X-ray Magnetic Circular Dichroism

Study of Multiferroic and Dilute

Magnetic Materials

Doctor Thesis

Virendra Kumar Verma

Department of Physics

Faculty of Science & Graduate School of Science

THE UNIVERSITY OF TOKYO

July, 2012

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Abstract

Spintronics materials like multiferroics and dilute magnetic

semiconductors (DMSs) have attracted a great deal of attention in

the scientific community from the viewpoints of both academic

research and practical applications. In order to elucidate the origin

of ferromagnetism of these spintronics materials, it is necessary to

investigate the electronic structure. Advances in experimental

techniques such as synchrotron radiation and electron

spectroscopies provide us with great opportunities to unravel the

underling physics producing the magnetic properties of the

materials. In this thesis, I have investigated the electronic

structure of spintronics materials using x-ray absorption

spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD).

Recently, thin films consisting of alternating ferrimagnetic

spinel-type NiFe2O4 (NFO) and ferroelectric perovskite-type

BaTiO3 (BTO) layers BTO/(NFO/BTO)n were prepared and

exhibited the coexistence of ferroelectric and ferromagnetic

ordering with strong magnetoelectric (ME) coupling between them

[1, 2], but the origin of ME coupling is still remains unclear because

in previous studies, the techniques used to characterize the ME

coupling of NFO/BTO heterostructures were mostly measure

macroscopic quantities. For that purpose, XAS and XMCD at the

both Ni and Fe 2 → 3 absorption edges are an ideal technique to

clarify this issue because they are element-specific microscopic

probes. Here, I have studied the local electronic and magnetic

states of Ni and Fe ions in the NFO/BTO multilayers grown on

(001)-SrTiO3 substrates using pulsed laser deposition with various

NFO and BTO thicknesses by XAS and XMCD in the bulk-sensitive

total-fluorescence yield (TFY) mode at room temperature. The

measured Ni 2p and Fe 2p spectra indicate that the Ni ions are

4

octahedrally coordinated by oxygen and are divalent (Ni2+) and that

the Fe ions are trivalent (Fe3+) and are tetrahedrally or

octahedrally coordinated by oxygen with opposite spin directions,

consistent with the inverse spinel structure of NiFe2O4. With

increasing number of layers, both Ni and Fe magnetic moments

decrease. With decreasing NFO layer thickness, the average

magnetic moment of the Ni ions decreases while the average

magnetic moment of the Fe ions remain unaffected, meaning that

Ni ions replaced by Fe ions at the interface lose ferromagnetic

ordering. I found clear correlation between the ME coupling

strength and the ferromagnetic moment of Fe. This suggests that

the enhancement of ME coupling occurs at interfacial Fe-TiO2

bonding. The Fe-TiO2 bonding creates oxygen vacancies and the

oxygen vacancies at the interface may play an important role to

enhance the ME coupling.

Further, I performed XAS and the XMCD studies of

Cd1-x-yMnxCryTe thin films with Cr content y varied from 0 to 0.04

while keeping the Mn content x fixed around 0.20 grown on GaAs

(001) substrates by the molecular beam epitaxy (MBE) technique.

The prototypical DMS Cd1-xMnxTe is a spin-glass (x<0.6) or

antiferromagentic (x>0.6). Shen et al. [3] found that Cr doping into

Cd1-xMnxTe turns the system from the antiferromagnetic spin glass

to a ferromagnet. In order to elucidate the origin of ferromagnetism,

interaction between Mn and Cr has to be clarified. I found that both

Cr and Mn ions are divalent and that the spins alignment of Cr and

Mn are parallel. The ferromagnetic moment of Mn increases with

Cr concentration. I found the equal concentration of ferromagnetic

Cr and Mn ions in Cd0.76Mn0.2Cr0.04Te sample. These results

suggest that in the presence of Cr ions in Cd1-x-yMnxCryTe, the

interaction between Mn spins changes from antiferromagnetic to

ferromagnetic mediated by the Cr ions and the possible mechanism

of ferromagnetic ordering between Mn and Cr ions is due to the

double exchange interaction.

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Next, I have studied the possible existence of orbital magnetic

moment in multiferroic MnWO4 using the Mn L2;3-edge XMCD.

Shanavas et al. [4] reported a finite but very small orbital magnetic

moment from ab initio electronic-structure calculations. By

applying the orbital sum rule to the XMCD spectrum, despite the d5

configuration of Mn2+ ion, a significantly large orbital magnetic

moment was deduced. The distorted (MnO6)10- octrahedra play an

important role in giving rise to the apparently large orbital

magnetic and electric polarization in MnWO4. The orbital magnetic

moment is in the same direction of the spin magnetic moment,

indicating that Mn 3d states are more than half-filled. Using the CI

cluster-model analysis, I found that the average 3d occupancy, n3d,

is equal to 5.09.

References

[1] C. Deng, Y. Zhang, J. Ma, Yuanhua Lin, and C. W. Nan, J. Appl.

Phys. 102, 074114 (2007).

[2] J. Liu, Y. Zhang, Yuanhua Lin, and C. W. Nan, J. Appl. Phys.

105, 083915 (2009).

[3] S. Shen, X. Liu, Y. J. Cho, J. K. Furdyna, M. Dobrowolska, Y. H.

Hwang, and Y. H. Um, Appl. Phys. Lett. 94, 142507 (2009).

[4] K. V. Shanavas, Debraj Choudhury, I. Dasgupta, Surinder M.

Sharma, and D. D. Sarma, Phys. Rev. B 81, 212406 (2010).

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Contents

Page

Abstract 3

Chapter 1: Introduction 11

1.1 Spintronics 11

1.2 Multiferroics 12

1.2.1 Multiferroic materials and magnetoelectric effect 12

1.2.2 Single-phase multiferroics 13

1.2.3 Physical properties of MnWO4 15

1.2.3.1 Structural properties of MnWO4 15

1.2.3.2 Multiferroic properties of MnWO4 16

1.2.4 Composite multiferroics (Two-phase systems) 20

1.2.4.1 Multiferroic CoFe2O4-BaTiO3

nanostructures

20

1.2.4.2 Multiferroic NiFe2O4-BaTiO3

Heterostructure

21

1.3 Diluted magnetic semiconductors 24

1.3.1 Transition metal doped CdTe 29

References 30

Chapter 2 Principles of x-ray magnetic circular dichroism 37

2.1 X-ray absorption spectroscopy 37

2.2 X-ray magnetic circular dichroism 39

2.3 XMCD sum rules 42

2.4 Configuration-interaction cluster model 43

References 48

Chapter 3 Experimental Details 51

3.1 NSRRC BL-11A 51

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3.2 KEK-Photon Factory BL-16A 55

3.3 SPring-8 BL23SU 58

References 60

Chapter 4 Origin of enhanced magnetoelectric coupling in

BaTiO3/NiFe2O4 multilayers studied by x-ray

magnetic circular dichroism

63

4.1 Introduction 64

4.2 Experimental 65

4.3 Results and discussion 67

4.4 Conclusions 77

References 78

Chapter 5 Cr-doping induced ferromagnetism in the

antiferromagnetic spin glass Cd1-xMnxTe

81

5.1 Introduction 82

5.2 Experimental 83

5.3 Results and discussion 84

5.4 Conclusions 94

References 95

Chapter 6 Orbital magnetic moment in multiferroic MnWO4

studied by x-ray magnetic circular dichroism

99

6.1 Introduction 99

6.2 Experimental 102

6.3 Results and discussion 102

6.4 Conclusions 108

References 110

Chapter 7 Summary and outlook 115

Appendix Orbital magnetic moment in FeCr2S4 studied by x-

ray magnetic circular dichroism

121

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A.1 Introduction 121

A.2 Experimental 122

A.3 Results and discussion 123

A.4 Conclusions 132

References 133

Acknowledgements 137

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Chapter 1

Introduction

1.1 Spintronics

Spintronics (or spin based electronics) also known as

magnetoelectronics, in which charge and spin degrees of freedom of

electrons, can be used simultaneously in order to develop a new

generation of useful electronics devices, and processes with new

functionality have become the subjects of growing interest for

researchers [1.1, 2, 3]. Spintronic devices are expected to develop

upon conventional electronics and photonic devices, allowing for

enhancement in the form of less power consumption, faster device

operation, and new forms of information computation. Possible

applications of spintronics devices include high-speed magnetic

filters, spin-polarized LEDs, sensors, spin-field effect transistors

(SFETs), quantum transistors, and spin-based qubits for quantum

computers [1.4, 5, 6, 7]. The key advantage of spintronic device is

that multiple functions can be integrated in the same chip, which

can function simultaneously as a memory and amplifier. The

utilization of electron spin in practical devices has already been

proven as spin valves, which are used as magnetic tunnel junctions

in magnetic random access memories (MRAM). Since electron spin

exists in either spin-up or spin-down configuration, spintronic

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materials would also open a door for the development of rewritable

devices that are nonvolatile.

1.2 Multiferroics

1.2.1 Multiferroic materials and magnetoelectric

effect

Multiferroic is a material that simultaneously possesses two

or more of the ferroic order. These ferroic order parameters are

ferroelectricity, ferromagnetism, and ferroelasticity. The phase

control in ferroics and multiferroics are shown in fig. 1.1. Coupling

interaction between the different order parameters could produce

additional effects, such as magnetoelectric (ME) effect. The

magnetoelectric (ME) effect is the phenomenon of inducing

magnetization by applying an external electric field and/or the

Figure 1.1: The phase control in ferroics and multiferroics [1.9]

phenomenon of inducing electric polarization by applying an

external magnetic field. Though the mechanisms that allow

ferroelectricity and ferromagnetism seem to be incompatible, there

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are a select few materials in which ferroelectricity and

ferromagnetism are both present. Theoretical studies have shown

that the usual atomic-level mechanisms driving ferromagnetism

and ferroelectricity are mutually exclusive, because

ferromagnetism and ferroelectricity require empty and partially

filled transition metal orbitals, respectively [1.8].

1.2.2 Single-phase multiferroics

ME response has been found in a relatively small number of

single-phase multiferroic materials. The first ferromagnetic

ferroelectric material to be discovered is nickel iodine boracite,

Ni3B7O13I [1.10]. This discovery was followed by the synthesis of

several multiferroic boracites compounds, all of these compounds

have complex structures with many atoms per formula unit and

more than one formula unit per unit cell. In these materials, the

essential factors of causing the multiferroicity and the nature of the

coupling between the magnetic and electric polarization and

structural order parameters are a number of inter-ionic interaction.

Nickel iodine boracite can be thought of as the ‘Rochelle salt’ of

magnetic ferroelectrics and it has wide applicability and contribute

to our increased understanding in the field. Other ferromagnetic

ferroelectrics materials were also searched by replacement of some

of the d0 B cations in ferroelectric perovskite oxides (ABO3) by

magnetic dn cations. (1-x)Pb(Fe2/3W1/3)O3 – xPb(Mg1/2W1/2)O3 [1.11]

is an example of first synthetic ferroelectric ferromagnetic material

in which Mg and W ions are diamagnetic and cause the

ferroelectricity, and the formally d5 Fe3+ ion is responsible for the

magnetic ordering. In the past few years, there has been renewed

interest in studying a number of other perovskite-based

multiferroic materials, such as manganites of small rare earth

elements [1.12, 13] and yttrium [1.14] and a few compounds in

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which Bi [1.15] is the large cation, which have large

magnetoelectric effects.

In the spiral type-II type of multiferroic materials, the

electrical polarization is caused by a spiral order of the magnetic

moments. The relevant exchange striction in these spiral magnets

is associated with the antisymmetric part of the exchange coupling,

known as the Dzyaloshinskii-Moriya interaction (DMI) [1.16].

Table 1.1 lists some of the known single phase multiferroic

materials adapted from [1.17]

Table 1.1 Example of single phase multiferroic materials that

exhibit ME effect.

Compound Type of

electric order

Type of

magnetic order

Tc (K) TN (K)

Pb (Fe2/3W1/3)O3 FE AFM 178 363

Pb (Fe1/2Nb1/2)O3 FE AFM 387 143

Pb (Co1/2W1/2)O3 FE WFM 68 9

Pb (Mn2/3W1/3)O3 AFE? AFM 473 203

Pb (Fe1/2Ta1/2)O3 FE AFM 233 180

Eu1/2Ba1/2TiO3 FE FM 165 4.2

BiFeO3 FE AFM 1123 650

BiMnO3 AFE FM 773 103

YMnO3 FE AFM 913 80

YbMnO3 FE AFM/WFM 983 87.3

HoMnO3 FE AFM/WFM 873 76

ErMnO3 FE AFM 833 79

Ni3B7O13I FE WFM 64 64

Ni3B7O13Br FE WFM 398 30, 40

Co3B7O13I FE WFM 197 38

*FE – Ferroelectric, AFE – Antiferroelectric, FM – Ferromagnetic, AFM –

Antiferromagnetic, and WFM – Weak ferromagnetic.

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Among the multiferroic manganates, MnWO4 takes a special

place as it does not contain a rare-earth ion, leaving Mn formally as

the only magnetic ion. It has already been proved that

ferroelectricity in MnWO4 can appear in the cycloidal-spiral spin

phase without centrosymmetry [1.18, 19]. Multiferroicity in which

electrical polarization is caused by the spiral order of magnetic

moments is explained by the spin-current model [1.20, 21]. If the

spin-rotation axis ê is not parallel to the magnetic propagation

vector Q, nonvanishing spontaneous polarization P α ê ×Q develop.

The research presented in chapter 6 is focused on the magnetic

properties of MnWO4.

1.2.3 Physical properties of MnWO4

1.2.3.1 Structural properties of MnWO4

The structure of MnWO4 is a wolframite structure, which

belongs to the monoclinic space group P2/c with β ~ 91º at room

temperature. Figure 1.2 (a) shows the crystal structure of MnWO4.

The crystal structure is characterized by alternative stacking of

manganese and tungsten layers parallel to the (100) plane as

shown in Fig. 1.2 (b). Mn2+ ions are surrounded by distorted oxygen

octahedra and aligned in zigzag chains along the c axis. Due to

distortion, the three different bond lengths of Mn-O in MnO6

octahedron are 2.09 Å, 2.18 Å, and 2.27 Å, respectively [1.23]. In

AF1 and AF3, magnetic moments collinearly align in the ac plane

forming an angle of about 35º with the a axis, whereas in AF2 an

additional component in the (010) direction exists, as shown in Figs.

1.2 (b) and 1.2 (c), respectively.

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Figure 1.2: (a) Crystal structure of MnWO4 viewed along the a axis:

each Mn atom (purple) is surrounded by an oxygen (red) octahedron.

W atoms (gray) separate zigzag chains of Mn atoms. (b) Collinear

magnetic structure in AF1 and AF3: magnetic moments lie in the ac

plane and canted to the a axis by about 35º. (c) Elliptical spiral spin

structure in AF2: basal plane of spiral is inclined to the ab plane.

[1.22]

1.2.3.2 Multiferroic properties of MnWO4

MnWO4 exhibits three different magnetic phase transitions at

T1 = 7.6 K, T2 = 12.7 K and TN = 13.5 K [1.24] to three long

wavelength antiferromagnetic (AF) ordering states. According to

neutron diffraction results, AF1 (T < T1), AF2 (T1 < T < T2), and AF3

(T2 < T < TN) are a commensurate collinear antiferromagnetic

phase, an incommensurate cycloidal spiral-spin phase, and an

incommensurate collinear antiferromagnetic phase, respectively.

