X-ray Magnetic Circular Dichroism Study of the Diluted...

X-ray Magnetic CircularDichroism Study of the Diluted

Magnetic SemiconductorZn1−xCrxTe

THESIS

Yo Yamazaki

Department of Physics, University of Tokyo

January, 2011

3

Abstract

Diluted magnetic semiconductors (DMS’s), which are semiconductorsdoped with magnetic ions dilutely, have been studied intensively since thediscovery of ferromagnetism in the III-V DMS Ga1−xMnxAs because the fer-romagnetic interaction between the Mn ions mediated by hole carriers enablesus to manipulate both the charge and spin degrees of freedom of electrons.Ever since the theoretical prediction that wide-gap DMS’s should show fer-romagnetism above room temperature, DMS’s based on wide-gap semicon-ductors such as GaN, ZnTe, TiO2 and ZnO have been investigated and, infact, there have been many reports on room temperature ferromagnetismin wide-gap DMS’s based on these materials. One of the most importantissue of DMS’s is the origin of ferromagnetism. In order to obtain a funda-mental understanding of the ferromagnetism, investigation of the electronicstructure of the doped magnetic ions and host semiconductor is necessary.

Soft x-ray spectroscopy enables us to extract the electronic structure inan element-specific way. In particular, x-ray magnetic circular dichroism(XMCD), which is defined as the difference between the core-level x-ray ab-sorption spectroscopy (XAS) spectra taken with right- and left-handed cir-cularly polarized x-rays, is very efficient to extract important informationabout the magnetic properties of the doped magnetic ions because XMCD issensitive only to magnetically active species of a specific element.

In the present thesis, we have performed x-ray absorption spectroscopy(XAS) and x-ray magnetic circular dichroism (XMCD) studies of the dilutedferromagnetic semiconductor Zn1−xCrxTe doped with iodine (I) or nitrogen(N), corresponding to electron or hole doping, respectively. From the shapeof the Cr 2p absorption peak in the XAS spectra, it was concluded thatCr ions in the undoped, I-doped and lightly N-doped samples are divalent(Cr2+), while Cr2+ and trivalent (Cr3+) coexist in the heavily N-doped sam-ple. This result indicates that the doped nitrogen atoms act as acceptorsbut that doped holes are located on the Cr ions. In the magnetic-field de-pendence of the XMCD signal at the Cr 2p absorption edge, ferromagneticbehaviors were observed in the undoped, I-doped, and lightly N-doped sam-ples, while ferromagnetism was considerably suppressed in heavily N-dopedsample, which is consistent with the results of magnetization measurements.

In Chapter 1, we introduce spintronics, diluted magnetic semiconductorand the physical properties of Zn1−xCrxTe. In Chapter 2, we describe the

4

principle of x-ray magnetic circular dichroism (XMCD). In Chapter 3, wedescribe experimental setup of XMCD measurements and samples which weremeasured in this thesis. In Chapter 4, we present the influences of cappingon the measurements of Zn1−xCrxTe. In Chapter 5, we present the effectsof iodine and nitrogen doping on the diluted ferromagnetic semiconductorZn1−xCrxTe by soft x-ray magnetic circular dichroism. In Chapter 6, wesummarize the results.

Contents

1 Introduction 11.1 Spintronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Magnetoresistance . . . . . . . . . . . . . . . . . . . . 11.1.2 Application of spintronics . . . . . . . . . . . . . . . . 2

1.2 Diluted magnetic semiconductors . . . . . . . . . . . . . . . . 31.3 Models for ferromagnetic DMSs . . . . . . . . . . . . . . . . . 41.4 Physical properties of Zn1−xCrxTe . . . . . . . . . . . . . . . . 9

2 Principles of X-ray magnetic circular dichroism 152.1 Principles of x-ray magnetic circular dichroism and sum rules . 15

2.1.1 X-ray absorption spectroscopy . . . . . . . . . . . . . . 152.1.2 X-ray magnetic circular dichroism . . . . . . . . . . . . 162.1.3 Total-electron yield (TEY) and total-fluorescence yield

(TFY) modes . . . . . . . . . . . . . . . . . . . . . . . 162.1.4 XMCD sum rules . . . . . . . . . . . . . . . . . . . . . 16

