

2000C

-1450C

(d)

Figure V.5: Dependence of dielectric constant and conductivity vs. frequency

for different temperatures: (a) - (b) the real part of the permittivity, (c) - (d) the ac-

conductivity for the two types of composites

In both types of ceramics, the permittivity decreases monotonically with

increasing frequency, with a pronounced increase at ultra-low frequencies

(Figure V.5 (a-b)), especially at high temperatures due to the Maxwell Wagner

effect, causing increased losses dielectric and permittivity. It is also observed the

presence of thermally activated relaxation phenomena in the range of

intermediate frequencies, which are similar in the two types of composites.

In the same temperature range, the two types of ceramics indicate a

similar dependence of conductivity as a function of frequency (Figure V.5 (c-d)).

Based on the Arrhenius analysis, the following were found:

(i) an activation energy Ea = 0,62 eV and relaxation time

characteristic τ = 6x10-13s for the sample sintered by the classical method

and respectively Ea = 0,64 eV and τ = 3,6x10-13s for SPS sintered

ceramics;

26

(ii) the plasma sintered sample also shows a second dispersion process,

specific to low temperatures, i.e. in the field (-145, -50)°C, characterized

by a lower activation energy, Ea = 0,31 eV and the characteristic

relaxation time τ = 2,8x10-14s.

V.3. Magnetic properties, nonlinear dielectric character and

magnetoelectric coupling

The values of magnetization are similar in the two types of ceramics,

having slightly higher values (by about 13%) in the case of ceramic sintered

ceramics due to the higher densification of this sample compared to the one

sintered by the traditional method.

0 100 200 300 400 500 600 700 800 9000

4

8

12

16

20

24

28

32

-10 -5 0 5 10-30

-20

-10

0

10

20

30

H (kOe)

Magnetisation

(emu/g)

BT-CZF (CM)

BT-CZF (SPS)

M (

em

u/g

)

Temperature (K)

T = 300K

Figure V.6: Magnetic properties of ceramic composites: magnetization as a

function of temperature. Inset: cycles of hysteresis M(H) at room temperature

Magnetic Curie temperature, determined as the temperature where

magnetization vanishes in the M(T) dependence, has the value of ~ 637°C for

both samples, being quite close to that of single-phase ferrite with the same

composition (reported values ~ 623° C [27]). The observed temperature

difference can be interpreted as being determined by a slight doping of the ferrite

with Ba or Ti ions at the interfaces between the two phases.

The hysteresis loops of the composites, recorded at room temperature

(inset in Figure V.6) show a ferrimagnetic character, with very low coercivity (

132 Oe), saturation magnetization of 24-27 emu/g and residual magnetization of

27

4 emu/g, both values being lower than those found in pure ferrite, as a result of

the "sum property", i.e. the "dilution" of the ferrite with 66% BaTiO3, a material

that has no magnetic order.

-25 -20 -15 -10 -5 0 5 10 15 20 25200

300

400

500

600

700

800

900

Perm

itti

vit

y

E (kV/cm)

BT-CZF (SPS)

BT-CZF (CM)

before

(a)

-25 -20 -15 -10 -5 0 5 10 15 20 25

200

300

400

500

600

700

800

900

(b)

Perm

itti

vit

y

E (kV/cm)

BT-CZF (SPS)

BT-CZF (CM)

the remaining state (after 10kOe)

0 5 10 15 20 25

1,0

1,2

1,4

1,6

1,8

2,0

(c)BT-CZF (CM)

before M

after M (10kOe)

Tu

nab

ilit

y, n

E (kV/cm)

0 5 10 15 20 25

1,0

1,2

1,4

1,6

1,8

2,0

(d)

before M

after M (10kOe)

BT-CZF (SPS)

Tu

na

bil

ity

, n

E (kV/cm)

Figure V.7: (a-b) Permittivity depending on the electric field dc applied to a

complete cycle of increase/decrease of the field for the virgin sample and in a state of

magnetic remanence, after applying a 10 kOe field and reducing it to zero;

(c-d) Tunability in electric field for the virgin sample and in a state of

magnetic remanence for composites BT-CZF (CM) și BT-CZF (SPS)

It can be observed (Figure V.7 (a)) that, in the virgin state, the BT-CZF

(MC) ceramic has a hysteretic ɛ(E) dependence symmetrical on the E = 0 axis,

still unsaturated at the maximum value of the applied field, while in the case of

the sintered plasma sample, the dependence is nonlinear but reversible (non-

hysteretic), almost linear, without a tendency to saturation.