The magnetic structure of the AF1 state is commensurate with the

propagation vector (−1/4, 1/2, 1/2), while that of the AF3 and AF2

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Figure 1.3: (a) Magnetic susceptibility for each crystal axis as a

function of temperature in 0.1 T. (b) Dielectric constant for E // b in

0 T. (c) Electric polarization ∆P // b in 0 T. During the measurement

of pyroelectric current to obtain ∆P, an electric field of 500 kV/m

was continuously applied along the b axis in a cooling process. ∆P

was calculated by integrating the measured pyroelectric current

with respect to time. [1.22]

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states are incommensurate with the propagation vector (−0.214, 1/2,

0.457). Figure 1.3 (a) shows the temperature dependence of the

magnetic susceptibility (χ) at magnetic field of 0.1 T parallel to the a,

b, and c axis, respectively. In all direction, a cusp is observed in χ at

Neel temperature TN ~ 13.5 K. Above TN, χ(T) follows the

Curie-Weiss law with a negative Weiss temperature (θ ~ 78 K). At

the transition temperature T1, χb shows a steep rise, while χa and χc

show a sharp drop. This behavior is consistent with the neutron

diffraction result that the easy axis of the Mn2+ moments is within

the ac plane in AF1 [1.24]. Figures 1.3 (b) and (c) display

temperature dependence of dielectric constant (εb) and electric

polarization (∆Pb) in zero magnetic field. εb shows a sharp peak at

T2, and a very small drop (~0.08%) at T1. As seen in Fig. 1.3 (c), the

spontaneous polarization exists in the AF2 phase between T1 and

T2. These results clearly indicate that MnWO4 becomes

ferroelectric simultaneously when the AF2 phase with the spiral

spin configuration appears.

Recently, Hollmann et al. [1.23] studied magnetic anisotropy

in multiferroic MnWO4 by soft x-ray absorption spectroscopy (XAS)

at room temperature. They found that Mn ions in MnWO4 are in

the high-spin Mn2+ electronic configuration and a crystal-field level

scheme is different from octahedral (Oh) for the MnWO4 system.

Figure 1.4 shows the room temperature Mn L2,3 XAS spectra of

MnWO4 taken with the E vector of the light parallel to the a, b, and

c crystallographic axes and found a small but clear polarization

dependence for MnWO4.

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Figure 1.4: Top panel: experimental and theoretical Mn L2,3 XAS

spectra of MnWO4 with the E vector of the light parallel to the a, b,

and c crystallographic axes. The spectrum of MnO is included as

reference. Bottom panels: a closeup revealing the polarization

dependence of the spectra. [1.23]

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1.2.4 Composite multiferroics (Two-phase systems)

The single-phase multiferroics are not very attractive because

none of the existing materials combine large and robust electric and

magnetic polarizations at room temperature. Composite

multiferroic materials with better design flexibility made by

combining ferroelectric and magnetic substances together in the

form of multilayers or self-organized nanostructures have drawn

significant interest in recent years due to their multifunctionality,

in which the coupling interaction between ferroelectric and

magnetic substances could produce a large ME response. In

composite multiferroics, each phase can be independently

optimized for room temperature performance. In general, two type

of ME coupling between ferroelectric and magnetic subsystems can

be expected: (i) owing to electronic effects at the interface, and (ii)

mediated by strain. This type of multiferroic materials can be

achieved in the form of composites laminates or epitaxial

multilayers. Zheng et al. and Ramesh et al. [1.25, 26] studied the

composite materials consisting of alternating ferroelectric and

ferromagnetic layers of nano-rods embedded in a matrix provide

strong ME coupling effect. Recently, ME properties of CoFe2O4 –

BaTiO3 [1.27] and NiFe2O4 – BaTiO3 [1.28, 29] heterostructures

studied by several groups. Table 1.2 lists some of the known

composite multiferroic materials adapted from Eerenstein et al.

[1.30].

1.2.4.1 Multiferroic CoFe2O4-BaTiO3 nanostructures

Zheng et al. used a different approach to produce multiferroic

thin films. They grew of alternating layers of the ferroelectric

material BaTiO3 and the ferro/ferrimagnetic material CoFe2O4 as

shown in Fig. 1.5 (A) and (B) and heterostructure consisting of

nanopillars of the ferro/ferrimagnetic phase embedded in a

ferroelectric matrix as shown in Fig. 1.5 (C) and (D). Such

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Table 1.2 Example of composite multiferroic materials

Morphology Materials Coupling constant

(mV.Cm-1Oe-1)

Composite BaTiO3 and CoFe2O4 50

Composite Terfenol-D and PZT in

polymer matrix

42

Laminated

composites

Terfenol-D and PZT in

polymer matrix/PZT in

polymer matrix

3000

Laminate Terfenol-D/PZT 4800

Laminate La0.7Sr0.3MnO3/PZT 60

Laminate NiFe2O4/PZT 1400

*PZT (Pb(Zr,Ti)O3) and BaTiO3 are piezoelectric, and terfenol-D

(TbxDy1-xFe2), the maganite and the ferrites are magnetostrictive.

nanostructures are deposited on (100) SrTiO3 (STO) substrates by

the pulsed laser deposition (PLD) technique from a single

0.65BaTiO3–0.35CoFe2O4 target. By using x-ray and electron

diffractions, they found that the films are epitaxial in the plane as

well as out of the plane, with self-assembled hexagonal arrays of

CoFe2O4 nanopillars embedded in a BaTiO3 matrix. In summary,

the CoFe2O4-BaTiO3 ferroelectromagnetic nanocomposite shows a

strong magnetoelectric coupling of the order parameters through

the heteroepitaxy of the two lattices.

1.2.4.2 Multiferroic NiFe2O4-BaTiO3 Heterostructure

Deng et al. [1.28] studied magnetic-electric properties of

epitaxial multiferroic NiFe2O4–BaTiO3 (NFO-BTO) heterostructure.

They found that NFO-BTO heterostructure show strong

ferroelectric and ferromagnetic responses simultaneously at room

temperature. Figures 1.6 (a) and (b) show good coexistence of

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Figure 1.5: (A) Superlattice of a spinel (top) and a perovskite

(middle) on a perovskite substrate (bottom). (B) Schematic

illustration of a multilayer structure on a substrate. (C) Epitaxial

alignment of a spinel (top left) and a perovskite (top right) on a

perovskite substrate (bottom). (D) Schematic illustration of a self

assembled nanostructured thin film formed on the substrate [1.25].

ferroelectric and ferromagnetic behaviors. The ferroelectric loops

show that the saturation polarization Ps and remanent polarization

Pr are about 29 and 10 μC/cm2, respectively. The ferromagnetic

hysteresis loops show that the saturation magnetization Ms is

about 80 emu/cm3. Figure 1.6 (c) show the magnetoelectric (ME)

response of the films. When an in-plane altering magnetic field

signal δH is applied, the single phase BTO and NFO films do not

produce any ME response, while the heterostructured NFO/BTO

film clearly yields the ME output following the magnetic

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Figure 1.6: (a) Ferroelectric hysteresis loops of the NFO/BTO

heterostructured film and the pure BTO film. (b) In-plane magnetic

hysteresis loops of the NFO/BTO film and the pure NFO film. (c)

The ME response of the films when the in-plane magnetic field is

applied. [1.28]

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excitation signal δH. In order to understand the microscopic origin

of the ME coupling at the BTO/NFO interfaces in the BTO/NFO

multilayer thin films, it is necessary to investigate the electronic

and magnetic properties of the Fe and Ni ions at the interfaces. In

the chapter 4, I have studied the origin of magnetoelectric coupling

in BaTiO3/NiFe2O4 heterostructures by XAS and x-ray magnetic

circular dichroism (XMCD).

1.3 Diluted magnetic semiconductors

Ferromagnetic semiconductors have been in the center of

much attention for more than a decade for research due to their

interesting physical properties and potentially useful spintronics

device. One aspect of spintronics of particular interest is the

creation and control of spin-polarized currents in semiconducting

material. The main idea is to combine conventional semiconductor

physics and magnetism to develop novel spin dependent

nanoelectronic devices including devices for quantum information

processing, which have been among the well established areas of

condensed matter physics. In dilute magnetic semiconductors

(DMSs), a fraction of the cations in the lattice are replaced

substitutionally by magnetic ions (Fig. 1.7) and are expected to

have both the properties of magnetic materials and semiconductors.

The coupling between the localized moments and delocalized

band-electrons renders unique properties of DMS, such as a giant

spin-splitting of electronic states and indirect ferromagnetic

exchange interactions between magnetic moments [1.31].

Semiconducting behavior in ferromagnetic material was first

uncovered by the discovery of the europium chalcogenides (EuO)

[1.32]. Eu-rich EuO shows ferromagnetic ordering while O-rich

EuO shows no metal-insulator transition (see Fig. 1.8). In 1986,

Story et al. [1.33] have reported that the alloy between the IV-VI

semiconductor and MnTe [(PbTe)1−x(SnTe)x]1−y[MnTe]y (y = 0.03)

25

shows ferromagnetic ordering when the hole carrier concentration

is above 1021 cm-3, as shown in Fig. 1.9.

Nonmagnetic semiconductor Magnetic semiconductor

Figure 1.7: (a) Semiconductor host doped with magnetic ions (b)

Magnetism in semiconductors: (Left) In normal, nonmagnetic

semiconductors, electronic energy does not depend on the spin

direction. (Right) In magnetic semiconductors, the d electrons of

magnetic ions influence the s and p electrons, and the conduction

band and valence band are split depending on the spin direction

(Zeeman splitting). [1.34]

(a)

(b)

26

Figure 1.8: Early reports on ferromagnetic semiconductors.

Conductivity of Eu-rich and O-rich EuO [1.32].

Figure 1.9: Carrier-concentration dependence of the ferromagnetic

Curie temperatures (TC) of PbSnMnTe [1.33].

Some of the reported DMS materials are summarized in Table

1.3. Most of the early dilute magnetic semiconductors such as

tellurides, selenides and sulfides were based on Mn-doped II-VI

semiconductors. The valence match (i.e. identical charge state) of

27

Table 1.3 Representative dilute magnetic semiconductors.

Material class Material Reference

II-VI p-Cd1-xMnxTe:N 1.40

II-VI p-Zn1-xMnxTe:N 1.38

II-VI Zn1-xCrxSe 1.41

II-VI Zn1-xCrxTe 1.42

II-VI Zn1-xCrxTe:I

Zn1-xCrxTe:N

1.43

IV-VI Pb1-x-ySnyMnxTe 1.44

III-V In1-xMnxAs 1.45

III-V Ga1-xMnxAs 1.46

III-V Ga1-xMnxN 1.47

IV Ge1-xMnx 1.48

Oxide Co-TiO2 1.49

Oxide Mn-ZnO 1.50

Oxide Cu-ZnO 1.51

Oxide Zn1-2xMnxCoxO 1.52

Oxide Co-SnO2 1.53

Oxide Fe-SnO2 1.54

Oxide Cr-In2O3 1.55

the cation of the II-VI host semiconductors to the dopant (Mn),

makes it easy to prepare samples with a large amount of Mn

[1.35-37]. The model materials (i.e. II-VI materials) in which

localized spins and delocalized holes can be introduced and

controlled independently, while dimensional effects can be tested

by using quantum heterostructures [1.38]. Sato and Katayama

Yoshida [1.39] employed first-principles calculation to investigate

ferromagnetism in both semiconductor and oxide spintronics. The

magnetic stability of transition metal-doped ZnO, ZnS, ZnSe, and

ZnTe was calculated using density functional theory (DFT) within

the framework of the local density approximation (LDA). The

28

random distribution of transitional metals impurity over the lattice

was inherently incorporated in the calculations by the coherent

potential approximation (CPA). Magnetic stability was calculated

by comparing the total energy difference between the ferromagnetic

and spin-glass state, the lower of the two representing the ground

state. In the case of Mn doped DMSs, the spin-glass state becomes

most stable while for V-, Cr-doped DMSs, the ferromagnetic states

are more stable than the spin-glass states.

Figure 1.10: Stability of the ferromagnetic states in (a) ZnO-, (b)

ZnS-, (c) ZnSe- and (d) ZnTe-based DMSs. V, Cr, Mn, Fe, Co or Ni is

doped as a magnetic impurity. The vertical axis is the energy

difference per one formula unit between the ferromagnetic state

and the spin-glass state. A positive energy difference indicates that

the ferromagnetic state is more stable than the spin-glass state

[1.39].

29

1.3.1 Transition metal doped CdTe

Galazkaet at el. [1.56] has investigated the magnetic

properties of Mn-doped CdTe single crystals, which could give rise

to paramagnetic, spinglass, and antiferromagnetic phase regions as

functions of temperature and Mn content. Compound or alloy doped

with two kind of transition element has rarely been studied.

Recently, Shen et al. [1.57] doped two transition metals in CdTe

crystal and studied the ferromagnetic behavior of Mn and Cr

co-doped CdTe bulk crystal. Figure 1.11 shows the magnetization

curve of Cr and Mn co-doped CdTe crystal. According to the

magnetic polaron model proposed by Shen et al. [1.57], a small

fraction of Cr atoms incorporated in (Cd,Mn)Te become Cr+ ions

acting as an acceptor and Mn spins are aligned by the holes

residing at this Cr+ acceptor level. The origin of ferromagnetism

and the interaction between Mn and Cr have not been clarified in

Shen et al. [1.57]. In chapter 5, I have studied the electronic

structure and magnetic properties of Mn- and Cr-doped CdTe by

x-ray magnetic circular dichroism, the element specific probe

technique.

Figure 1.11: SQUID magnetization for CdMnCrTe sample with 1%

Cr. Inset: field-cooled and zero-field-cooled temperature

dependences of remanent magnetization measured by SQUID.

[1.57]

30

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35

36

37

Chapter 2

Principles of x-ray magnetic

circular dichroism

X-ray magnetic circular dichroism (XMCD) technique is the

unique tool for an element-specific as well as a symmetry selective

probe of microscopic magnetic properties. XMCD is the difference

between left-handed and right-handed circularly polarized x-ray

absorption spectra (XAS). Nowadays, XAS is easy to measure at

synchrotron radiation facilities with excellent x-ray energy

resolution. This chapter is focusing on the principles of x-ray and

XMCD.

2.1 X-ray absorption spectroscopy

XAS and x-ray photoelectron spectroscopy (XPS) are the two

most widely used core electron spectroscopies. The measurements

of photo-absorption by excitation of a core-level electron into

unoccupied states as a function of photon energy is called x-ray

absorption spectroscopy. We can obtain information about the

valence states and the local environments surrounding each

elements by XAS spectra. The advantage of XAS technique is that it

can probe samples in a surface-sensitive (electron yield detection)

mode or bulk-sensitive (transmission) mode, which is of great

38

importance for artificially made multilayer structures. Figure 2.1

shows the principles of x-ray absorption spectroscopy, using a

one-electron model for the case of L edge absorption in a d band

transition metal. The absorption intensity from the initial state

can be calculated by summing all possible final states :

, (2.1)

where hν is the photon energy of the x-ray, T is the dipole

transition operator. The 2p core-level XAS spectra of transition

metal compounds well reflect the 3d electronic states in the 3d

transition-metal compounds including the symmetry and the

crystal field splitting of the 3d orbitals. Thole et al. [2.1] performed

the calculations of transition-metal 2p XAS spectra of 3d

transition-metal ions in a crystal field.

Figure 2.1: Principles of X-ray absorption spectroscopy, using a

one-electron model for the case of L edge absorption in a d band

transition metal. [2.2]

There are two measurement modes for XAS, the

transmission-mode and the total-yield mode. Transition mode is the

most direct measurement mode, in which the intensity of the x-ray

39

is measured before and after the sample and the ratio of the

transmitted x-rays is counted. Transmission-mode experiments are

standard for hard x-rays, though for soft x-rays, they are difficult to

perform because of the strong interaction of soft x-rays with the

sample. The total-yeild method can be classified in two modes, the

total-electron yield (TEY) and the total-fluorescence yield (TFY)

modes. TEY and TFY denote measuring the current flow and the

fluorescence of the sample radiated by x-ray, respectively. TEY

mode is surface sensitive while TFY mode is bulk sensitive and the

probing depth of TEY and TFY modes are ~5 nm and ~100 nm,

respectively. The TFY mode suffers from self-absorption effect

because of its long probing depth. In the present work, TEY and

TFY modes were employed.