3 Experiment 193.1 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1.1 NSRRC BL11A . . . . . . . . . . . . . . . . . . . . . . 193.2 samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.2.1 ZnTe-capped samples . . . . . . . . . . . . . . . . . . . 213.2.2 Al-capped samples . . . . . . . . . . . . . . . . . . . . 21

4 Influences of capping layers on Zn1−xCrxTe 234.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . 24

4.3.1 ZnTe capped sample . . . . . . . . . . . . . . . . . . . 24

i

ii Contents

4.3.2 Al capped sample . . . . . . . . . . . . . . . . . . . . . 264.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5 Effect of co-doping of donor and acceptor impurities ... 295.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . 305.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . 305.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6 Summary 37

References 41

Chapter 1

Introduction

1.1 Spintronics

Spintronics is defined as technologies which utilize both the spin and chargedegrees of freedom of an electron. Spintronics emerged from the discover-ies in the 1980s of spin-dependent electron transport phenomena in solid-state physics. The use of semiconductors for spintronics can be traced tothe theoretical proposal of a spin field-effect-transistor. Electrons are spin-1/2 fermions, and therefore, constitute a two-state system with up-spin anddown-spin. In order to make a device which is based on spintronics, thereare several conditions. First, it is required that a system that can generate acurrent of spin-polarized electrons comprising of one spin species more thanthe other. Second, it is required that a system is sensitive to the spin polar-ization of electrons. The former one can be utilized as a spin injector, andthe latter one can be utilized as a spin detector.

1.1.1 Magnetoresistance

One of the most important phenomenon utilized in spintronics is magnetore-sistance. Magnetoresistance is the property of a material that the value ofits electrical resistance changes when an external magnetic field is applied toit.

1

2 Chapter 1. Introduction

Giant magnetoresistance (GMR)

GMR is magnetoresistance effect observed in thin film structures composedof alternating ferromagnetic and non-magnetic metallic layers. The 2007Nobel Prize in physics was awarded to two physicists who discovered GMR.

The effect is observed as a significant change in the electrical resistancedepending on whether the magnetization of adjacent ferromagnetic layersare parallel or anti-parallel. The overall resistance is relatively low for theparallel alignment and relatively high for anti-parallel alignment. GMR effectis broadly used for hard disk drives in the computers.

Tunnel magnetoresistance (TMR)

TMR is a magnetoresistive effect that occurs in magnetic tunnel junctions.TMR is observed in a system which consists of two ferromagnets separatedby a thin insulating layer. The magnetization of one of the ferromagnetsis fixed, while, the other is not fixed so that one can change the directionof its magnetization by applying magnetic field. The former is called fixedlayer and the latter is called free layer. If the insulating layer is thin enough,typically a few nanometers, electrons can tunnel from one ferromagnet tothe other. If the magnetizations of ferromagnets are in parallel orientations,it is more likely that electrons will tunnel through the insulating film thanif they are in the opposite orientations. Consequently, such a junction canbe switched between two states of electrical resistance, one with low and theother with high resistance.

Anisotropy magnetic resistance (AMR)

AMR is the property of a material in which the electrical resistance dependson the angle between the direction of electric current and the orientation ofmagnetic field. AMR effect is used in sensors for measurement of Earth’smagnetic field (electronic compass), for electric current measurement, trafficdetection, and linear position and angle sensing.

1.1.2 Application of spintronics

Here, as examples of applications of spintronics, magnetic random accessmemory (MRAM) and spin field effect transistor are explained.

1.2. Diluted magnetic semiconductors 3

Magnetic random access memory (MRAM)

MRAM is a non-volatile computer memory that has been under developmentsince the 1990s. Unlike conventional random access memory (RAM), data inMRAM is not stored as electric charges or current flows, but as magnetiza-tion. Each element is formed from two ferromagnetic plates, each of whichcan hold a magnetic field, separated by a thin insulating layer. One of thetwo plates is a permanent magnet set to a particular polarity, the other canbe changed to match the external field to store information. The memorydevice is built from a grid of such cells.