After the application of a static magnetic field of 10 kOe and its reduction

28

to zero, under remanence conditions, the ceramics maintained their nonlinear

dielectric character, i.e. the variation of the permittivity with the applied electric

field, but the permittivity value itself and the tunability were considerably

reduced (Figure V.7 (b)) in the state of magnetic remanence.

Figure V.7 (c-d) shows how the tunability in the electric field is affected

by the application of the magnetic field in the case of the two types of samples.

It is observed that for both types of ceramics, in the state of magnetic remanence,

the tunability is strongly reduced and in particular, for the sintered plasma

sample, it is almost canceled (Figure V.7 (d)).

Next, to complete the magnetoelectric characterization of the ceramics

investigated in this chapter, the S11 reflection coefficients in the microwave range

in the range (2-6) GHz were measured. Resonant structures containing composite

ceramics as active material were made, using a vector analyzer, in two situations:

(i) without applying a magnetic field;

(ii) under the action of a magnetic field of 1.9 kOe (about 10 times larger

than the coercive field).

1 2 3 4 5 6-35

-30

-25

-20

-15

-10

-5

0

H=0

Hdc=1.9kOe

BT-CZF

(CM)

S1

1 (

dB

)

Frequency (GHz)

(a)1,0 1,5 2,0 2,5 3,0

-35

-30

-25

-20

-15

-10

-5

0

(b)

H=0

Hdc=1.9kOe

BT-CZF

(SPS)

S1

1(d

B)

Frequency (GHz)

Figure V.8: Variation of the coefficient S11 with the frequency for sintered

composite ceramics by the classical method: BT-CZF (CM) and BT-CZF (SPS)

respectively in the absence and in the presence of a magnetic field dc of 1.9 kOe

In both types of composite ceramics, the resonance can be shifted under

the action of a magnetic field dc having values higher than the coercive one. The

displacements of the resonance curves towards higher values, respectively lower

in the two cases, can be explained by the combined effect of the variation of

permittivity and tunability with the magnetic field, i.e. by a bi-tunable character

and by the opposite sign of magnetocapacity in the two types of composite

ceramics investigated.

29

VI. Study of the laminar composite

0.33BaTiO3 – 0.33Co0.8Zn0.2Fe2O4 – 0.33BaTiO3

In this last chapter, the properties of a laminar system having the

composition 0.33BT-0.33CZF-0.33BT, which was densified by SPS sintering,

are presented. Its properties can be compared with those of the system prepared

under similar conditions: 0.33BT-0.33CF-0.33BT which was presented in

Chapter IV, in which the observed differences will be due to the compositional

difference of the ferrite in the intermediate layer, but also with those of the

magnetoelectric composite with the same composition, but having a random

mixture of phases, which was presented in Chapter V.

VI.1 Microstructural characterization

The microstructure of this type of ceramic can be seen in Figure VI.1,

which was performed by scanning electron microscopy in cross section in the

fresh fracture of multi-layer ceramics.

Figure VI.1: Microstructures obtained by SEM microscopy of the laminar composite

0.33BT-0.33CZF-0.33BT, made in fresh fracture, in which a region of the interface and

the microstructures of the constituent phases are observed: BT and CZF.

30

A very good densification of the composite ceramic is observed, having

distinct areas characteristic of the two phases, BT and CZF respectively. In the

ferrite and ferroelectric layers, respectively, the compaction is very good, but at

the interfaces that separate them there are still areas with a certain degree of

porosity.