2.2 X-ray magnetic circular dichroism

When the relativistic electrons in the storage ring are

deflected by the bending magnets that keep them in a closed

circular orbit, they emit highly intense beams of linearly polarized

x-rays in the plane of the electron orbit (bremsstrahlung). On the

other hand, they emit circularly or elliptically polarized light out of

the plane. Currently, a number of alternative sources for circularly

polarized synchrotron radiation are under development. The most

notable ones are so-called insertion devices like helical wigglers

[2.3] and crossed [2.4] undulators. Both wiggler and undulator are

complex arrays of magnets with which the electrons in a storage

ring are made to oscillate in two directions that are perpendicular

to their propagation direction, with the result that they emit

circularly polarized light.

Figure 2.2 shows schematic diagram of XMCD. Using

circularly polarized light in XAS, the absorption intensity depends

on the helicity of the incident light. When right- and left-handed

circularly-polarized x-rays are irradiated on a sample under applied

40

magnetic fields, differences in the absorption intensity are observed

because of differences of transition matrix elements (or selection

rule). XMCD is defined as the difference in the absorption spectra

when the helicity of the x-rays are parallel and antiparallel to the

magnetization direction of a material. XMCD is an element specific

measurement because of core-level excitation and is sensitive to

magnetically active components. The line shape of XMCD spectra

reflects electronic structure related to the magnetism. In addition,

by applying XMCD sum rules described below, one can estimate

spin and orbital magnetic moments separately from integrated

intensities of XAS and XMCD spectra. Therefore, XMCD is a

powerful tool to investigate magnetic properties of materials.

Figure 2.2 (c) shows the transition probability of 2p → 3d

absorption with circularly polarized x-rays. The transition

probability is proportional to

(2.2)

where is the Gaunt coefficient and mp and md are the

magnetic quantum numbers of the 2p and 3d states with md = mp+1

or md = mp-1. The circular polarization is expressed by , where

and have helicity 1 and -1, respectively. The selection rule is

derived from the equation. The selection rule of the transition

induced by circularly polarized light with helicity is change in the

magnetic quantum number by ±1 while the spin moment is

conserved. The final 2p hole is located at the 2p3/2 and 2p1/2 states,

and two absorption edges related to the 2p3/2 to 3d and 2p1/2 to 3d

transitions, called L3 and L2, respectively. The 2p3/2 and 2p1/2 have

four and two degenerated states, respectively. When we apply

magnetic field on the magnetic material, the final state d orbitals

are split according to the spin and are slightly affected by the

magnetic quantum number. Thus, the final state for both spin

directions can be classified as the states state. For 2p3/2

41

(c)

Figure 2.2: Schematic diagram of x-ray magnetic circular dichroism

(XMCD). (a) Experimental set up for XMCD measurements.

(b) Circularly polarized x-ray absorption spectra. (c) Transition

probability of 2p → 3d absorption with circularly polarized x rays

for less-than-half filled 3d electronic configuration.

42

and positive helicity, the transition probability ratios for each

are 18, 6 and 1. The transition probability ratios for 2p3/2

with opposite helicity are 3, 6 and 6 as shown in figure. Similarly,

the transition probability ratios for 2p1/2 with positive and negative

helicities are 3 and 2, and 3 and 12, respectively. The difference of

the transition probabilities between positive and negative helicities

provides the XMCD.

2.3 XMCD sum rules

XMCD reflects the spin and orbital polarization of local

electronic states. By using integrated intensity of the L2,3-edge XAS

and XMCD spectra of a transition-metal atom, one can separately

estimate the value of orbital and spin magnetic moments. Let us

consider 2p → 3d excitation as an example. Using representation of

hole number of the each 3d states hmd, the orbital and spin moments

are related to hmd as follows:

The relative transition intensity is proportional to product of the

transition matrix element and the hole number, and can be

calculated by using Fermi Golden Rule. Therefore, for less than half

filled 3d states, the difference in the 2p → 3d transition intensity

between the right-handed and left-handed circular polarizations

can be expressed for the for L3 and L2 edges as follows, respectively.

ΔIL3 ∝ 18h+2 + 6h+1 − 2h0 − 6h−1 − 6h−2, (2.5)

ΔIL2 ∝ 3h+1 + 2h0 − 3h−1 − 12h−2, (2.6)

Then, sum of the transition intensities is proportional to the orbital

moment as given below:

43

ΔIL3 + ΔIL3 ∝ 18h+2 + 9h+1 − 9h−1 − 18h−2,

∝ 9(2h+2 + h+1 − h−1 − 2h−2),

∝

∝

As well as the orbital moment, one can estimate the spin moment

from the transition matrix element of circularly polarized x-rays.

Thole et al. [2.5] and Carra et al. [2.6] have derived formulae to

calculate the spin and orbital magnetic moments of the atoms from

the XMCD spectra. For the 2p-3d XMCD analysis, the formulae of

the spin and orbital magnetic moments are given by

(2.8)

where μ+ and μ- are the absorption intensities for the positive- and

negative polarizations, respectively, and Nd is the number of

electrons in the 3d band. MT is the magnetic dipole moment, which

can be neglected in the case of perovskite-type oxide with cubic

symmetry [2.7, 8]. The orbital magnetic moment (Morb) and the spin

magnetic moment (Mspin) both are in units of μB/atom, where μB is

the Bohr magneton.

2.4 Configuration-interaction cluster model

In order to consider the correlation between electrons, we

consider the hybridization between the Slater determinants instead

of a single Slater determinant. The hybridization between the

Slater determinants leads to so called configuration interaction (CI).

In this thesis, core-level and valence-band spectra will be analyzed

using CI calculation. The CI cluster-model analysis has been a

44

useful framework for understanding the electronic structure of

magnetic materials [2.9-11]. According to Tanaka and Jo [2.12], the

Hamiltonian of the cluster is assumed to be

H = H1 + H2 (2.9)

where H1 is the non-multiplet term which describes the valence and

core states and the effective interaction between electrons, which is

adopted from the early stages of analysis of PES and XAS data

[2.13], given by

where denotes the transition metal 3d symmetry group in the

crystal field, e.g., t2g and eg representation in the octahedral (Oh)

configuration, denotes the basis of each representation, and

stands for the orbital and spin state of the core orbit. The energy

difference between the t2g and eg is given by 10Dq for the 3d orbit.

The first, second, third, fourth, fifth and sixth term in equation

(2.10) denote the energy of the 3d orbit of the TM ion, of the ligand

molecular orbit, the core orbit, the effective 3d-3d interaction, the

attractive core hole potential acting on the 3d electron and the

hybridization between the ligand and 3d orbit, respectively. The

charge transfer energy is defined as ∆ = , where

and are the average energies of and

configuration, respectively. Here, denotes a hole in the ligand p

orbitals in the cluster.

45

H2 describes the full-multiplet 3d-3d (Hdd) and 3d-core

interaction (Hdc) is given by

(2.11)

is the spin-orbit interaction in the 3d state with the coupling

constant and is the spin-orbit interaction in core

state.

The wave function of the ground state ψg for N-electron state,

is spanned by linear combinations of charge transfer state as given

by

(2.12)

The final state wave functions of transition-metal 2p core-level ψc

and transition-metal 3d valence-band ψv are also spanned by linear

combinations of charge transfer state as,

(2.13)

(2.14)

where c and L denotes holes in the transition-metal 2p core level

and ligand p orbitals, respectively, and n is the number of d

electrons for the ground state of the transition metal. The

charge-transfer energy for anion-to-3d orbital is defined by Δ≡

E(dn+1)−E(dn), and the 3d-3d Coulomb interaction energy is defined

by U≡E(dn+1)+E(dn-1) − 2E(dn). It is also possible to define the

charge-transfer energy Δeff and the Coulomb interaction energy Ueff

with the lowest term of each multiplet. The multiplet splitting is

expressed using Racah parameters A, B, and C or Kanamori

parameters u, u’, j, and j’ for the multiplet splitting of the dn

configuration due to intra-atomic Coulomb and exchange

46

interactions. There is relationship between these four parameters;

u’=u−2j, j’=j. The charge-transfer energy Δ and Coulomb energy

U as well as the Slater-Koster parameters are adjustable

parameters in the cluster-model calculation.

The Slater parameters which describe the d electrons and the

2p electrons of created the core hole and the spin-orbit coupling

parameter for the d and for the 2p states are also used in the

cluster model calculation. The Slater parameters and the spin-orbit

coupling can be obtained from Hartree-Fock calculations [2.14, 15].

The tetrahedral and octahedral symmetry are shown in figure 2.3.

For Td crystal field symmetry, transfer integrals T(Γi) between the

3d orbitals and ligand p orbitals are given by Slater-Koster

parameters (pdσ) and (pdπ) as

(2.15)

(2.16)

Figure 2.3: Octahedral (a) and tetrahedral (b) crystal symmetry

47

In the case of the Oh crystal field symmetry, one electron transfer

integrals T(Γi) between the 3d orbitals and the anion ligand orbitals

can be expressed in terms of the Slater-Koster parameters (pdσ)

and (pdπ) as follows:

(2.17)

(2.18)

To perform cluster model calculation, I used the program Xtls

8.5 developed by Prof. A. Tanaka. First I set the electronic

configurations of the transition metal ion. In chapter 6, I have

employed three charge-transfer states such as 2p63d5, 2p63d6L, and

2p63d7L2, where, denotes a hole in the ligand p orbitals. Thus the

initial state is expanded by a linear combination of these three

states and the final state is described by a linear combination of

2p53d6, 2p53d7L, and 2p53d8L2.

There are five important adjustable parameters which are

also used to perform the cluster model calculation: the

charge-transfer energy (∆), the Coulomb interaction energy (Udd)

between the 3d electrons, the Coulomb attraction energy (Udc)

between the 2p core hole and 3d electron, the hybridization energy

(Veg) and the crystal field splitting (10Dq).

48

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50

51

Chapter 3

Experimental Details

One of the most powerful techniques to investigate the

magnetism of magnetic materials is x-ray magnetic circular

dichroism (XMCD). This technique requires circular polarized

x-rays and variation of photon energy which are only fulfilled by

synchrotron radiation at synchrotron facility. Synchrotron

radiation is very popular for scientific applications because it is

available in a large photon energy range and useful for a wide

range of physical, chemical, and biological experiments. The

experiments of the present work were carried out at synchrotron

facilities. We have performed the XAS and XMCD measurements at

beamlines BL-11A (Dragon beamline) of National Synchrotron

Radiation Research Center (NSRRC), Taiwan, BL-16A2 of Photon

Factory (PF), High Energy Accelerator Research Organization

(KEK) and BL-23SU of SPring-8. This chapter describes details of

the experiments.

3.1 NSRRC BL-11A

A Dragon beamline 11A at National Synchrotron Radiation

Research Center (NSRRC) has been designed for x-ray absorption

spectroscopy (XAS), x-ray photoelectron spectroscopy (XPS),

magnetic circular dichroism (MCD) spectroscopy, and magnetic

52

linear dichroism (MLD) spectroscopy. Figure 3.1 shows the

schematic diagram of beamline 11A. The light source is a bending

magnet. There are two mirrors close to each other, the one closest to

the source is the horizontal focusing mirror (HFM), and the other is

the vertical focusing mirror (VFM). Both are spherical mirrors. The

top view shows the HFM focusing the source inside the storage ring,

and the side view shows the VFM focusing the source into the

entrance slit. The optical function of these two mirrors is decoupled.

The advantage of decoupled VFM and HFM is that one can

visualize the possibility of adjusting these two elements to

compensate the drift in the position of the source inside the storage

ring. After the entrance slit there is grating (6m-SGM) which

diffracts and focuses the photon beam vertically into the exit slit.

The exit slit is movable to achieve high resolving power. The

monochromatic light was reflected by toroidal refocusing mirror

(RFM), and introduced to the end station. Two vertical plane

mirrors (VPM) between the gratings and the exit slit to extend the

lowest photon energy to 10 eV. In this beamline, six spherical

gratings are used to cover an energy range from 10 eV to 1700 eV.

In practical measurements, the photon energy was scanned using a

grating which have 1200 lines/mm and covers the photon energy

range 400 − 1200 eV. Photon flux is 1×1010 with the energy

resolution E / ΔE = 10,000. The measurement chamber is located at

the end station of the beamline as shown in Fig. 3.1. The chamber

for XMCD measurements is equipped with an electromagnet with a

water-cooling system and magnetic field up to H = 1 T can be

achieved. The angle between the beam axis and the magnetic field

is 30o. The sample holder can be rotated freely around the vertical

axis by using differentially pumped rotary feedthrough (DPRF).

Both the total electron yield (TEY) and total fluorescence yield

(TFY) measurement modes can be performed. Figure 3.2 (a) show

the experimental geometry of XMCD measurements. For different

elements the resonant energy is different as shown in Fig. 3.2 (b).

53

Figure 3.1: Measurement system in BL-11A. (a) Schematic layout of

beamline [3.1, 2]. (b) Overview of the measurement system at

BL-11A. (c) Data acquisition system.

54

Figure 3.2: (a) Experimental geometry of XMCD measurements.

(b) L2,3-edge XAS of Fe, Co, Ni, and Cu transition metals measured

at BL 11A, NSRRC [3.3].

55

3.2 KEK-Photon Factory BL-16A

The beamline BL-16A at the Photon Factory (PF) has been

designed for XMCD, photoemission spectroscopy (PES), and

resonance-soft x-ray scattering measurements. Figure 3.3 shows

the schematic diagram of beamline BL-16. The light source is a

double-array variable polarization undulator of APPLE-II type. The

variation of the phase between the two magnet arrays leads to

change of the polarization of the light. It can generate horizontally

and vertically linear, left- and right-handed circularly-polarized

light. This undulator covers photon energies ranging from hν = 300

eV to 1000 eV in the circular polarization mode by the first

harmonic radiation. The photon energy was scanned using a

varied-line-spacing plane grating (VLSPG) grazing-incidence

monochromator (600 lines/mm) [3.5]. This type of monochromator is

very popular because of its high resolution, high throughput, simple

scanning mechanism and fixed exit slit. The photon flux is better

than 1 × 1011 photons/sec with energy resolution E / ∆E = 8,000. The

degree of circular Pc was evaluated to be Pc = ± 95% ± 4% on BL-16A

of the Photon Factory. The XMCD chamber for measurements [3.7]

is equipped with a superconducting magnet with a coil made of a

NbTi wire, and magnetic field up to H = 5.8 T can be achieved. The

superconducting magnet is designed so that it can be baked to

reach a vacuum as good as ~10–10 Torr. He-gas-flow cryostat is used

to cool the sample and the achievable lowest sample temperature is

5 K. Using a differentially pumped rotary feedthrough (DPRF), the

angle between the sample normal and the magnetic field θ can be

varied in the range of 0o ≤ θ ≤ 60o. Figure 3.4 (a) shows the

experimental geometry of the x-ray magnetic circular dichroism

(XMCD) measurements and the XAS and XMCD spectra measured

at BL 16A are shown in Fig. 3.4 (b). The typical XAS and XMCD

spectra obtained for a Fe/Ni/Cu multilayer, indicating magnetism

56

observed in the top thin Fe layer consisting of only 0.9 times of a

monolayer.

Figure 3.3: Measurement system at BL-16A of Photon Factory.

(a) Schematic layout of the beamline [3.4]. (b) Overview of the

XMCD measurement chamber. (c) Controller of BL-16A2 station.

57

Figure 3.4 (a) Experimental geometry of the x-ray magnetic circular

dichroism (XMCD) measurements [3.6]. (b) Fe L2,3-edge XAS and

XMCD spectra for an Fe(0.9 ML)/Ni(6 ML)/Cu(100 ML) film

measured at BL 16, KEK-PF [3.4].