Spin field effect transistor (Spin FET)

Spin FET is a magnetically-sensitive transistor. The spin FET consists oftwo ferromagnetic parts, source and drain, channel which is placed betweenthe source and drain, and gate-electrode by which one can apply electricfield in the channel. The magnetizations of source and drain are fixed inthe same direction. Without electric field, the electrons flow from sourceto drain because magnetizations of both source and drain are aligned. Onthe other hand, when one applies electric field, the direction of the spin ofelectrons inside the channel are rotated, and as a result, the element showshigh resistance.

1.2 Diluted magnetic semiconductors

Diluted magnetic semiconductors (DMS’s) are semiconductors doped withmagnetic ions (3d transition metals or 4f lanthanides) dilutely and are ex-pected to have both the properties of magnetic materials and semiconductors.In particular, in ferromagnetic DMS’s, it is considered that the ferromagneticinteraction between the local magnetic moments of doped ions is mediatedby the spins of charge carriers of the host semiconductors. This type offerromagnetism is called carrier-induced ferromagnetism. Because this prop-erties enable us to manipulate both the spin and charge degrees of freedomof electrons, ferromagnetic DMS’s are key materials for “spin electronics”or “spintronics” [1, 2], which is intended to manipulate both the spin andcharge degrees of freedom of electrons in semiconductor devices.

DMSs having TC above room temperature are strongly desired for prac-tical applications of spintronic devices. In 2000, Dietl et al. [3] predicted


Figure 1.1: Computed values of the Curie temperature for various p-typesemiconductors containing 5% of Mn and 3.5 × 1020 holes per cm3 [3].

that DMS’s based on semiconductors with wide band gaps such as GaN andZnO should show ferromagnetism having TC exceeding room temperature ifthey contain 5% of Mn and 3.5 × 1020 hole per cm3 [Fig. 1.1]. After thetheoretical prediction, indeed, ferromagnetism having TC above room tem-perature was found in wide-gap DMS’s. That is, in 2001, Reed et al. [4]found that Ga1−xMnxN thin films show room-temperature ferromagnetismand anomalous Hall effects. The discovery of room-temperature ferromag-netism in Ga1−xMnxN triggered the investigation of wide-gap DMSs.

1.3 Models for ferromagnetic DMSs

In DMSs, the 3d levels of the doped transition-metal ions substituting forcation sites are split by a crystal field due to ligand anions, and hybridizewith the ligand bands, resulting in bonding, antibonding, and non-bondingstates. Electronic structure near the Fermi level (EF) is important to un-derstand carrier-induced ferromagnetism. Here, sp− d exchange interaction,double exchange interaction, and percolation of bound magnetic polarons areexplained as examples of carrier-induced ferromagnetism.

1.3. Models for ferromagnetic DMSs 5

sp-d exchange − The original idea has been provided by Zener [5]. If the3d states are localized and the EF is located near the top of the valence band,interaction between the 3d orbitals and the ligand p band shifts the up-spin pband opposite to the down-spin one, leading to a half-metallic band structure,as shown in Fig. 1.2(a). Therefore, the hole carriers are highly polarized andmediate the ferromagnetic interaction between the local magnetic moments.This interaction is called p-d exchange and is characterized p-d exchangeconstant Nβ, where N is the number of cations per unit volume. In the casewhere EF is located near the bottom of the conduction band, interactionbetween 3d orbitals and the conduction s band split the s bands as shownin Fig. 1.2(b). The interaction is called s-d exchange interaction and ischaracterized s-d exchange constant Nα. The exchange constants are causedby direct exchange interaction between the sp band and the 3d electrons andhybridization between the sp bands and 3d orbitals [6]. The s-d exchangeinteraction Nα is smaller than the p-d exchange interaction Nβ, and is almostindependent of host material with tetrahedral symmetry. It is likely that thes,p-d exchange interaction is long ranged because the carriers are itinerant.One can estimate the p-d exchange constant Nβ, which is given by [7]

Nβ =16

S

(1

δeff

− 1

δeff + 4j

) (1

3(pdσ) − 2

√3

9(pdπ)

)21

3

+16

S

(− 1

δeff + 6j− 0.64

−δeff + u′ − j

) (1

3(pdσ) − 2

√3

9(pdπ)

)22

3,

where u′ and j are Kanamori parameters, δeff = ∆eff + WV /2, ∆eff is theeffective charge-transfer energy, and WV is the width of the host valenceband.