The micrograph of the interface also shows very well the dimensional

contrast of the two oxide phases, which have very different average granulations,

namely: 150 nm for the corresponding BT regions and respectiv ~ 1.15 µm for

the corresponding CZF region, respectively. Both in the area corresponding to

ferroelectric and CZF spinel, the ceramic granules are faceted, well crystallized

and compact, without intragranular porosity and having perfect triple points,

which indicates a very good sintering of composites and a good compatibility of

the two oxide phases.

The SEM-EDX elemental chemical analysis performed in the three areas

of the studied composite, indicates a clear separation of the component phases;

no more detailed analysis was performed at the interface, as it has a slightly

irregular structure.

VI.2 Weak field electrical properties

The electrical properties were studied by the method of impedance

spectroscopy, which allows the explanation of dielectric and conduction

properties in relation to the microstructure and composition, taking into account

the contributions of different ceramic components (grains, their boundaries, the

interface between ceramic and electrode, etc.) .

The complex impedance diagram at room temperature for the BT-CZF

layered composite (SPS) shows two components.

The electrical properties at room temperature are shown in Figure VI.2.

100

101

102

103

104

105

106

102

103

104

BT-CZF (layered)

SPS

Real p

art

of

perm

itti

vit

y

Frequency (Hz)

(a)

10

010

110

210

310

410

510

610

0

101

102

103

104 (b)BT-CZF (layered)

SPS

Imag

inary

part

of

perm

itti

vit

y

Frequency (Hz)

31

100

101

102

103

104

105

106

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

(c)

104

105

106

0.00

0.05

0.10

0.15

0.20

0.25

BT-CZF (layered)

SPS

tg

Frequency (Hz)10

010

110

210

310

410

510

60,000

0,001

0,002

0,003

0,004

0,005

0,006

(d)

M''

BT-CZF (layered) SPS

Frequency (Hz)

100

101

102

103

104

105

106

10-7

10-6

10-5

10-4

(e)

Co

nd

ucti

vit

y (

S/m

)

BT-CZF (layered) SPS

Frequency (Hz)

=A f n

n=0.4

Figure VI.2: Dependence of dielectric properties as a function of frequency at

room temperature for BT-CZF laminated composite sintered in plasma arc (SPS):

(a) the real part of permittivity; (b) imaginary part of permittivity;

(c) dielectric loss; (d) imaginary part of dielectric module; (e) electrical conductivity.

.

A monotonous decrease in permittivity can be observed from giant

values of 1.2x104, for a frequency of 1 Hz, with a saturation tendency at a value

of ~ 55 for a frequency of 1 MHz (Figure VI.2 (a)).

The imaginary part of the permittivity shows a decrease with frequency,

from 14000 (f = 1Hz) to 1.4 for a frequency of 1 MHz (Figure VI.2 (b)). The

frequency dependence of the imaginary part provides information on the charge

transport mechanisms and conductivity relaxations, allowing the distinction

between dielectric relaxation and conductivity processes.

The conduction is indicated by the presence of a maximum in M"(f),

which is not accompanied by a maximum in ɛ"(f), while a dielectric relaxation

would determine maximums in both dependencies.

32

Dielectric losses show quite high values in the frequency range (10,103)

Hz, with a maximum due to relaxation phenomena (Figure VI.2 (c)), of 3.84 at

a frequency of 37 Hz, which indicates a semiconductor character of the

dielectric in this frequency range.

The frequency conductivity dependence is represented in Figure VI.2

(e), a curve that is subject to the universal law of Jonsker's dielectric relaxation

[29]: σ = Afn, where n is a frequency and temperature dependent exponent, in

general [28,29] with values between 0 and 1, which in this case has the value n

= 0,4.

Permittivity and dielectric losses at high temperatures increase above

the Curie temperature (Figure VI.3), especially at low frequencies, due to the

relaxation of thermally activated slow species, generated by space charges

(Maxwell Wagner relaxation). Comparatively, at a fixed frequency of 500 kHz,

the permittivity of the BT-CZF laminated composite (SPS) varies between

(55÷135) and the permittivity of the randomly mixed BT-CZF (SPS) composite

between (210÷397). A greater variation of the temperature permittivity is

presented by the laminated composite BT-CZF (SPS) at the frequency of 1kHz,

between (55÷4783), essentially feeling the contribution of the ferroelectric

phase of BT.