58

3.3 SPring-8 BL23SU

The beamline BL-23SU at SPring-8 is designed for a wide

variety of scientific research areas such as spectroscopic studies on

actinide compounds, semiconductor surfaces, and biological

materials, etc., in the soft x-ray region [3.8 – 10]. The name of

“SPring-8” comes from Super Photon ring for 8 GeV. A schematic

diagram of the beamline is shown in Fig. 3.5. The light source is a

double-array variable undulator of APPLE-II (advanced planar

polarized light emitter) type. The variation of the phase between

the two magnet arrays leads to change of the polarization of the

light. It generates horizontally and vertically linear, left- and

right-handed circular polarized light. This undulator covers photon

energy range of hν = 500 eV to 3000 eV in the circular polarization

mode. Furthermore, it is possible to switch the direction of circular

polarization periodically at each photon energy [3.11]. This enables

us to measure the XMCD spectra much more precisely than the

conventional measurement mode with fixed polarization. The

photon energy was scanned using a varied-line-spacing plane

grating (VLSPG) grazing-incidence monochromator (600 lines/mm).

This consists of an entrance slit, spherical mirrors,

varied-line-spacing plane gratings, an exit slit, a post-focusing

mirror, and refocusing toroidal mirrors. The photon flux is better

than 1 × 1011 with the energy resolution E /∆E = 10,000. The PES

station and the XAS-MCD station are located at the end station

ST3 shown in Fig. 3.5. At each station, preparation chamber for

sample surface cleaning was connected to the measurement

chamber to enable transfer without breaking the ultra-high

vacuum. In PES measurements, photoelectrons were corrected by a

commercial Scienta SES-2002 analyzer.

59

Figure 3.5: Measurement system in BL-23SU. (a) Schematic layout

of beamline [3.9]. (b) Overview of the measurement system at

BL-23SU [3.12]. (c) Experimental geometry of XMCD

measurements [3.6].

60

References

[3.1]http://140.110.203.42/manage/fck_fileimage/file/bldoc/11ADRA

SGM.htm

[3.2] NSRRC Activity Report 2002/2003,

http://140.110.201.35/djhuang/download/research/XAS_1.pdf

[3.3] http://140.110.201.35/djhuang/research_xas.html

[3.4] KEK annual report 2008, Vol. 1, page 24,

http://ccdb5fs.kek.jp/tiff/2008/0822/0822001.pdf

[3.5] K. Amemiya and T. Ohta, J. Synchrotron Rad. 11, 171 (2004).

[3.6] T. Kataoka, Doctor thesis (2010).

[3.7] T. Koide, T. Shidara, and H. Fukutani, Rev. Sci. Instrum. 63,

1462 (1992).

[3.8] A. Yokoya, T. Sekiguchi, Y. Saitoh, T. Okane, T. Nakatani, T.

Shimada, H. Kobayashi, M. Takao, Y. Teraoka, Y. Hayashi, S.

Sasaki, Y. Miyahira, T. Harami, and T. A. Sasaki, J. Synchrotron

Rad. 5, 10 (1998).

[3.9] Y. Saitoh, T. Nakatani, T. Matsushita, A. Agui, A. Yoshigoe, Y.

Teraoka, and A. Yokoya, Nucl. Inst. Meth. A 474, 253 (2001).

[3.10] J. Okamoto, K. Mamiya, S.-I. Fujimori, T. Okane, Y. Saitoh,

Y. Muramatsu, A. Fujimori, S. Ishikawa, and M. Takano, AIP Conf.

Proc. 705, 1110 (2004).

[3.11] A. Agui, A. Yoshigoe, T. Nakatani, T. Matsuhisa, Y. Saitoh, A.

Yokoya, H. Tanaka, Y. Miyahara, T. Shimada, M. Takeuchi, T.

Bizen, S. Sasaki, M. Takao, H. Aoyagi, T. P. Kudo, K. Satoh, S. Wu,

Y. Hiramatsu, and H. Ohkuma, Rev. Sci. Instrum. 72, 3191 (2001).

61

[3.12]http://www.spring8.or.jp/wkg/BL23SU/instrument/lang-en/IN

S-0000000367/instrument_summary_view

62

63

Chapter 4

Origin of enhanced

magnetoelectric coupling in

BaTiO3/NiFe2O4 multilayers

studied by x-ray magnetic

circular dichroism

NiFe2O4 (NFO)/BaTiO3 (BTO) multilayer heterostructures

grown on (001)-SrTiO3 substrates with alternating ferroelectric

BTO and ferrimagnetic NFO layers exhibit magnetoelectric

coupling which increases with the number of layers, namely, with

the number of interfaces. I have studied the local electronic and

magnetic states of Ni and Fe ions in the NFO/BTO multilayers with

various NFO and BTO thicknesses by x-ray absorption

spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD)

at room temperature. The measured Ni 2p and Fe 2p spectra

indicate that Ni ions are octahedrally coordinated by oxygen and

that are divalent (Ni2+) and Fe ions are trivalent (Fe3+) and are

tetrahedrally and octahedrally coordinated by oxygen with opposite

spin direction, consistent with the inverse spinel structure of

NiFe2O4. With increasing number of layers, both Ni and Fe

magnetic moment decrease. With decreasing NFO layer thickness,

64

the average magnetic moment of Ni ions decreases while the

average magnetic moment of Fe ions remain unaffected. I found

good correlation between magnetoelectric coupling per interface

(αE/n) and Fe XMCD which means that the Fe magnetic moment at

the interface governs the ME coupling. Specifically, the charge

transfer between the hybridized Fe-TiO2 bonding at the interface

contributes to the density of the trapped electrons, which could

favour the enhancement of the ME coupling.

4.1 Introduction

Multiferroic materials, a class of materials which exhibit

ferroelectric, ferromagnetic, and/or ferroelastic properties

simultaneously have drawn increasing interest due to their

significant potentials for applications as next-generation

multifunctional devices [4.1-4]. If there is magnetoelectric (ME)

coupling in multiferroic materials, by applying an external

magnetic field, the materials will produce electric polarization,

while an external electric field will induce magnetization. ME

coupling in single-phase compounds is, however, generally weak.

On the other hand, it has been demonstrated that Composite

materials consisting of alternating ferroelectric and ferromagnetic

layers [4.5] or ferromagnetic nano-rods embedded in a ferroelectric

matrix [4.6, 7] provide strong ME coupling.

Recently, thin films consisting of alternating NiFe2O4 (NFO)

and the ferroelectric perovskite-type BaTiO3 (BTO) layers

BTO/(NFO/BTO)n were prepared and exhibited the coexistence of

ferroelectric and ferromagnetic ordering with strong ME coupling

between them [4.8, 9]. It was found that the magnetic moment

decreases and ME coupling increases with increasing number of

layers n for a fix total thickness of the sample, and both

observations were attributed to the interfaces since the relative

contributions of the interfaces increases with n. Here, BTO is a well

known ferroelectric oxide. Structural compatibility of perovskites

65

with various magnetic oxides such as those with the spinel

structures allows one to combine thin layers of ferroelectric and

magnetic materials to design multifunctional heterostructures.

NFO is one of the well known inverse spinel-structure oxides,

represented by Fe3+[Ni2+Fe3+]O4. Half of the Fe3+ ions occupy the

tetrahedral sites while Ni2+ ions and the other half of the Fe3+ ions

occupy the octahedral sites. Similar to the Fe ions between the

tetrahedral and octahedral sites in Fe3O4, the Ni2+ and Fe3+ ions at

the tetrahedral sites are coupled antiferromagnetically through

superexchange interaction via oxygen ions as the

Kanamori-Goodenough (KG) rule predicts [4.10, 11].

In order to understand the microscopic origin of the ME

coupling at the BTO/NFO interfaces in the BTO/NFO multilayer

thin films, it is necessary to investigate the electronic and magnetic

properties of the Fe and Ni ions at the interfaces. For that purpose,

X-ray absorption spectroscopy (XAS) and x-ray magnetic circular

dichroism (XMCD) are ideal techniques because they are element

specific probes of the electronic and magnetic properties of

transition-metal ions [4.12]. I have studied the electronic and

magnetic states of the Fe and Ni ions in samples with various BTO

and NFO layer thicknesses. I have found that interfacial Ni atoms

lose ferromagnetic moment while the Fe magnetic moments remain

unaffected at the interface and the loss of the Ni ferromagnetic

moment is well correlated with the enhancement of ME coupling in

the NFO/BTO multilayers. I discuss possible scenarios which lead

to an interfacial structure having both weakened Ni ferromagnetic

moment and strong ME coupling simultaneously.

4.2 Experimental

In this study, two sets of BTO/(NFO/BTO)n (n=1, 2, 3, 4)

heterostructures were prepared on (001)-oriented 0.7% Nb-doped

SrTiO3 (STO) single crystal substrates by using the pulsed laser

66

deposition technique. The structures of the samples are shown in

Fig. 4.1. The total thicknesses of these films were kept about 120

nm. I prepared two sets of samples. In set 1, the volume ratios of

BTO to NFO in the films are kept 4:1 while in set 2, the thicknesses

of the NFO and BTO layers were the same and therefore the

thickness ratio of BTO to NFO varies between samples.

The XAS and XMCD measurements were carried out at the

Dragon monochrometer beamline 11A of National Synchrotron

Radiation Research Center (NSRRC), Taiwan. XAS and XMCD

spectra were collected in the total-fluorescence yield (TFY) mode by

using a photodiode. The probing depth of the fluorescence yield

detection is about 100 nm. The direction of the applied magnetic

field was reversed to obtain XMCD spectra. XAS spectra for

positive and negative magnetic fields μ± with fixed photon helicity

were taken, and XAS and XMCD spectra were obtained by taking

the sum and the difference between μ+ and μ-, respectively. The

samples were positioned with the plane parallel to the magnetic

field. All the measurements were performed at room temperature

with magnetic fields up to 1 T.

FIG. 4.1: Sample structures of BTO/(NFO/BTO)n heterostructures

of sets 1 and 2.

67

4.3 Results and discussion

In Fig. 4.2, the XAS and XMCD spectra of the Ni 2p core level

are shown and compared with reference data. Each absorption

spectrum of Ni shows two groups, the L3 (2p3/2) and L2 (2p1/2) edges,

separated by the spin-orbit splitting of the 2p core level of ~17 eV.

The double peak structure at the L3 edge and a partially resolved

doublet structure at the L2 edge are the characteristic features of

the high-spin (S=1) Ni2+ ion in the octahedral crystal field [4.13, 14].

In panel (a), the Ni 2p XAS spectrum measured for the

BTO/NFO/BTO sample is compared with the reference data of Ni

metal [4.15], PrNiO3 (Ni3+) [4.16], NiO (Ni2+) [4.17], and a

calculated spectrum of Ni2+ in an octahedral crystal field. The Ni 2p

XAS spectrum of BTO/NFO/BTO is different from those of Ni metal

and PrNiO3 and similar to the spectra of NiO and calculated Ni2+ in

an octahedral crystal field, indicating that the valance of Ni in

BTO/NFO/BTO is 2+ like Ni in NiO. Similarly, as shown in Fig. 2

(b), the Ni 2p XMCD of BTO/NFO/BTO is different from that of Ni

metal and well agrees with the calculated XMCD spectrum of Ni2+

in the octahedral crystal field, indicating that the ionic Ni atoms

with localized 3d electrons contribute to the magnetism in this

sample.

Figure 4.3 shows the Fe 2p XAS and XMCD spectra of the

BTO/(NFO/BTO)1 films. The core–level spin–orbit coupling splits

the XAS into the L3 (2p3/2) and L2 (2p1/2) edges. In Fig. 3(a), the Fe

2p XAS spectrum of the BTO/NFO/BTO is compared with the XAS

spectra of Fe metal [4.18], GaFeO3 [4.19], γ-Fe2O3 [4.20] and

calculated Fe3+ in Oh and Td crystal field. The peak position of Fe3+

is similar to the calculated Fe3+ in Oh and Td peak position. The Fe

valance state in γ-Fe2O3, GaFeO3, and BTO/(NFO/BTO)1 are all

trivalent but the Fe ions are located at sites of different local

symmetries. The line shape of the Fe 2p XAS spectrum of

BTO/NFO/BTO is rather similar to both γ-Fe2O3 and GaFeO3 but is

68

FIG. 4.2: Comparison of the Ni 2p XAS (a) and XMCD (b) spectra of

BTO/NFO/BTO with those of Ni metal [4.15], PrNiO3 (Ni3+) [4.16],

NiO (Ni2+) [4.17], and calculated Ni2+ in octahedral crystal field.

69

FIG. 4.3: Comparison of the Fe 2p XAS (a) and XMCD (b) spectra of

BTO/NFO/BTO with those of Fe metal [4.18], GaFeO3 [4.19],

γ-Fe2O3 [4.20] and calculated Fe3+ in Oh and Td crystal field.

70

quite different from that of Fe metal, indicating that the valence of

the Fe ions in BTO/(NFO/BTO) are in 3+. The difference of the XAS

of BTO/NFO/BTO from those of γ-Fe2O3 and GaFeO3 is the lower

intensities of structures A and C in BTO/NFO/BTO. Because peaks

A and C are due to Fe3+ ions at the octahedral (Oh) sites and peak B

to those at the tetrahedral (Td) site, weaker A and C mean that the

number of Fe3+ at the octahedral positions is smaller. This

corresponds to the facts that the octahedral to tetrahedral ratio

decreases from 5/3 in γ-Fe2O3 to 1 in NiFe2O4, and that all Fe3+ go to

the octahedral sites in GaFeO3. The Fe 2p3/2 XMCD spectra of the

BTO/NFO/BTO samples show three sharp negative, positive, and

negative peaks at hν =708.15, 709.30, and 710.25 eV, denoted by A,

B, and C, respectively, corresponding to the directions of the spins

of the Fe3+ ions at the tetrahedral and octahedral sites which are

coupled antiferromagnetically in NFO. The negative peaks A and C

come from the Fe3+ ions at the octahedral sites and the positive

peak B comes from the Fe3+ ions at the tetrahedral sites. The

XMCD spectrum is also compared with those of γ-Fe2O3, GaFeO3

and Fe metal. The XMCD spectral line shape of the BTO/NFO/BTO

sample is clearly different from that of Fe metal. The positive peak

B due to Fe3+ at the Td sites is absent in GaFeO3 because all the

Fe3+ ions occupy only the octahedral sites in GaFeO3. The peak

positions of A, B, and C coincide with those of γ-Fe2O3 and indicate

that Fe3+ ions are both at the Td and Oh sites. γ-Fe2O3 has the spinel

structure with Fe (Oh) vacancies represented by Fe3+[□1/3Fe3+5/3]O4,

in which the ratio of the octahedral Fe3+ to the tetrahedral Fe3+ is

5/3. NiFe2O4 is represented by Fe3+[Ni2+Fe3+]O4 and the ratio of

octahedral Fe3+ to tetrahedral Fe3+ is 1. Peak C is weaker in NFO

because of the smaller Fe3+(Oh) to Fe3+(Td) ratio than γ-Fe2O3.