Double exchange − The basic concept has been also proposed by Zener formanganese perovskite oxides [8]. In La1−xCaxMnO3, Mn3+ ions are replacedby Mn4+ ions by substituting Ca2+ ions for La3+. Let us consider the transferof an electron between Mn3+ (d4) and Mn4+ (d3) ions through an interveningO2− (p6) ion, and denote the wave functions representing the configurationof the system before (ψi) and after the electron transfer (ψf ) as follows:

ψi : Mn3+ O2− Mn4+,ψf : Mn4+ O2− Mn3+.

(1.1)

The electron transfer from one Mn3+ ion to the adjacent Mn4+ ion is consid-ered as the transfer of an electron from the left Mn3+ to the central O2− ion


simultaneously with the transfer of an electron from the central O2− ion tothe right Mn4+ ion. This transfer is called double exchange. Because, whenthe magnetic moments of the transition-metal ions are aligned antiparallel,the spin of the transferred electron is opposite to that of the t2g orbitalsand hence the electron loses energy due to Hund’s coupling, the double ex-change interaction leads to a ferromagnetic alignment of the local magneticmoments. The double exchange interaction gains energy through a d-electrontransfer and occurs when EF crosses d band, as shown in Fig. 1.2(c). It islikely that double exchange interaction is short ranged because carriers haved-band character.

Percolation of bound magnetic polarons − For DMS with low carrier con-centration, the carriers tend to localize around the magnetic ions and ex-change interaction of carriers with the magnetic ions leads to the formationof bound magnetic polarons. Percolation of magnetic polarons is consideredto lead to ferromagnetism in DMS’s. Figure 1.3 shows schematic pictures forpercolation of bound magnetic polarons [9]. Kaminski and Sarma [10] havedeveloped an analytic polaron percolation theory for DMS’s in the limit oflow carrier density and have obtained reasonable agreement with experimen-tal results for TC and M . Coey et al. [11] have proposed a donor impurityband exchange model for ZnO-based DMS’s. In oxides, oxygen vacanciesact as double donors. The donor electrons tend to be localized around thevacancies and are confined in hydrogenic orbitals centered on the vacancies.The overlap of the hydrogenic orbitals leads to the formation of an impurityband, as shown in Fig. 1.4(a). When minority- or majority-spin d states liein the impurity band, interaction of the magnetic ions with the hydrogenicelectrons in the impurity band leads to the formation of bound magnetic po-larons. The overlap of bound magnetic polarons splits the spin states of theimpurity band, as shown in Figs. 1.4(b) and (c), resulting in high TC. Themodel can explain the magnetic moment of ZnO-based DMS’s thin filmsdoped with 3d element from Sc to Cu. Figure 1.4(c) shows the magneticmoment of Zn0.95M0.05O (M = Sc − Cu) thin films measured at room tem-perature. There are two peaks centered at Ti and Co. It is possible that forTi- and Co-doped ZnO, the majority- and minority-3d states overlaps withthe impurity band, respectively. It is likely that for the Cr-, Mn-, and Cu-doped ZnO, there may be no overlap between the 3d states and the impurityband.

Practically, the magnetic properties of DMS’s can be understood as dueto competition between ferromagnetic interaction such as double exchange

1.3. Models for ferromagnetic DMSs 7

and antiferromagnetic interaction such as superexchange.


EF

E

(a) p-d exchange

d level

p band

EF

E

(b) s-d exchange

d level

s band

EF

E

(c) double exchange

d levelp band

Figure 1.2: Schematic diagram for carrier-induced ferromagnetism in DMS[5, 8]. (a) p-d exchange interaction. (b) s-d exchange interaction. (c) doubleexchange interaction. Dashed lines denote non-hybridized or ionic electronicstates. The upper density of state (DOS) indicates the up-spin component,and the lower DOS indicates the down-spin component.

(a) (b) (c)

Figure 1.3: Schematic pictures for bound magnetic polarons [9]. (a) Randomorientations of local magnetic moments of the doped magnetic ions. (b)Formation of a bound magnetic polaron due to carrier localization around amagnetic ion. (c) Percolation of bound magnetic polarons.