-150 -100 -50 0 50 100 150 200

102

103

(a)

Perm

itti

vit

y

BT-CZF (layered) SPS

Temperature (0C)

1MHz

500kHz

100kHz

50kHz

10kHz

1 kHz

-150 -100 -50 0 50 100 150 200

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

(b)

tg

1MHz

500kHz

100kHz

50kHz

10kHz

1kHz

BT-CZF (layered)

SPS

Temperature (0C)

Figure VI.3: Temperature dependence of permittivity (a) and dielectric loss

(b) for BT-CZF laminated ceramic composite sintered in plasma arc (SPS)

Dielectric losses (Figure VI.3 (b)) increase with increasing temperature,

their maximums shifting to high temperatures as the frequency increases.

33

VI.3 Electrical properties at different temperatures

Figure VI.4 shows the frequency dependencies of the electrical

properties at the temperature variation in the range (-145C, 198C). It can be

observed for the laminated ceramic composite BT-CZF (SPS) a monotonous

decrease of the permittivity with the increase of the frequency from the room

temperature to T = 198 ° C, in the range of low frequencies with very high

values. This behavior is mainly due to the Maxwell-Wagner effect, which

causes an increase in permittivity and dielectric loss. Comparatively, at room

temperature, the permittivity of the BT-CZF laminated composite (SPS) varies

in the range of 90 ÷ 6400, and in the case of the randomly mixed BT-CZF (SPS)

composite between 350 ÷ 800, in the analyzed frequency range.

100

101

102

103

104

105

106

0

5000

10000

15000

20000

25000

30000

T=-145C

T=-126C

T=-101C

T=-76C

T=-51C

T=-25C

T=-1C

T=23C

T=48C

T=73C

T=98C

T=123C

T=148C

T=173C

T=198CRe

al p

art

of

pe

rmit

tiv

ity

Frequency (Hz)

BT-CZF (layered) SPS

(a)

100

101

102

103

104

105

106

0

2000

4000

6000

8000

10000

(b)

BT-CZF (layered) SPS

Frequency (Hz)

Ima

gin

ary

pa

rt o

f p

erm

itti

vit

y

C

T=-126C

T=-101C

T=-76C

T=-51C

T=-25C

T=-1C

T=23C

T=48C

T=73C

T=98C

T=123C

T=148C

T=173C

T=198C

Figure VI.4: Frequency dependence of the real part of the permittivity (a),

the imaginary part of the permittivity (b) and the conductivity ac (c)

100

101

102

103

104

105

106

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

(c)

Frequency (Hz)

BT-CZF (layered) SPS

Co

nd

uc

tiv

ity

(s

/cm

)

T=-145C

T=-126C

T=-101C

T=-76C

T=-51C

T=-25C

T=-1C

T=23C

T=48C

T=73C

T=98C

T=123C

T=148C

T=173C

T=198C

34

High dc conductivities were obtained, as in the case of the laminated

compound BT-CF (SPS), in the range (10-13, 10-8) S/cm, which indicates the

existence of uncompensated electrical charges located at the interfaces between

the two ferroelectric and magnetic phases (Maxwell-Wagner phenomena), due

to the polarization difference between the two types of oxide materials. The

frequency dependence of the permittivity and conductivity, corroborated with

high dielectric losses in the analyzed frequency range, indicates a more

semiconductor than dielectric character of the analyzed laminated composite.

Using the law Arrhenius ln(τ) vs. 1/T, we obtained from the maxima of

the tangent of the loss angle an activation energy of Ea = 0,67 eV, and from the

maxima of the imaginary part of the dielectric module, an activation energy Ea

= 0,58 eV, values comparable to the activation energy determined in the case of

the randomly mixed composite BT-CZF sintered in SPS plasma arc, Ea = 0,64

eV (chapter V of this thesis).