Figures 4.4(a) and 4(b) show the Fe 2p and Ni 2p XMCD

spectra of the BTO/NFO/BTO thin film of set 1 measured at various

magnetic fields. Figures 4.4(c) and 4.4(d) show the Fe 2p XMCD

intensity of Oh and Td and comparison of the Ni 2p XMCD intensity

71

FIG. 4.4: Magnetic field dependence of the Ni 2p (a) and Fe (b) 2p

XMCD spectra of the BTO/NFO/BTO thin films of set 1, and

comparison of the SQUID results with the Ni 2p XMCD intensity of

Oh (c), and Fe 2p XMCD Intensity of Td and Oh of BTO/NFO/BTO

(d) of set 1.

with the magnetization measured using a superconducting

quantum interference device (SQUID) magnetometer of the

BTO/NFO/BTO sample of set 1, respectively. Because Ni2+ (Oh) and

Fe3+ (Oh) are antiferromagnetically coupled with Fe3+ (Td), the

difference in the total magnetic moments at the Oh and Td sites

would give rise to ferrimagnetism. In NFO, the magnetic moments

of the Ni2+ ions at the Oh sites are responsible for the macroscopic

net magnetization because the magnetic moments of the Fe3+ ions

at the Td and Oh sites cancel out macroscopically. The Ni 2p XMCD

72

intensities follow the magnetization result. The magnetic field

dependence of XMCD spectra of the BTO/(NFO/BTO)n

heterostructures clearly show ferromagnetism at room

temperature.

Figure 4.5 shows how the XMCD spectra change between the

different samples with different numbers and thicknesses of NFO

and BTO layers. The XMCD spectra of set 1 at the Fe and Ni L2,3

edges of various n’s are shown in Figs. 4.5(a) and 4.5(b). The XMCD

intensity of the Fe and Ni L2, 3 edges decreases with increasing

number of NFO and BTO layers as shown in Figs. 4.5(c) and 4.5(d).

FIG. 4.5: Ni 2p (a) and Fe 2p (b) XMCD spectra of BTO/(NFO/BTO)n

thin films (where n=1, 2, 3) of set 1, and the Ni (c) and Fe (d) 2p

XMCD intensities versus the number of layers of BTO/(NFO/BTO)n

thin films of sets 1 and 2.

73

This observation means that, as we increase the interface-volume

ratio of the NFO layers, the magnetization decreases, which implies

the degradation of the ferromagnetic order at the interfaces. By

changing the layer numbers, not only the interfaces but also the

strain from the BTO layer may influence the magnetization of NFO

because the NFO layers would be in a highly compressively

strained state due to the lattice mismatch between NFO and BTO

layers. Liu et al. [4.9] showed that when the number of layers

increases, the c parameter of NFO increases, which means that the

compressive strain within the ab plane in NFO layers increases. I

also obtained a similar trend of decreasing magnetization in set 2 of

BTO/(NFO/BTO)n multilayer thin films but, set 1 and set 2 behave

differently as functions of the layer (interface) number, which

indeed indicates that the strain itself affects the magnetization in a

complicated way.

In order to see how the Fe and Ni magnetizations are

affected by the number of interface and the NFO thickness, I have

plotted the XMCD intensities of Fe and Ni against NFO thickness

(x-axis) and number of layers (y-axis) in Figs. 4.6 with appropriate

interpolation and extrapolation. In the x-axis (NFO thickness)

direction, the NFO layer thickness increases and the volume ratio

of the interface in the NFO layer decreases. In the y-axis direction,

the number of layers increase which means that number of

interfaces between layers increase. Both Ni and Fe 2p XMCD

intensity decreases with the number of layers because the NFO

layer suffered by compressive strain due to the residual stress

originating from the lattice mismatch and different thermal

expansion coefficients between the NFO and BTO layers. The Fe 2p

XMCD intensity was found to be independent of the NFO thickness

while Ni 2p XMCD intensity depends on the NFO thickness. The Ni

ion is responsible for the net magnetization in the multilayered

films. The gradual reduction in the magnetization of Ni in

BTO/NFO/BTO with decreasing NFO thickness could be

74

understood by considering the negative magnetostriction of NFO

that it contracts when magnetized. The independency of Fe XMCD

intensity on NFO thickness is still unknown. The possible reason

may be that the some of Ni is replaced by Fe with decreasing NFO

thickness and Fe XMCD intensity remains constant (or little

increase) while Ni XMCD intensity decreases. The magnitude of

magnetostriction constant of Fe3O4 (–19×10–6) [4.21] is less than

NiFe2O4 (–45×10–6) [4.22] means that Fe ions less contract in Fe3O4

than Ni ions in NFO when it magnetized. With decreasing NFO

thickness, some of Ni ion in NFO is replaced by Fe and the

magnitude of magnetostriction coefficient reduces, may be the

possible reason of independent of Fe magnetization on NFO

thickness.

As for ME coupling, single-phase films of NFO and BTO

have no obvious ME output but the heterostructure consisting of

NFO and BTO layer clearly yields ME output [4.9]. The interface

therefore should play a crucial role in inducing the ME coupling.

The ME coupling constant increase as the interface density and

number of interface in the heterostructure increases as shown in

Fig. 4.6 (d). Here, the ME voltage coefficient αE is defined by

δV/(δHt), where δV is the induced voltage signal and t is the total

film thickness. From Fig. 4.6 (b) and (d), it is clear that ME

coupling increases with the decrease of Ni ferromagnetism at the

interface. The Ni ferromagnetism is degraded at the interface but

does not degrade the ME coupling. αE/n is a ME coupling constant

per interface and plotted against NFO thickness and number of

layers as shown in Fig. 4.6 (e). I found good correlation between αE/n

and Fe XMCD which means that the Fe magnetic moment at the

interface governs the ME coupling. Recently, microscopic

mechanism of ME coupling at the Fe/BTO interface was studied by

the element specific x-ray magnetic scattering and Ti atoms at the

interface were found to have finite spin polarization [4.23]. The

details about the nature of the ME coupling in the NFO/BTO

75

FIG. 4.6: Fe (Td) (a) Ni (Oh) (b) Fe (Oh) (c) 2p XMCD intensity, ME

coupling coefficient αE (d) and ME coupling coefficient per interface

αE/n (d) plotted against the NFO thickness and the No. of layers.

76

heterostructures are currently unknown, but we may speculate

that strong ME coupling occurs at interfacial defects where Ni

atoms are replaced by Fe atoms and Fe and Ti atoms are located

close to each other at the interface. The Ti and O atoms in the

ferroelectric BTO are sensitive to ferroelectric displacements and

the magnitude of ME coupling constant depends on the interface

termination. The Fe-TiO2 bonding at the interface creates oxygen

vacancies. The oxygen vacancies at the interface may play an

important role in ferroelectric displacement and hence determining

the strength of the ME coupling. In addition, the charge transfer

between the hybridized Fe-TiO2 bonding may possibly contribute to

the density of the trapped electrons, which could favour the

enhancement of the electric polarization by aligning the defect

dipoles and the dynamic exchange interaction of trapped electrons

in the ordered polarons derives the enhancement of the FM. To

improve the ME coupling of the device, to increase the number of

interfaces is important. With increasing n, αE/n decreases rather

than increases. This is probably because the distance between NFO

layers become too short and the surface anisotropy at the interface

increases. Thus, the surface anisotropy contribution increasingly

dominates over the ME contribution and ME coupling decreases.

Finally, with persistently growing demand for strong ME

composites, these materials offer a potential usage as building

blocks for multiferroic materials and can be exploited in

multifunctional devices such as tunable phase shifters, ME sensors,

and resonators. The fast growing research to increase ME coupling

in multiferroic compounds will ultimately tell us much more about

the usefulness of these amazing multiferroic materials. We need

more careful treatment to prepare samples and further

investigations remain in need of investigation for understanding

and designing the ME coupling.

77

4.4 Conclusions

Multilayered heterostructures, BTO/(NFO/BTO)n epitaxially

grown on (001)-STO substrates via pulsed laser deposition

technique were studied by XAS and XMCD. The Ni ions are in the

2+ states at the Oh position while Fe ions are in the 3+ states at Oh

and Td positions. The ferromagnetic moment of Ni2+ is parallel to

Fe3+(Oh) but are antiparallel to Fe3+(Td) following the inverse spinel

structure of NiFe2O4, but the Ni moment is found to decrease at the

interface. With decreasing NFO thickness, some of Ni ion in NFO is

replaced by Fe and the magnitude of magnetostriction coefficient

reduces may be the possible reason of the degradation of Ni

moment. I found clear correlation between the ME coupling

strength and the ferromagnetic moment of Fe. This suggests that

the enhancement of ME coupling occurs at interfacial Fe-TiO2

bonding. The Fe-TiO2 bonding creates oxygen vacancies and the

oxygen vacancies at the interface and the charge transfer between

the hybridized Fe-TiO2 bonding contributes to the density of the

trapped electrons which may play an important role to enhance the

ME coupling.

78

References

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80

81

Chapter 5

Cr-doping induced

ferromagnetism in the

antiferromagnetic spin glass

Cd1-xMnxTe

The prototypical diluted magnetic semiconductor Cd1-xMnxTe

is a spin-glass (x<0.6) or antiferromagentic (x>0.6), but becomes

ferromagnetic upon doping a small amount of Cr ions. In this

chapter, in order to investigate the origin of the ferromagnetism in

Cd1-x-yMnxCryTe, I have studied the element specific magnetic

properties of its thin films by x-ray absorption spectroscopy (XAS)

and x-ray magnetic circular dichroism (XMCD). I have studied thin

films with a fixed Mn content x ~ 0.2 and varied Cr contents in the

range of y = 0 ~ 0.04 grown by molecular beam epitaxy. The

measured Cr 2p and Mn 2p spectra indicate that both Cr and Mn

ions are divalent and that the spins alignment of Cr and Mn are

aligned in the parallel directions. The ferromagnetic moment of Mn

increases with increasing Cr concentration. These results suggest

that in the presence of Cr ions in Cd1-x-yMnxCryTe, the interaction

between Mn spins changes from antiferromagnetic to ferromagnetic

mediated by the Cr ions and possible mechanism of ferromagnetic

ordering between Mn and Cr ions is due to the double exchange

82

interaction.

5.1 Introduction

II-VI semiconductor compounds based diluted magnetic

semiconductors (DMS’s) [5.1-3] have attracted much attention

because of their relevance to spintronic applications by utilizing

both charge and spin of electrons [5.4, 5]. In II-VI DMS such as

ZnTe, ZnSe, CdSe, and CdTe, magnetic ions with valency 2+ can

easily be incorporated at the II-cation sites of the host lattice [5.6-8].

The replacement of a fraction of the lattice of the nonmagnetic

semiconductor compounds by magnetic ions, paticularly Mn, brings

about interesting magnetic and optical properties such as giant

Faraday rotation, spin glass behavior, formation of magnetic

polarons and so on, which arise from sp–d interaction of the

magnetic ions with band electrons, and/or from interaction between

the magnetic ions themselves, which makes DMS’s as a CdMnTe for

fabricating magneto-optical devices. Galazka et al. [5.9] and Nagata

et al. [5.10] have investigated the magnetic properties of Mn-doped

CdTe and Mn-doped HgTe single crystals, respectively, which could

give rise to paramagnetic, spinglass, and antiferromagnetic phase

regions as functions of temperature and Mn content. Cr is also an

interesting 3d transition element to be incorporated in II-VI

compounds. Cr-based magnetic semiconductors were theoretically

investigated by Sato et al. [5.11] and Blinowski et al. [5.12]. The

ground state of the Cr2+ ion has the 3d4 (e2↑t2

↑e0↓t0

↓) configuration,

which opens the possibility of hopping of both spin-down and

spin-up valence band electrons. Sato and Blinowski et al. predicted

that the p-d exchange between the valence band p holes and the d

electrons of the Cr ions become ferromagnetic. Recently, Shen et al.

[5.13] found that Cr doping into Cd1-xMnxTe turns the system from

the antiferromagnetic spin glass to a ferromagnet. They studied the

ferromagnetic behavior of Cd1-x-yMnxCryTe bulk crystals using a

superconducting quantum interference device (SQUID), but the

83

magnetization measured by a SQUID does not provide the element

specific information about the interaction between Mn and Cr,

which is necessary to elucidate the origin of ferromagnetism have

not been clarified yet. To clarify the current issue, x-ray absorption

spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD)

are ideal techniques because they are element specific magnetic

probes of the electronic and magnetic properties of transition-metal

ions. In this chapter, I have studied the electronic structure and

magnetic properties of Mn and Cr ions embedded in the lattice of

Cd1-x-yMnxCryTe thin films by XAS and XMCD experiments.

5.2 Experimental

The samples studied here are thin films of Cd1-x-yMnxCryTe

with Cr contents y varied from 0 to 0.04 while keeping Mn content x

fixed around 0.20. The thin films were fabricated on GaAs (001)

substrates by the molecular beam epitaxy (MBE) technique. First, a

buffer layer of CdTe of thickness ~ 700 nm was grown on a GaAs

(001) substrate and after that a Cd1-x-yMnxCryTe layer of thickness

~ 300 nm was successively grown on the CdTe buffer layer. The

sample surface was capped with a 2 nm thick Al layer to avoid

oxidization when the sample was exposed to air. X-ray diffraction

(XRD) studies confirmed that the thin films of Cd1-x-yMnxCryTe were

grown epitaxially without formation of any secondary phases.

Mn and Cr 2p core-level XAS and XMCD measurements were

done at BL-16 of KEK-Photon Factory (PF), Japan. XAS and XMCD

spectra were collected in the total-fluorescence yield (TFY) mode

using a photodiode. The probing depth of the fluorescence yield

detection is about 100 nm. A fixed magnetic field is used to obtain

the XAS spectra and the direction of the photon helicity was

reversed to obtain XMCD spectra. The samples were positioned

with the plane perpendicular to the magnetic field. All the

measurements were performed at 15 K with magnetic fields up to 5

T.

84

5.3 Results and discussion

Figure 5.1 shows the Mn 2p-3d XAS and the XMCD spectra

for Cd1-x-yMnxCryTe thin films, taken in the TFY mode at T = 15K.

The main two groups of the peaks shown in the XAS spectra are

due to the Mn 2p3/2 (L3 edge) and 2p1/2 (L2 edge) spin-orbit

components and are in good agreement with the Mn XAS of

Ga1-xMnxN reported by Hwang et al. [5.14]. From this finding, I

conclude that the doped Mn ions in the Cd1-x-yMnxCryTe thin films

are in the Mn2+ state. Figure 5.1(b) shows the Mn 2p3d XMCD

spectra of the Cd1-x-yMnxCryTe sample measured at various

magnetic fields at T=15 K. It is shown that the peak intensity of the

XMCD spectra is dependent on the magnetic field.

Figure 5.2 shows the Cr 2p XAS and XMCD spectra of the

Cd1-x-yMnxCryTe thin films taken in the TFY mode at T = 15K. Here,

μ+ and μ- indicate absorption spectra for photon helicity parallel and

antiparallel to the Cr 3d spin, respectively. The major two peaks in

the spectrum structures around hν = 579.4 and 589.1 eV are due to

absorption from the Cr 2p3/2 and Cr 2p1/2 core levels, respectively.

The XAS and XMCD spectra at the Cr 2p edge show multiplet

structures, indicating the localized nature of the Cr 3d electrons in

a crystal field. The Cr 2p absorption overlaps with the tail of the

broad absorption due to Te 3d Te 5p transition. Therefore, one

can assume that the Te 3d core absorption spectrum behaves as a

background in Cr 2p XAS and that Te does not show magnetization

so that XMCD is unaffected by the Te core-level absorption. It is

found that the intensity of μ- is larger than that of μ+ in the 2p3/2

core absorption region, whereas the intensity of μ- is smaller than

that of μ+ in the 2p1/2 region. The difference of μ+ and μ- XAS derives

the XMCD (μ+-μ-) structures as shown in Fig. 5.2(b). Figures 5.1(b)

and 5.2(b) reveal the features that the polarity of Mn 2p XMCD is

the same to that of the Cr 2p XMCD, which indicates the parallel

alignment of the spin moments between the Mn and Cr ions.

85

FIG. 5.1: Mn 2p XAS spectra of Cd0.76Mn0.2Cr0.04Te compared with

GaMnN (Mn2+) [5.14] (a) and XMCD spectra at various magnetic

fields (b) of the Cd1-x-yMnxCryTe thin film.