(a) (b) (c)

(d)

Figure 1.4: Donor impurity band exchange model [11]. (a)-(c) Schematicband structure for non-hybridization between 3d bands and impurity band(low TC), minority-, and majority-spin 3d bands hybridized with the impurityband (high TC), respectively. (d) The magnetic moments of Zn0.95M0.05O(M = Sc − Cu) thin films measured at room temperature.

1.4. Physical properties of Zn1−xCrxTe 9

The discovery of ferromagnetism at relatively high temperatures (up to110 K) in dilutely Mn-doped GaAs was amazing phenomenon. Many theo-retical and experimental investigations have been done to reveal the originof its ferromagnetism, and it was concluded that the origin of ferromag-netism in Mn doped GaAs is so called carrier-induced ferromagnetism. Incarrier-induced ferromagnetism, short-ranged magnetic exchange interactionis mediated by carriers which is running through the materials as itinerantstates. To understand the origin of the ferromagnetism in DMSs will enableus to design new materials which have higher Curie temperature (TC).

Recently, in addition to the carrier-induced ferromagnetism, relationshipbetween ferromagnetism and spinodal decomposition has attracted much at-tention. The spinodal decomposition may explain unsolved phenomena whichhave been observed in DMSs. Till now, so many synthetic experiments ofDMS have been done, however, the experimental results related to ferromag-netism vary from system to system. For example, in the Mn-doped GaN case,experimental reports about ferromagnetism vary from paramagnetic to TC

up to 900 K, whereas, ab initio calculation [12] suggests that TC of GaMnNis at most 50 K. In general, DMS systems exhibit a solubility gap and showphase separation in thermal equilibrium. As is well known in alloy physics, inthe quenching process of such systems spinodal decomposition occurs, thatis, local density fluctuations are amplified and finally lead to spatial patternsof high and low concentration regions of magnetic ions. The reason for theinconsistencies between theory that assumes homogeneous distribution andexperiment seems to be due to the spatially inhomogeneous distribution ofmagnetic ions due to the spinodal decomposition. In addition, consideringthat the degree of spinodal decomposition under the nonequilibrium crystalgrowth conditions, the differences of growth process of spinodal decomposi-tion explain the variability of synthetic experimental reports.

1.4 Physical properties of Zn1−xCrxTe

The II-VI semiconductor ZnTe crystallizes in the zinc-blend structure asshown in Fig. 1.5(a), has a band gap of ∼ 2.4 eV, and shows p-type electricalconductivity. Cr-doped ZnTe compound, in which the Cr concentration isbelow 1%, has been investigated before the discovery of ferromagnetism inheavily Cr-doped ZnTe thin films. Infrared absorption [13], which is shownin Fig. 1.6, and electron spin resonance [14] studies of bulk Zn1−xCrxTe have


Zn2+ Te2- Cr2+

(a) (b)

Figure 1.5: Crystal structure of Zn1−xCrxTe. (a) Zinc-blend structure. (b)[CrTe4]

−6 cluster.

suggested that the Cr ions are divalent and are subject to tetragonal Jahn-Teller distortion. MCD measurements in visible-to-ultraviolet region on bulkZn1−xCrxTe compounds have revealed a positive p-d exchange constant Nβ,that is, the exchange interaction between the hole spin and the local magneticmoment is ferromagnetic [15]. Saito et al. [16] have succeeded to prepareZnTe thin films doped with high concentration of Cr atoms (x ∼ 20%) by themolecular beam epitaxy (MBE) method. The Zn1−xCrxTe thin films showedferromagnetism at room temperature and their MCD signals observed at theabsorption edge of ZnTe showed magnetic-field (H) and temperature (T )dependences which follow these of magnetization (M) as shown in Fig. 1.8,indicating that there is strong interaction between the spins of the host s,p-band electrons and the magnetic moments of the doped Cr ions [16, 17].Therefore, Zn1−xCrxTe has attracted much attention as an intrinsic DMSwith strong s,p-d interaction.

So far, x-ray magnetic circular dichroism studies in total-electron yieldmode have been done on Zn1−xCrxTe to investigate the electronic structure ofCr ions [18, 19]. From the results of XMCD measurements, it was found thatmost Cr ions are in the 2+ state and that ferromagnetism in Zn1−xCrxTethin films are intrinsic from magnetic field dependence of magnetizations ofCr ions.