In the case of BT-CF laminate composite (SPS), two thermally activated

processes were obtained, the relaxation process identified by activation energies

in the range (0,47÷0,50) eV and another process corresponding to a higher

activation energy, (0,55÷0,63) eV.

Activation energies in the field (0,5÷1) eV, are attributed in the

literature [30,31] to the presence of oxygen vacancies or the phenomenon of

hopping conductivity Maxwell-Wagner.

VI.4 Magnetic properties

The maximum value obtained for magnetization in the case of the BT-

CZF laminate compound (SPS), rather high (~ 39 emu/g) (Figure VI.5 (a)), is

due to the very good densification achieved by the sintering method used. For

the magnetic Curie temperature (the temperature at which the magnetization is

canceled in the dependence M(T)) the value of 639 K was obtained (Figure VI.5

(b)).

35

0 150 300 450 600 750 9000

10

20

30

40

BT-CZF layered

(SPS) (a)

T (K)

M (

em

u/g

)

400 500 600 700 800 900-0,30

-0,25

-0,20

-0,15

-0,10

-0,05

0,00 BT-CZF layered

(SPS) (b)

T (K)

dM

/dT

(e

mu

/g/K

)

639K

Figure VI.5: Magnetic properties of ceramic composites: (a) magnetization

as a function of temperature; (b) the temperature dependence of the dM/dT

Figure VI.6 comparatively represents the temperature dependencies of

the magnetization and the dM/dT, corresponding to the samples with the same

composition but with different phase arrangement, the two being densified by the

same SPS method. It can be seen that in both composites, the magnetization of

(30-39) emu/g is reduced compared to the known values for pure ferrite

Co0.8Zn0.2Fe2O4 [32], this being a consequence of the sum property in composites

(Figure VI .6 (a)). For both composites the same magnetic Curie temperature of

~ 639 K was found, which is relatively close to that of the single-phase ferrite

with the same composition for which values of ~ 623 K were reported [33]. The

temperature difference obtained may be due to a slight doping of the ferrite with

Ba or Ti ions at the interfaces between the two phases.

Due to the large losses in the laminated composite BT-CZF (SPS), it

was not possible to measure the nonlinear dielectric properties below the high

dc field (tunability) and it was not possible to perform measurements of

ferroelectric hysteresis P(E). In conclusion, this system also needs to be

optimized in order to reduce losses and to be able to support the application of

intense electric fields, in order to meet the conditions of laminated

magnetoelectric material with ferroelectric character and nonlinear dielectric

at room temperature.

VII. General conclusions

The research carried out in this doctoral thesis focused on the study of

magnetoelectric composites consisting of a magnetostrictive oxide material and

36

a piezo/ferroelectric one, having two types of arrangements of the constituent

phases: (i) in the form of multilayer and (ii) under form of solid material with

randomly mixed phases.

Studies on magnetoelectric materials aim to obtain magnetoelectric

materials having simultaneously dipole and magnetic order (ferroelectric and

ferro, feri- or antiferomagnetic properties) in the same structure and high

magnetoelectric coefficient, in the field of temperatures of interest for

applications, with corrosion resistance high and mechanical hardness, and can

be made at relatively low prices. There are very few single-phase

magnetoelectric materials, and existing ones usually have these properties at

cryogenic temperatures and as such, there is a permanent interest in finding new

single-phase materials or combining composite materials to sum up these

properties at ambient temperature.

In this paper, combinations were chosen between the two materials in

which the ferrite concentration is higher, at the percolation limit, namely 33

vol.%, to maintain a strong magnetic response and also trying to maintain the

dielectric character in the composite. In order to understand the role of

microstructure on their dielectric and magnetic properties, combinations were

made with the same composition of the two phases (66% ferroelectric - 33%

ferrite), but distributed differently in the volume of the composite. Thus, the

magnetoelectric materials studied in this paper were mixed composites with the

formula 0.33CF - 0.66BT and 0.33CZF - 0.66BT, respectively, having a random

phase mixture, as well as laminated composites (tri-layer) type 0.33BT -

0.33CZF - 0.63 BT and 0.33BT - 0.33CF - 0.63BT, respectively. The properties

of these composite ceramics, having the same composition but with the

constituent phases located in different ways in the volume of the ceramics

(random mixture or in laminar structures) were analyzed comparatively and

described by finite element modeling.