86

FIG. 5.2: Cr 2p XAS spectra (a) and XMCD spectra at various

magnetic fields (b) of the Cd1-x-yMnxCryTe thin film.

87

The energy integral of Mn and Cr 2p XMCD are shown in Fig.

5.3 (a) and (b). Although the application of XMCD sum rules [5.15,

16] to TFY data does not give accurate results because of the

self-absorption effect, I have attempted to deduce the magnetic

moments of the Mn and Cr ions by applying the XMCD sum rules to

the TFY data. Following the orbital sum rule [5.16], the orbital

moment of the Mn ion is almost quenched and consistent with the

expectation while the orbital moment of the Cr ion has large

negative XMCD integral. We can understand the orbital moment by

the electronic configuration of Mn and Cr. Mn2+ has 5 electrons in

3d states and all t2g levels are filled, results the quenching of Mn

orbital magnetic moments. For Cr2+ (3d4), one level of t2g is vacant,

provides the orbital magnetic moment.

Figure 5.4 (a) shows the XMCD intensity of the Cr and Mn 2p

core-levels of Cd1-x-yMnxCryTe thin film versus magnetic field. The

Cr and Mn XMCD intensities show finite slope and can be

extrapolated to a finite value at zero field. This means that the Cr

and Mn XMCD intensities contain both the paramagnetic (PM) and

ferromagnetic (FM) components. I assume that the PM component

is linear in the magnetic field, and obtain the FM components by

subtracting the PM components from the XMCD spectra as shown

in Fig. 5.4 (b). From the line shapes, I conclude that both the FM

and PM components of Cr and Mn are originated from the Cr2+ and

Mn2+ ions, respectively, and that the ferromagentic moment is

larger for the Cr ions than the Mn ions. This suggests that the Cr

ions drive the ferromagnetic ordering of both Cr and Mn ions in the

present system.

88

FIG. 5.3: Mn (a) Cr (b) 2p XMCD of Cd0.76Mn0.2Cr0.04Te and its

energy integral at 5T. (c) Single-electron energy levels in a Td for

Mn2+ (3d5) and Cr2+ (3d4).

89

FIG. 5.4: Cr and Mn 2p XMCD intensity (a) and FM and PM

component of Cr and Mn (b) versus magnetic field of the

Cd1-x-yMnxCryTe thin film.

90

The magnetic field dependences of the Cr and Mn spins are

decomposed into the paramagnetic (PM) component and the

ferromagnetic (FM) component as shown in Fig. 5.4 (b). The slope of

the PM component of Mn is larger than the slope of the PM

component of Cr, indicating that the PM contribution of Mn in

magnetism is larger than the Cr. From the paramagnetic magnetic

susceptibility formula, χ=(gμB)2S(S+1)/3kBT, I calculated the

theoretical susceptibility (χThe.) for Mn2+ (S=5/2) and Cr2+ (S=2),

where g is the lande factor and kB is the Boltzmann constant. I

found that the χThe. for Mn2+ and Cr2+ are 0.52 and 0.35 μB.T-1/ion,

respectively, and are larger than the experimental susceptibility

(χexp.), which suggests the antiferromagnetically (AFM) coupling of

Mn2+ and Cr2+ ions in the Cr-doped CdMnTe. (χThe./ χexp.)Mn > (χThe./

χexp.)Cr indicates that the antiferromagnetic (AFM) coupling of Mn

ions is larger than the Cr ions.

Figure 5.5(a) shows the Mn 2p XMCD spectra of the

Cd1-x-yMnxCryTe sample with various Cr concentration. The figure

indicates that the Mn2+ XMCD intensity depends on the Cr

concentration. Figure 5.5 (b) shows the magnetic moments of the

Mn ions versus magnetic field for various Cr concentrations. From

Fig. 5.5 (b), it is clear that Mn ions without Cr show paramagnetic

behavior as reported by Galazkaet at el. [5.9] and Nagata et al.

[5.10]. After incorporating a small concentration of Cr ions in

Mn-doped CdTe, the dramatically change in the Mn magnetic

moment are observed. The magnetic moment of Mn increases with

the increase of Cr ions in the thin films. A similar trend has also

been observed by Shen et al. [5.13] and Ishikawa et al. [5.17] and

they measured magnetization by using a SQUID. Ishikawa et al.

[5.17] observed a negative Curie-Weiss temperature ΘP in

Cd1-xMnxTe and the ΘP turns to be positive after incorporating only

0.46 % of Cr. Therefore, small content of the Cr present in thin

films changes ΘP from negative to positive value indicating that the

magnetic interaction between Mn spins becomes dominating by

91

FIG. 5.5: Mn 2p XMCD spectra (a) and magnetic moment of Mn (b)

with various Cr content of Cd1-x-yMnxCryTe thin film.

92

ferromagnetic interaction, in contrast to the predominantly

antiferromagnetic Mn-Mn interaction in Cd1-xMnxTe. In the

presence of Cr ions, there should be some mechanism by which the

interaction between Mn becomes ferromagnetic mediated by Cr.

According to Shen et al. [5.13], a small fraction of Cr + ions are

present and the hole bound to the Cr+ acceptor can mediate FM

ordering between the magnetic ions contained within the Bohr orbit

of the hole and this orbit would include both the Cr and the nearby

Mn ions (bound magnetic polaron mechanism). However, the

energy level of the Cr+ charge state is located 1.34 eV above Ev

[5.18], which is too deeply located within the band gap. Another

possible mechanism of the ferromagnetic ordering DMSs doped

with two kind of transition metal atoms is proposed by Akai et al.

[5.19]. They proposed a model for the case of co-doped transition

metals in ΙΙ–ΙV compound semiconductors that the

antiferromagnetic interaction between different kinds of transition

metals results in ferrimagnetic ordering, due to a mechanism

similar to the double-exchange interaction. However, it is not

certain whether this mechanism would be really effective in the

present case where the contents of Mn and Cr are much different

because in Akai et al. [5.19], equal amount of transition metals are

doped. Furthermore, magnetic interaction between Mn and Cr in

the present case is ferromagnetic and not antiferromagnetic which

could lead to ferrimagnetism in Akai et al's model.

Figure 5.6 shows a possible mechanism and a schematic

picture of magnetic interaction between Mn-Mn in Mn-doped CdTe

and Mn-Cr in Cr-doped CdMnTe. In Mn-doped CdTe, the Mn spins

are antiferromagnetically coupled as shown in Fig. 5.6 (a) but after

small amount of Cr doping the interaction between Mn ions near a

Cr ions are changed and aligned parallel with a net magnetic

moment. I calculated the number of ions aligned parallel in samples.

The expected FM component of Cr2+ (3d4) and Mn2+ (3d5) are 4μB/ion

and 5μB/ion, respectively. I found the FM component of Cr and Mn

93

~0.4μB/ion and 0.1μB/ion at 5 T, respectively (see Fig. 5.4 b). It

means only 10% of Cr and 2% of Mn ions are aligned parallel and

the concentration of ferromagnetic Cr and Mn ions in

Cd0.76Mn0.2Cr0.04Te sample aligned parallel is 0.004 ions. The equal

number for Mn and Cr indicates that possible mechanism of

ferromagnetic ordering is due to the double exchange interaction

between Mn and Cr ions as shown in Fig. 5.6 (b). This situation is

similar to doubly and equally doped DMS in Akai et al. [5.19] where

an electron may be exchanged between two species of transition

metal. The double exchange process is mediated by the conduction

band of the host crystal via a resonance state that mixes same spin

d orbitals of the Cr and Mn ions and extended conduction band

states. The wave functions of the state of Cr2+ and Mn2+ ions

coupled with the same spin direction because the d orbitals of the

Cr2+ and Mn2+ ions are less than half filled and half filled,

respectively, and provide the ferromagnetic interaction between Cr

and Mn ions.

FIG. 5.6: Schematic picture of magnetic interaction between (a)

Mn-Mn in Mn-doped CdTe and (b) Mn-Cr in Cr-doped CdMnTe.

94

5.4 Conclusion

I have performed an XAS and XMCD study on

Cd1-x-yMnxCryTe thin films with a fixed Mn content x ~ 0.2 and

varied Cr contents in the range of y = 0 ~ 0.04. From the XAS and

XMCD measurements, the valence of Cr and Mn ions were found to

be in the 2+ state and the spin alignment between Cr and Mn ions

are parallel. From the sum rule analysis, the orbital moment of the

Mn ion is almost quenched while the Cr ion has a large value. The

magnetic moment of Mn depends on Cr concentration in the film

and it increases with increasing the Cr concentration. In the

presence of Cr ions, the interaction between Mn ions becomes

ferromagnetic which is mediated by Cr ions. I found the equal

concentration of ferromagnetic Cr and Mn ions in

Cd0.76Mn0.2Cr0.04Te sample. The equal number for Mn and Cr

indicates that possible mechanism of ferromagnetic ordering is due

to the double exchange interaction between Mn and Cr ions similar

to doubly and equally doped DMS in Akai et al. [5.19].

95

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98

99

Chapter 6

Orbital magnetic moment in

multiferroic MnWO4 studied

by x-ray magnetic circular

dichroism

In this chapter, I have investigated the existence of orbital

magnetic moment in the multiferroic MnWO4 using the Mn

L2;3-edge x-ray magnetic circular dichroism (XMCD) in the

paramagnetic state. I confirmed that the Mn ions in MnWO4 are in

the 2+ states and that the Mn site has lowered symmetry. By

applying the orbital sum rule to the XMCD spectrum, despite the d5

configuration of Mn ion in MnWO4, a finite large orbital magnetic

moment was deduced: Morb/Mspin ~ 0.057. The large value of

Morb/Mspin is due to lower symmetry of Mn sites in the distorted

[MnO6]10- octahedron of MnWO4.

6.1 Introduction

Recently there have been intensive studies on multiferroic

materials due to their intriguing magnetoelectric properties and

potential applications for storage devices, sensors, traducers,

100

actuators, etc [6.1−4]. Very few multiferroic material exist in nature

because in most ferroelectrics such as BaTiO3 ferroelectricity is

driven by the hybridization of empty d orbitals with the occupied p

orbitals of the octahedrally coordinated oxygen ions while

ferromagnetism arises from the partially filled d shell of transition

elements [6.5]. Also ferroelectricity can be induced if the

spin-lattice interaction is large enough to displace the nuclei

cooperatively in particular direction giving rise to a macroscopic

electric polarization. Recently, a new type of multiferroics such as

rare-earth perovskite RMnO3 (R = Y, Tb, Gd, Dy) [6.6−8], in which

magnetic ions are directly involved in a spontaneous electrical

polarization and show a strong interplay between electric

polarization and magnetic order, have been investigated in detail.

Manganese tungstate (MnWO4) is a special compound among the

multiferroic manganates which does not contain a rare-earth ion. It

has already been proved that ferroelectricity can appear in the

cycloidal-spiral spin phase without centrosymmetry [6.9, 10].

Multiferroicity in which electrical polarization is caused by the

spiral order of magnetic moments is explained by the spin-current

model [6.11, 12]. If the spin-rotation axis ê is not parallel to the

magnetic propagation vector Q, nonvanishing spontaneous

polarization P α ê ×Q develop. In these spiral magnets, correlation

between the ferroelectric polarization and cycloidal-spiral spin

structure is suggested to be associated with the antisymmetric part

of exchange coupling, known as Dzyaloshinskii-Moriya interaction

(DMI) [6.12−14].

The multiferroic compounds TbMnO3 [6.15], Ni3V2O8 [6.16],

and MnWO4 [6.17] are prominent examples for inversion symmetry

breaking magnetic ordering. In these multiferroic oxides,

non-collinear spiral spin structure without centro-symmetry may

induce macroscopic spontaneous electric polarization. Among the

spin-spiral multiferroics, MnWO4 is one of the prototypical

multiferroic oxide that exhibits ferroelectricity which is induced by

101

its spiral spin structure, and has been studied by several groups

[6.17, 18]. Taniguchi et al. [6.17] and Arkenbout et al. [6.18] have

reported that the noncollinear spin configuration of the sample is

caused by the electric polarization. Moreover, the electric

polarization induced by the magnetic field flops from the b to the a

axis of the crystal structure. MnWO4 is crystallized in a wolframite

structure (Fig. 1.2), which belongs to the monoclinic space group

P2/c and the MnO6 and WO6 octahedra in the crystal structure

share their edges to form zigzag chains along the c direction. This

compound exhibits three different magnetic phase transitions at T1

= 7.6 K, T2 = 12.7 K and TN = 13.5 K [6.19] to three long wavelength

antiferromagnetic (AF) ordering states. According to neutron

diffraction results, AF1 (T < T1), AF2 (T1 < T < T2), and AF3 (T2 < T

< TN) are a commensurate collinear antiferromagnetic phase, an

incommensurate cycloidal spiral-spin phase, and an

incommensurate collinear antiferromagnetic phase, respectively.

The magnetic structure of the AF1 state is commensurate with the

propagation vector (−1/4, 1/2, 1/2), while that of the AF3 and AF2

states are incommensurate with the propagation vector (−0.214, 1/2,

0.457), while other wolframite FeWO4, CoWO4, and NiWO4 show

only one magnetic transition to the commensurate magnetic state

with a propagation vector (1/2, 0, 0).

The valence state of Mn ions in MnWO4 is 2+ and the orbital

magnetic moment of the Mn2+ ion with the half filled 3d5

configuration is supposed to be quenched. Recently K. V.

Shanavas et al. [6.20] reported a finite orbital magnetic moment

and the origin of ferroelectricity using ab initio electronic-structure

calculations in MnWO4. In this chapter, I report on Mn L2,3 x-ray

absorption spectra (XAS) and x-ray magnetic circular dichroism

(XMCD) experiments of MnWO4 polycrystalline sample in order to

study the electronic structure and magnetic properties of Mn in

MnWO4.

102

6.2 Experimental

Polycrystalline MnWO4, which is expected to have higher

conductivity than the single crystals and good for the measurement

of XAS and XMCD in the TEY mode. Polycrystalline MnWO4

pellets sintered under anisotropic pressures in pure Ar atmosphere.

The sample was then arc melted to create an oxygen vacancies and

to make the sample less resistive.

The XAS and XMCD measurements were carried out at the

undulator beam line BL23-SU of SPring-8, Japan. XAS and XMCD

spectra were collected in the TEY mode. The probing depth of the

total electron yield detection was about 5 nm. In order to avoid

charging effect, the measurements were performed at room

temperature in an applied magnetic field of 8 Tesla. XAS and

XMCD spectra were obtained by taking the sum and the difference

between the spectra taken with different helicities μ+ and μ−,

respectively. The monochromator resolution was E /ΔE > 10000.

The base pressure of the measuremet chamber was about 10−9 Torr.

The sample was cleaved in-situ in the vacuum to obtain a clean

surface.

6.3 Results and Discussion

In Fig. 1, the XAS and XMCD spectra of the Mn 2p core level

are shown and compared with data of reference compounds. Each

absorption spectrum of Mn shows two groups, the L3 (2p3/2) and L2

(2p1/2) edges separated by the spin-orbit splitting of the core level of

~ 11 eV. In Fig. 1, the Mn 2p XAS spectrum measured for the

MnWO4 sample is compared with the Mn 2p XAS data of SrMnO3

(Mn4+, Oh) [6.22], LaMnO3 (Mn3+, Oh) [6.23], Ga1-xMnxN (Mn2+, Td)

[6.24], MnO (Mn2+, Oh) [6.23], and MnWO4 (Mn2+, Oh) [6.25]. The

Mn 2p XAS spectrum of MnWO4 is distinctly different from those of

SrMnO3, and LaMnO3. However, the line shape of the Mn 2p XAS

103

FIG. 1. Mn 2p XAS spectra of MnWO4 compared with those of

SrMnO3 (Mn4+, Oh) [6.22], LaMnO3 (Mn3+, Oh) [6.23], Ga1-xMnxN

(Mn2+, Td) [6.24], MnO (Mn2+, Oh) [6.23], and MnWO4 (Mn2+, Oh)

[6.25].