Figure 1.6: Optical absorption coefficient versus photon wave number forCr-doped ZnTe.

Recently, effects of doping on the ferromagnetism of Zn1−xCrxTe havebeen intensively investigated [20, 21, 22]. Iodine (I), which is expected tobe an electron dopant, enhances the ferromagnetism while nitrogen (N),which is expected to be a hole dopant, suppresses it [20, 21], as shown inFig. 1.9. One of the candidates of the origin of ferromagnetism in Zn1−xCrxTeis carrier-induced ferromagnetism. However, carrier-induced ferromagnetismin Zn1−xCrxTe is doubted because the Zn1−xCrxTe films are highly insulat-ing. Furthermore, the tendency that N doping increases hole carrier concen-tration and suppresses the ferromagnetism is opposite to the observation forGa1−xMnxAs [23], in which the ferromagnetic property is enhanced by the in-crease of hole concentration. Recently, spatially inhomogeneous distributionof the Cr ions, that is, spinodal decomposition has been pointed out to influ-ence the magnetic properties [24, 25, 26]. Spatially resolved energy-dispersivex-ray spectroscopy study has recently revealed that co-doping with I inducesthe formation of spatially inhomogeneous Cr-rich (Zn,Cr)Te nano-regions,whereas co-doping with N results in homogeneous Cr-ion distributions [22][Figs. 1.9(b) and (c)].


-20

-10

0

XM

CD

(ar

b. u

nits

)

590585580575570Photon Energy (eV)

100

50

0

XA

S (a

rb. u

nits

)

600

500

400

300

200

100

0

XA

S Integral

-20

0

XM

CD

Integral XMCD integ.

XAS integ. r

p q

XAS

XMCD

(a)

Zn1-xCrxTe

(b)

t2

e

Cr2+

(d4) Td Cr

2+ (d

4) D2d Cr

+ (d

5)

b2

e

b1

a1

Figure 1.7: Cr 2p XAS and XMCD spectra of Zn1−xCrxTe thin films. (a)XAS, XMCD spectra and the application of the XMCD sum rules for theirspectral integrals. (b) Candidates for the electronic configurations of the Crions in Zn1−xCrxTe. The orbital degree of freedom of the Cr2+ ion in the Td

symmetry crystal field (left column) is quenched in the D2d symmetry crystalfield (middle column) or in the Cr+ configuration (right column).


(a) (b)

Figure 1.8: Ferromagnetic properties of Zn1−xCrxTe thin film (x = 0.20).(a) Magnetic-field dependence of magnetization at various temperatures [16].The inset shows the Arrott plots of the magnetization data. (b) Arrott plotsof MCD intensity at the absorption edge of ZnTe (E = 2.2 eV) with vary-ing temperature. The inset shows the T dependence of the MCD intensity,together with that of the saturation magnetization (solid circles).

I doped N doped

(a)

(b) (c)

Figure 1.9: Doping effects on Zn1−xCrxTe. (a) Magnetic-field dependence ofthe magnetization of I- and N-doped Zn1−xCrxTe [20]. (b) and (c) Cross-sectional mapping images of the Cr Kα emission intensity of I- and N-dopedZn1−xCrxTe thin films, respectively [22].

Chapter 2

Principles of X-ray magneticcircular dichroism

2.1 Principles of x-ray magnetic circular dichro-

ism and sum rules

2.1.1 X-ray absorption spectroscopy

The measurements of photo-absorption by excitation of a core-level electroninto unoccupied states as a function of photon energy is called x-ray absorp-tion spectroscopy. The photo-absorption intensity is given by

I(hν) =∑

f

|⟨f |T |i⟩|2δ(Ei − Ef − hν), (2.1)

where T is the dipole transition operator. In the 3d transition-metal com-pounds, transition-metal 2p XAS spectrum reflects the 3d states such as thevalence, the spin state and the crystal-field splitting.

There are three measurement modes for XAS, that is, the transmissionmode, the total electron-yield mode and total fluorescence-yield mode. In thetransmission mode, the intensity of the x-ray is measured before and afterthe sample and the ratio of the transmitted x-rays is counted. Transmission-mode experiments are standard for hard x-rays, while for soft x-rays, theyare difficult to perform because of the strong interaction of soft x-rays withthe sample. In the present work, the total electron-yield mode and totalfluorescence-mode were employed.