It was also investigated for the same composition and how the sintering

method modifies the microstructural characteristics (porosity, granulation) and

functional properties, in case of using two different sintering methods:

traditional method sintering and plasma arc sintering, having randomly mixed

phases.

37

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List of original publications:

40

1. Alexandra Guzu, Cristina E. Ciomaga, Mirela Airimioaei, Leontin

Padurariu, Lavinia P. Curecheriu, Ioan Dumitru, Felicia Gheorghiu,

George Stoian, Marian Grigoras, Nicoleta Lupu, Mihai Asandulesa and

Liliana Mitoseriu, Functional properties of randomly mixed and layered

BaTiO3 - CoFe2O4 ceramic composites close to the percolation limit, Journal

of Alloys and Compounds, 796, 55-64, (2019)

IF:4.175, AIS: 0.601

2. Cristina E. Ciomaga, Alexandra Guzu, Mirela Airimioaei, George

Stoian, Mihai Asanduleasa, Lavinia P. Curecheriu, Ovidiu Avadanei and

Liliana Mitoseriu, Comparative study of BaTiO3–Co0.8Zn0.2Fe2O4 ceramic

composites sintered by classical method and by Spark Plasma Sintering,

Ceramics International (2019).

IF: 3.45, AIS: 0.454.

TOTAL AIS: 1.055

International conferences:

41

1. M. Airimioaei, C. E. Ciomaga, A. Guzu, N. Horchidan, L. P. Curecheriu,

N. Lupu, F. M. Tufescu, L. Mitoseriu, Study of microstructure and functional

properties of layered BaTiO3– ferrite–BaTiO3 magnetoelectric composites

obtained by SPS method, ECerS 2017,15th Conference & Exhibition of the

European Ceramic Society Budapest, Hungary, July 9–13, 2017 (poster

presentation)

2. C. E. Ciomaga, M.Airimioaei, A. Guzu, O. Avadanei, N. Lupu, L.

Mitoseriu, Study of functional properties of ferroelectric-magnetic ceramic

composites obtained by different synthesis method, International Conference

CIEC 16, Torino, Italy, 9-11 September 2018 (poster presentation)

3. C. E. Ciomaga, M. Airimioaei, A. Guzu, F. Gheorghiu, G. Stoian, M.

Grigoraș, M. Asănduleasa, L. Pădurariu, L. Mitoșeriu, Comparative study of the

functional properties magnetoelectric composites, ISAF-ICE-EMF-IWPM-PFM

Joint Conference, July 14-19, 2019, Lausanne, Switzerland (oral presentation)

National conferences:

1. Alexandra Guzu, Cristina E. Ciomaga, Lavinia P. Curecheriu, Mirela

Airimioei, Nădejda Horchidan, Petronel Postolache and Liliana Mitoseriu,

Studies on structural, electrical and magnetic behavior of CoZn ferrite and

BaZr0.15Ti0.85O3 ferroelectric ceramic composites, FARPHYS (Fundamental and

Applied Research in Physics) 29 October, Iasi, Romania, 2016 (poster

presentation)

2. Alexandra Guzu, Lavinia P. Curecheriu, Mirela Airimioei, Nădejda

Horchidan, Petronel Postolache, Cristina E. Ciomaga and Liliana Mitoseriu,

Dielectric and magnetic properties of BaZr0.15Ti0.85O3 and Co-Zn ferrite ceramic

composites, CNFA (National Conference on Applied Physics), 26, 27 November,

Iasi, Romania, 2016 (poster presentation)

3. Alexandra Guzu, Cristina E. Ciomaga, Mirela Airimioei, Felicia

Gheorghiu and L. Mitoseriu, Comparative study of functional properties of

BaTiO3-based magnetoelectric composites, a XLVII-a FTEM (National

Conference Physics and Modern Educational Technologies), Iasi, May19-20,

2018 (poster presentation).