104

spectrum of MnWO4 is somewhat similar to Ga1-xMnxN. The

difference of XAS of MnWO4 from Ga1-xMnxN is the peak at

structure A. Peak A in MnWO4 is due to the Mn2+ ions at octahedral

site. On the other hand, the spectral features of the Mn 2p XAS of

MnWO4 and MnO are similar to each other with the L3 main peaks

at identical energies. This clearly indicates that the Mn ions in

MnWO4 are also in the high-spin state of the d5 electronic

configuration as the Mn ions in MnO, and the valence of Mn ions in

MnWO4 are in 2+ state at the octahedral position. The clear

differences between the spectra of MnWO4 and MnO are the

shoulder height at low and high energies of the L3 edge. The

differences of peak height suggest that the crystal field in the

MnWO4 system has a lower symmetry than octahedral.

Figure 2 shows the Mn L2,3-edge XAS and XMCD spectra of

the MnWO4 polycrystal. We evaluated the orbital magnetic moment,

morb (Mn), and the spin magnetic moment, mspin (Mn), of the Mn2+

ion using the XMCD sum rules [6.26, 27],

(6.2)

where μ+ and μ− stand for the absorption coefficients for the photon

helicity, parallel and antiparallel to the Mn 3d majority spin

direction, respectively, Nd is the number of electrons in the 3d band,

and MT is the magnetic dipole moment. The orbital magnetic

moment (Morb) and the spin magnetic moment (Mspin) both are in

units of μB/atom, where μB is the Bohr magneton. The spin and

orbital magnetic moments of the Mn ion are thus found to be

0.07±0.01 and 0.004±0.001 μB/ion. Thus the ratio between the

orbital and spin magnetic moments becomes Morb/Mspin = 0.057. The

105

orbital magnetic moment is found to be large and in the same

direction of the spin magnetic moment, indicating that Mn 3d

states are more than half-filled [6.28].

FIG. 2. Mn L2,3-edge XAS and XMCD of MnWO4. (a) XAS

spectrum (sum of μ+ and μ− XAS spectra) and its energy integral (b)

XMCD spectrum and its energy integral. Here, μ+ and μ− stand for

the absorption coefficients with photon helicity parallel (↑↑) and

antiparallel (↑↓) to the Mn 3d majority-spin direction.

106

The large orbital magnetic moment obtained from the

experiment is due to the distortion of [MnO6]10- octahedra. I have

carried out configuration interaction (CI) calculations for MnO6

cluster with the Mn2+ ion in a D3d symmetry crystal field in the

magnetic field of 8 T at a temperature of 300 K. The D3d crystal

field is nothing but the distortion of the [MnO6]10- octahedron.

Under the trigonal distortion, the local symmetry group around the

Mn site changes from the cubic Oh to D3d. The calculated XAS and

XMCD spectra are shown in Fig. 3, where they are compared with

the experimental XAS and XMCD spectra of the Mn2+ ion. The line

shape of the experimental Mn L2,3 XAS is reproduced very well by

the calculated spectrum in the D3d crystal field with Udd = 4.0 eV,

Udc = 4.4, ∆ = 7.0 eV, 10Dq = 0.4 eV, and Veg = 2.0 eV. Here, ∆ is the

charge-transfer energy, Udd is the on-site Coulomb interaction

energy between two 3d electrons, Udc is the Coulomb energy

between the 2p core hole and 3d electron, 10Dq is the octahedral

crystal field splitting, and Veg is the p-d hybridization energy for

the eg electrons of the Mn2+ ion. The charge-transfer energy

between the O 2p and Mn 3d orb i tals i s def ined as

∆=E(dn+1L1)−E(dn), where E(dn) is the multiplet averaged energy of

the Mn 3dn configuration and E(dn+1L1) denotes the same for a

configuration obtained by transferring one electron from one of the

O 2p orbitals to a Mn 3d level having n electrons. I have also

calculated the spin and orbital magnetic moment from the

calculated XAS and XMCD spectra using the sum rules (Equ. 6.1

and 6.2) and found that Mspin=0.07 and Morb=0.002 μB/ion,

respectively, and the ratio of Morb/Mspin=0.029. The experimental

value of Morb/Mspin is large than the calculated Morb/Mspin. One of the

possible reason for the large experimental value of Morb/Mspin may

be the distorted [MnO6]10- octahedron forms lower symmetry than

the D3d. Second possible reason for the large experimental value of

Morb/Mspin is the hybridization between Mn 3d and W 5d orbitals

through the oxygen atoms. The spin and magnetic moments are in

107

the same direction. From the calculation, I found the average 3d

occupancy, n3d, to be 5.09 with 89.10 % d5 character, 10.58% d6L1

character, and 0.32% d7L2 character. These values are in good

agreement with the values obtained by Shanavas et al. [6.20], the

average occupancy of 3d state is 5.14, with 86.48% d5 character,

FIG. 2. Comparison of the experimental and calculated XAS and

XMCD spectra at the Mn L2,3-edge of Mn2+. Experimental and

calculated XAS (a) and XMCD (b) spectra.

108

12.99% d6L1 character, and 0.52% d7L2 character. The significant

population of Mn 3d level more than half-filled in MnWO4 might

provide a way to understand the presence appreciable orbital

moment in MnWO4.

Shanavas et al. [6.20] calculated orbital magnetic moment for

the MnWO4 system in the magnetically ordered state and found

0.001 μB with the same direction as the spin magnetic moment by

using ab initio electronic-structure calculations. This orbital

moment is several times smaller than the observed one. Similar to

the present case of the MnWO4 system, Kim et al. [6.29] also found

large orbital moment for the FeO6 octahedra in the GaFeO3 system.

In GaFeO3, the Fe ion shifts along a certain direction from the

center of the FeO6 octahedron, the parity symmetry of the orbitals

becomes broken for axes orthogonal to the direction and creates

orbital moment according to Kim et al. [6.29]. In the MnWO4

system too, the Mn sites are surrounded by oxygen octahedra and

each oxygen octahedron is significantly distorted and the distorted

octahedra give rise to inequivalent bond lengths. Due to the

distortion, the three different bond lengths of Mn-O in [MnO6]10- are

2.285 Å, 2.160 Å, and 2.104 Å, respectively. The distorted

arrangement of oxygens leads to the distortion of Mn positions and

forms a noncentrosymmetric structure for each individual

octahedron, which is considered to be responsible for the apparently

orbital magnetic moment and electric polarization. Alternatively,

the broken inversion symmetry induce hybridization between Mn

3d and Mn 4p orbitals, which may distort the L2,3 edge XAS and

XMCD in a subtle way, and the orbital sum rule may become

inaccurate.

6.4 Conclusion

In order to investigate the orbital magnetic configuration of

MnWO4, I carried out XAS and XMCD measurements at the Mn

L2,3 edges. The spectral features of Mn 2p XAS indicate that Mn

109

ions are in the high-spin electronic configuration in the 2+ states.

From the sum rule analysis, the spin and orbital magnetic

moments of the Mn ion are found to be 0.07±0.01 and 0.004±0.001

μB/ion, respectively. The orbital magnetic moment is in the same

direction as the spin magnetic moment, indicating that Mn 3d

states are more than half-filled. Using the CI cluster-model

analysis, I found that the average 3d occupancy, n3d, is equal to 5.09.

The distorted (MnO6)10- octrahedra play an important role in giving

rise to the apparently large orbital magnetic and electric

polarization in MnWO4. Further investigation of large orbital

moment in MnWO4 system is needed due to the complex nature of

the coupling between structural and electronic degrees of freedom.

110

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113

114

115

Chapter 7

Summary and Outlook

In the preceding chapters, I have presented the x-ray

absorption spectroscopy (XAS) and x-ray magnetic circular

dichroism (XMCD) studies on the NiFe2O4 (NFO)/BaTiO3 (BTO)

multilayer thin films, Cr-doped CdMnTe thin films, and MnWO4

crystal, in order to obtain the understanding of these materials

from the electronic structure points of view. XMCD is a powerful

technique to study transition metal ions of the magnetic structure

of complex systems and the spin-orbit splitting of L2,3 absorption

edges enables the determination of the spin and orbital magnetic

moments by using sum rules. XMCD technique also enables to

study the electronic structure and the orientation of spin direction.

In Chapter 4, I have performed the Ni and Fe 2p XMCD

measurements on the multiferroic BTO/(NFO/BTO)n multilayer

thin films grown on (001)-SrTiO3 substrates using pulsed laser

deposition with various NFO and BTO thicknesses to understand

the microscopic origin of the magnetoelectric (ME) coupling at the

BTO/NFO interfaces. The XAS and XMCD of the Ni and Fe ion of

the NFO/BTO heterostructures were recorded in the bulk-sensitive

total-fluorescence yield (TFY) mode at room temperature. I found

that the Ni ions are in the 2+ states at the octahedral (Oh) position

116

while Fe ions are in the 3+ states at both Oh and Tetrahedral (Td)

positions. The ferromagnetic moment of Ni2+ is parallel to Fe3+(Oh)

but are antiparallel to Fe3+(Td) following the inverse spinel

structure of NiFe2O4. With decreasing NFO layer thickness, the

average magnetic moment of the Ni ions decreases while the

average magnetic moment of the Fe ions remains unaffected. The

strong ME coupling occurs at interfacial defects where Ni atoms are

replaced by Fe atoms and Fe and Ti atoms are located close to each

other at the interface. The Ti and O atoms in the ferroelectric

BaTiO3 are sensitive to ferroelectric displacements and the

magnitude of ME coupling constant depends on the interface

termination. The Fe-TiO2 bonding at the interface creates oxygen

vacancies. The oxygen vacancies at the interface may play an

important role in ferroelectric displacement and hence determining

the strength of the ME coupling. To improve the ME coupling of the

device, to increase the number of interfaces is important. With

increasing n, αE/n decreases rather than increases. This is probably

because the distance between NFO layers become too short.

meaning that Ni ions at the interface lose ferromagnetic ordering. I

found clear correlation between the ME coupling strength and

decreases of the ferromagnetic moment of Ni. This suggests that

the enhancement of ME coupling occurs at interfacial Ni atoms

whose ferromagnetic behavior is weakened and hybridization with

the BTO electronic states are enhanced.

In Chapter 5, I have studied the XAS and the XMCD of

Cd1-x-yMnxCryTe thin films with Cr content y varied from 0 to 0.04

while keeping the Mn content x fixed around 0.20 grown on GaAs

(001) substrates by the molecular beam epitaxy (MBE) technique to

clarify the origin of the ferromagnetism in the thin films. From the

XAS and XMCD measurements, the valence of Cr and Mn ions were

found to be in the 2+ state and the spin alignment between Cr and

Mn ions are parallel. From the XMCD spectra, I obtained the

ferromagnetic (FM) and paramagnetic (PM) component and found

117

that both the FM and PM components of Cr and Mn are originated

from the Cr2+ and Mn2+ ions, respectively, and that the

ferromagentic moment is larger for the Cr ions than the Mn ions.

From the sum rule analysis, the orbital moment of the Mn ion is

almost quenched while the Cr ion has large value. The magnetic

moment of Mn depends on Cr concentration in the film and it

increases with increasing the Cr concentration. In the presence of

Cr ions, the interaction between Mn ions becomes ferromagnetic

which is mediated by Cr ions.

In Chapter 6, I investigated the possible existence of orbital

magnetic moment in multiferroic MnWO4 using the Mn L2;3-edge

XMCD. The XAS and XMCD of the Mn L2;3-edge was measured in

total-electron yield (TEY) mode at room temperature. From the

spectral features of Mn 2p XAS, I conclude that Mn ions are in the

high-spin electronic configuration in the 2+ states. By applying the

sum rule to the XMCD spectrum, despite the d5 configuration of

Mn2+ ion, a significantly large orbital magnetic moments was

deduced and the orbital magnetic moment is in the same direction

as the spin magnetic moment, indicating that Mn 3d states are

more than half-filled. The distorted (MnO6)10- octrahedra play an

important role in giving rise to the apparently large orbital

magnetic and electric polarization in MnWO4.

Finally, prospects on possible future spintronics research are

mentioned. In this thesis, I studied the magnetoelectric coupling of

NFO/BTO heterostructures. Since BTO is a strong ferroelectric

material, we may consider heterostructure of other ferromagnetic

ferrites such as MnFe2O4, CoFe2O4, and MnCr2O4, etc., with the

ferroelectric BTO to understand the microscopic origin of the ME

coupling at the interfaces. I also studied the Cr and Mn interaction

in Cr and Mn co-doped CdTe. DMS materials doped with two kinds

of magnetic elements have rarely been studied and little has been

known about the interaction between different kinds of magnetic

elements. The present work will highlight new development in

118

doubly doped DMSs in the spintronics. In this thesis, I have also

demonstrated the orbital magnetic moment studies on MnWO4 and

hope that this work will capture some of exciting new developments

in the field of spintronics at a rapid pace, throwing further light

onto the details of these materials. Overall, we have demonstrated

that soft x-ray spectroscopic studies on spintronics materials

provide rich information about the electronic and magnetic

structures. In particular, the present XAS and XMCD studies show

that the interplay between the two transition-metal ions (Ni and Fe,

Mn and Cr, Mn and W) is very rich and is important to create new

spintronics functionalities with significant spin dependent

electronic and magnetic properties. I hope that the present work

will promote soft x-ray spectroscopy on multiferroic and dilute

magnetic semiconductors, leads to further understandings of

physics of spintronics material and could help researcher in the

next generation of smaller, more affordable and more power

efficient devices from spintronics materials.

119

120

121

Appendix

Orbital magnetic moment in

FeCr2S4 studied by x-ray

magnetic circular dichroism

A.1 Introduction

Transition metal-based spinel compounds (AB2X4) have been

a hot topic of experimental and theoretical studies for researchers

[A.1-3]. Within the last decade, a number of new physical

phenomena were discovered in these compounds such as colossal

magneto-resistance [A.4], complex spin order and spin dimerization

[A.5, 6], spin-orbital liquids [A.7], and orbital glasses [A.8], colossal

magneto-capacitive coupling [A.9], and multiferroic behavior [A.10].

These novel phenomena in transition metal-based spinel

compounds present an increased interest for spintronic and

advanced multifunctional device applications. Among them,

FeCr2S4 is a ferromagnetic semiconductor with TC ~ 170 K, and

shows large negative magnetoresistance near the Curie

temperature [A.4]. Due to the strong coupling among spin, charge,

orbital and lattice degrees of freedom, FeCr2S4 displays abnormal

low-field magnetic behavior [A.11] and fascinating physical effects,

e.g., colossal magnetoresistance [A.4] and gigantic Kerr rotation

[A.12]. Park et al. [A.13] reported a band-structure calculation for

FeCr2S4 in which each sublattice of the Fe and Cr transition metals

122

orders ferromagnetically, while the two sublattices are coupled

antiferromagnetically to each other. Recently, the electronic

structure of FeCr2S4 has been studied by density functional

calculation [A.14]. Sarkar et al. [A.14] found small orbital moment

of Cr ion while Fe ion shows large orbital moment. In previous

years, there have been several theoretical and experimental studies

on this compound except for the orbital magnetic moment in

FeCr2S4 by element-specific XMCD technique. To study the orbital

magnetic moment, it is necessary to elucidate the electronic and

magnetic structure of FeCr2S4 by microscopic element-specific

measurement technique that provides the direct information on

valence states, spin and orbital magnetic moments of each

transition-metal element. Here, I report on Fe and Cr L2,3 x-ray

absorption spectroscopy (XAS) and x-ray magnetic circular

dichroism (XMCD) measurements of FeCr2S4 single crystal in order

to study the electronic structure and magnetic properties of Fe and

Cr ions and I find that the Fe ions show a large orbital magnetic

moment at 80K.

A.2 Experimental

The sample studied here is a single crystal of FeCr2S4. The

crystal was grown by a chemical vapor transport method with CrCl3

as a transport agent. The octahedral-shaped crystals were

characterized by powder x-ray diffraction and inductively coupled

plasma spectrometry, which confirms that the obtained crystals are

single phase with expected chemical formula. The sample was

cleaved in-situ to obtain a clean surface. Fe and Cr 2p core-level

XAS and XMCD measurements were done at BL-16 of KEK-Photon

Factory (PF), Japan. XAS and XMCD spectra were collected in the

total-electron yield (TEY) mode (probing depth ～5 nm) at T = 80K

by measuring the sample current and normalizing it with the

mirror current. A fixed magnetic field is used and the direction of

the photon helicity was reversed to obtain XMCD spectra. The base

123

pressure of the measurement chamber was about 10−9 Torr and the

energy resolution was E / ∆E = 8,000.

A.3 Results and Discussion

Figure A.1 (a) shows comparison of the Fe 2p XAS spectrum of

FeCr2S4 with those of FeO (Fe2+) [A.15], Fe metal [A.16], and

γ-Fe2O3 (Fe3+) [A.17]. It is clearly seen that the the Fe 2p XAS

spectra of FeCr2S4 are different from that of γ-Fe2O3 and somewhat

similar to that of FeO. However, in contrast to FeO, no (or very

weak) multiplet structures are observed in FeCr2S4 in both the L3

and L2 peaks, resulting in the line shape which is very similar to

that of Fe metal. The absence of multiplet structures in FeCr2S4

indicates that the Fe 3d electrons are strongly hybridized with the

valence electrons of the S p electrons. The Fe 2p XAS spectrum of

Fe metal is more broadened than the Fe 2p XAS spectra of FeO and

FeCr2S4. Figure A.1 (b) shows comparison of the Fe 2p XMCD

spectra of FeCr2S4 and Fe metal [A.16]. The Fe 2p XMCD spectrum

of FeCr2S4 shows some multiplet features indicated by arrows in

Fig. A.1 (b) while the multiplet features are absent in Fe metal.

From Figs. A.1 (a) and (b), one can conclude that the Fe ions are

mainly in divalent (2+) states in FeCr2S4. The valence state of Fe2+

in FeCr2S4 crystal is also confirmed by the Mossbauer spectroscopy

measurements performed by Chen et al. [A.18].

Figure A.2 shows comparison of the Cr 2p XAS spectrum of

FeCr2S4 with those of Cr2O3 (Cr3+) [A.19] and Cr metal [A.20]. The

Cr 2p XAS spectra of FeCr2S4 are qualitatively similar to that of

Cr2O3, but very different from that of Cr metal, indicating that the

Cr ions in FeCr2S4 are mainly trivalent (3+). On the other hand, the

multiplet structures in FeCr2S4 are not so sharp compared to the

Cr3+ XAS of Cr2O3. These differences in multiplet structures reflect

that the character of the Cr–S bonding is not a simple ionic bonding,

but rather close to a covalent bonding as compared to the more ionic

Cr–O bonding in Cr2O3. This is probably due to the weaker

124

electronegativity of S ions than the O ions.

Figure A.1: (a) Comparison of the Fe 2p XAS spectra of FeCr2S4

with those of FeO (Fe2+) [A.15], Fe-metal [A.16], and γ-Fe2O3 (Fe3+)

[A.17]. (b) XMCD spectrum of FeCr2S4 compared with Fe metal

[A.16].

125

Figure A.2: Comparison of the Cr 2p XAS spectra of FeCr2S4 with

those of Cr2O3 (Cr3+) [A.19] and Cr metal [A.20].

The Fe 2p XAS spectra of FeCr2S4 for incident radiation with

both positive and negative helicities (μ+ and μ-) taken in the TEY

mode are shown in Fig. A.3 (a). The major two peaks in the spectral

structures around hν = 707.9 and 720.6 eV are due to absorption

from the Fe 2p3/2 and 2p1/2 core levels, respectively. The intensity of

μ+ is larger than that of μ- in the 2p3/2 (L3-edge) core absorption

region, whereas the intensity of μ+ is smaller than that of μ_ in the

2p1/2 (L2-edge) region. The difference between the μ+ and μ- XAS

derives the XMCD (μ+-μ-) structures as shown in Fig. A.3 (b). The Fe

2p3d XMCD spectra of the FeCr2S4 crystal measured at various

magnetic fields are shown in Fig. A.3 (b). The inset of the Fig. A.3

(b) shows the enlarge Fe 2p XMCD of FeCr2S4 at 0 T. The XMCD

126

intensity down to H = 0 T clearly indicates the ferromagnetism by

Fe ions of the sample.

Figure A.4 (a) shows Cr 2p XAS spectra of FeCr2S4 for

incident radiation with photon helicity parallel (μ+) and antiparallel

(μ-) to the Cr 3d spin, respectively, taken in the TEY mode. The

major two peaks in the spectrum structures around hν = 579.6 and

587.9 eV are due to absorption from the Cr 2p3/2 and Cr 2p1/2 core

levels, respectively. It is found that the XAS spectrum with positive

helicity (μ+) is more broadened than that of negative helicity (μ-). In

addition, the spectral weight of μ+ is shifted to higher energy

compared to μ- both in the 2p3/2 and 2p1/2 regions. This derives the

complicated XMCD (μ+-μ-) structures. The Cr 2p XMCD spectra of

the FeCr2S4 measured at various magnetic fields are shown in Fig.

A.4 (b). The inset of Fig. A.4 (b) shows an enlarged Cr 2p XMCD of

FeCr2S4 at 0 T. The XMCD intensity of Cr 2p down to H = 0 T

clearly indicates ferromagnetism by Cr ions in the sample. From

Figs. A.3 and A.4, one can see the polarity of Fe 2p XMCD is

opposite to that of Cr 2p XMCD, which indicates the antiparallel

alignment of the spin moments between the Fe and Cr ions, which

is consistent with the band-structure calculation for FeCr2S4

reported by Park et al. [A.13].

Figures A.5 and A.6 show the Fe and Cr 2p XAS and XMCD

spectra with its energy integral. The black dotted lines in Figs. A.5

(a) and A.6 (a) show arctangent background to subtract [A.21]. A

determination of the spin magnetic moment using XMCD sum rules

for lighter transition metal elements such as Cr is questionable

because the 2p3/2 and 2p1/2 edges are not well separated and overlap

each other. Here, I have evaluated the orbital magnetic moments of

Fe and Cr ions using the XMCD sum rules [A.22, 23]

127

Figure A.3: Fe 2p XAS spectra at 1T (a) and XMCD spectra at

various magnetic field (b) of FeCr2S4. Inset shows an enlarged

spectrum of Fe 2p XMCD at 0 T in the inset of Fig. A.3 (b).

128

Figure A.4: Cr 2p XAS spectra at 1T (a) and XMCD spectra at

various magnetic field (b) of FeCr2S4. Inset shows an enlarged

spectrum of Cr 2p XMCD at 0 T in the inset of A.4 (b).

129

where Nd is the number of electrons in the 3d band. The orbital

magnetic moment (Morb) is given in units of μB/atom, where μB is

Bohr magneton. From the sum rule analysis, the orbital magnetic

moments of Fe and Cr ions are found to be 0.225 and 0.016 μB/ion,

respectively. In the Fe ions, the spin and orbital moments have the

same sign, whereas opposite sign is found for the Cr ions. This is

because the d states of Fe2+ are more than half filled while those

Cr3+ is less than half filled. The orbital moment of the Cr ions is

found to be small due to the d3 configuration of Cr. However, the

large orbital moment of the Fe ions is remarkable since the

spin-orbit interaction within e levels of Fe should be small. A

possible reason is that the crystal-field splitting is small in FeCr2S4.

According to first-principles calculation by Sarkar et al. [A.14], the

orbital magnetic moments of Fe and Cr ions are 0.077 and 0.024 for

GGA+SO calculation, and 0.134 and 0.026 for GGA+U+SO

calculation, respectively. Here, GGA, U, and SO stands generalized

gradient approximation, Hubbard, and spin-orbit interaction,

respectively. Our experimental orbital moment of the Fe ion is a

little larger while the orbital moment of the Cr ion is a little smaller

than the theoretical value given by Sarkar et al. [A.14].

Han et al. [A.24] performed XAS and XMCD measurements of

Fe0.5Cu0.5Cr2S4 and found that the spin and orbital magnetic

moment of Cr3+ (d3 system) are aligned in the same direction (Fig.

A.7). According to Hund's third rule, the spin and orbital moment

should be antiparallel for systems with less than a half-filled shell,

and parallel for systems with more than a half-filled shell. I found

that the spin and orbital magnetic moments are aligned in the

opposite direction in Cr3+ (d3 system), in disagreement with to the

result in Han et al. and in agreement with Hund’s third rule. Park

et al. and Sarkar et al. also found that the opposite alignment of the

spin and orbital moments of Cr ions from band structure

calculations for FeCr2S4.

130

Figure A.5: Fe 2p (μ++μ-) XAS spectrum of FeCr2S4 and its energy

integral (a) and XMCD (μ+-μ-) spectrum and its energy integral (b)

at a magnetic field of 1T. Black dotted line in Fig (a) shows the

arctangent background subtraction.

131

Figure A.6: Cr 2p (μ++μ-) XAS spectrum of FeCr2S4 and its energy

integral (a) and XMCD (μ+-μ-) spectrum and its energy integral (b)

at a magnetic field of 1T. Black dotted line in Fig (a) shows the

arctangent background subtraction.

132

Figure A.7: Cr 2p XAS and XMCD spectra of Fe0.5Cu0.5Cr2S4 [A.24].

A.4 Conclusion

FeCr2S4 crystal grown by a chemical vapor transport method

with CrCl3 as a transport agent was studied by an XAS and XMCD.

The valence states of Cr and Fe ions are nearly trivalent (Cr3+) and

divalent (Fe2+), respectively. On the other hand, the Fe 2p XAS

spectra of FeCr2S4 do not exhibit the multiplet structures,

indicating the strong hybridization between the Fe 3d and S p

electrons. From the magnetic field dependence XMCD

measurement of Fe and Cr ions, I can conclude that the Fe and Cr

ions ordered ferromagnetically in FeCr2S4. From the sum rule

analysis, the two sublattices in FeCr2S4 are coupled

antiferromagnetically to each other and agree with the Hund’s

third rule and the orbital magnetic moments of Fe and Cr ions were

0.225 and 0.016 μB/ion, respectively.

133

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137

Acknowledgments

It is my great pleasure to express my deep regards and special

gratitude to the following people for their help concerning my

doctor thesis.

First, my special cordial gratitude goes to Prof. Atsushi Fujimori,

for his advice, encouragement, as well as for always making himself

available for discussion and guidance whenever it was needed

throughout the whole project. My Ph. D. thesis would not have been

possible without his excellent supervision. I have appreciated his

clear advice, which comes from his deep knowledge and insight in

the field of condensed matter physics.

I would also like to express my deeply thanks to Dr. Teppei

Yoshida. His valuable advice were indispensable to this work. I

have learned a lot from his energetic action and thoughtfulness. I

would like to say my special thanks to Dr. Takashi Kataoka and Dr.

Vijay Raj Singh who have introduced me the enchanted world of

DMS and multiferroic materials and their enthusiasm and effort

took me step into the field of synchrotron radiation spectroscopy in

the very beginning. I also thank Dr. Toshiharu Kadono, Mr. Keisuke

Ishigami, Mr. Yo Yamazaki, Mr. Goro Shibata, and Mr. Takayuki

Harano for their great help during beamtime and the fruitful

discussion we had together. I have spent great time with all of them

during our study.

The experiments at NSRRC, Photon Factory, and SPring-8 were

supported by a number of people. I am particularly grateful for

members of NSRRC: Mr. Fan-Hsiu Chang, Dr. Hong-Ji Lin, Prof.

Di-Jing Huang and Prof. C. T. Chen for their helpful technical

138

support during the beamtimes at NSRRC. The experiments at

Photon Factory BL-16A2 were supported by Prof. Tsuneharu Koide,

who has given me a lot of educative advice about experimental

technique and XMCD measurements. I would like to thank for their

valuable technical support during the beamtime. I am deeply

thankful to SPring8 members: Dr. Yukiharu Takeda, Dr. Tetsuo

Okane, Dr. Yuji Saito, and Prof. Hiroshi Yamagami for their kind

support during the beamtimes at SPring8.

I have to express my great thanks to Prof. Yuanhua Lin, Prof.

Ce-Wen Nan, Dr. Yi Zhang, and Dr. Jing Liu for providing us with

the high-quality samples of the BTO/(NFO/BTO)n multilayer thin

films and warm encouragement. I am very thankful to Prof. Shinji

Kuroda and Mr. Kôichirô Ishikawa for providing us with the

interesting samples Cr-doped CdMnTe thin films and valuable

discussions. I would also like to thank Prof. Sugata Ray and Mr.

Somnath Jana for providing excellent MnWO4 crystal and their

kind support. I would like to thank Prof. Kenya Ohgushi and Prof.

Yoshinori Tokura for the high-quality FeCr2S4 crystal and valuable

discussions.

I express my special thanks to Prof. Arata Tanaka for his

theoretical support about CI cluster-model analysis of the x-ray

absorption spectra, teaching me his codes of cluster model

calculation, and enlightening discussions.

My warmest thanks go to all the former and current members of

Fujimori group for useful advice and supporting my daily research

life: Dr. Shin-ichiro Ideta, Dr. Kohei Yoshimatsu, Mr. Leo Cristobal

C. Ambolode II, Mr. Shin-ichi Aizaki, Mr. Ichiro Nishi, Mr. Wataru

Uemura, Mr. Hakuto Suzuki, Mr. Fumio Takahashi, and Mr.

Masafumi Horio. I have spent great time with all of them during

our study and in many other additional activities. Also I thank

them all for being ready to help me all the time. I would like to

thank Ms. Emiko Murayama, Ms. Yuko Shimazaki, Ms. Ami Ito,

and Ms. Miki Ueda for dealing with a lot of business stuff and

139

giving me encouragement.

I express my thanks to Ms. Emiko Gosho and other staffs of

International Liaison Office (ILO) for dealing with matters such as

admission & enrollment registration for International Research

Students, an arrival orientation, arrangements of scholarships,

housing, visas and organizing various intercultural events.

My stay in Tokyo makes a lot of friends who shared with me

everything and a lot of discussions. List is very long, in no

particular order Dr. Kirpa Ram, Dr. Md. Rizwan, Dr. Md. Waseem

Akhtar, Dr. Zainul aabdin Khan, Dr. Gautam Singh, Mr. Saurabh

Sharma, Mr. Raghavendra Jain, Mr. Akki Reddy, Mrs. Alka Gupta

and Mr. Deepu. I would also like to thank the family members of

University of Tokyo-Indian Students Association (UTISA) who

made my days at University of Tokyo really enjoyable and cheerful.

My great thanks are also to the Japanese Embassy in India and

the Japanese Ministry of Education, Culture, Sports, Science and

Technology (MEXT) for choosing me to come to Japan and receive

the Monbukagakusho scholarship and continue my PhD study at

The University of Tokyo. I would also like to thank Department of

Physics's special program (Yusen Haichi Waku in Japanese) for

inviting me for Ph. D. course.

I would like to thank the whole University of Tokyo family which

made my stay in University and Tokyo a memorable one.

Finally, I express my best regards and thanks to my father Sri.

Ram Anuj Verma, mother Smt. Gyanmati Verma, brother Mr. Jai

Prakash Verma, sister Sarita Verma, and other family members

Smt. Poonam, Neha, Anisha, Utkarsh and Divyansu for their love,

understanding, support and encouragement.

University of Tokyo Virendra Kumar Verma

July 2012