15

16 Chapter 2. Principles of X-ray magnetic circular dichroism

2.1.2 X-ray magnetic circular dichroism

When the relativistic electrons in the storage ring are deflected by the bend-ing magnets that keep them in a closed circular orbit, the electrons emithighly intense beams of linearly polarized x-rays in the plane of the electronorbit (bremsstrahlung). On the other hand, they emit circularly ellipticallypolarized light out of the plane. Currently, a number of alternative sourcesfor circularly polarized synchrotron radiation are under development. Themost notable ones are so-called insertion devices like helical wigglers [27] andcrossed [28] undulator, which are complex arrays of magnets with which theelectrons in a storage ring are made to oscillate in two directions that areperpendicular to their propagation direction, with the result that they emitcircularly polarized light.

Using circularly polarized x-rays in XAS, x-ray magnetic circular dichro-ism (XMCD) is defined as the difference in absorption spectra between right-handed and left-handed circularly polarized x-rays when the helicity of thex-rays are parallel and antiparallel to the magnetization direction of the mag-netic materials such as ferromagnet or ferrimagnet. XMCD is sensitive tomagnetic polarization, and therefore enable us to study the magnetic prop-erties of particular orbitals on each element.

2.1.3 Total-electron yield (TEY) and total-fluorescenceyield (TFY) modes

For XMCD measurements, there are two measurement modes. One is theTEY mode, and the other is the TFY mode. Figure 2.2 (a) shows a schematicdiagram of the TEY mode. In the TEY mode, Auger electrons which arecreated when the incident X-ray creates a core hole and the core hole decaysvia Auger transition are measured as a sample current. The detection depthof the TEY mode is about 5 nm. Figure 2.2 (b) shows a schematic diagramof the TFY mode. In the TFY mode, emitted light which is created whenan outer electron makes a transition into core hole state is measured. Thedetection depth is about 100 nm.

2.1.4 XMCD sum rules

XMCD reflects the spin and orbital polarization of local electronic states.Using integrated intensity of the L2,3-edge XAS and XMCD spectra of a

2.1. Principles of x-ray magnetic circular dichroism and sum rules 17

B

x ray

E

sample

+2md

+3/2 +1/2 -1/2 -3/2

+1/2 -1/2

mj

mj

2p3/2

2p1/2

+1 -2-10

2p-3d XMCD σ−σ+

183

6

1

26 63

312

(a)

(b)

Ab

sorp

tio

n

Photon Energy

0

Photon Energy

0

Photon Energy

<Sz>d

(c)

XM

CD

=

µ

+ −

µ−

µ+

µ−

(µ+ −

µ−)d

ω

<Lz>d

A1+2A2

A1

A2

Figure 2.1: Schematic diagram of x-ray magnetic circular dichroism(XMCD). (a) Experimental set up for XMCD measurements. (b) Transi-tion probability of 2p → 3d absorption with circularly polarized x rays forless-than-half filled 3d electronic configuration. (c) Circularly polarized x-rayabsorption spectra.

transition-metal atom, one can estimate the orbital [29] and spin [30] mag-netic moments separately by applying XMCD sum rules given by

Morb = −4∫

L3+L2(µ+ − µ−)dω

3∫

L3+L2(µ+ + µ−)dω

(10 − Nd), (2.2)

Mspin +7MT = −6∫

L3(µ+ − µ−)dω − 4

∫L3+L2

(µ+ − µ−)dω∫L3+L2

(µ+ + µ−)dω(10−Nd), (2.3)

where Morb and Mspin are the spin and orbital magnetic moments in unitsof µB/atom, respectively, µ+(µ−) is the absorption intensity for the positive(negative) helicity, Nd is the d electron occupation number of the specific

18 Chapter 2. Principles of X-ray magnetic circular dichroism

Figure 2.2: Schematic diagram of the TEY and TFY modes. (a) Schematicdiagram of the TEY mode. (b) Schematic diagram of the TFY mode.

transition-metal atom. The L3 and L2 denote the integration range. MT isthe expectation value of the magnetic dipole operator, which is small whenthe local symmetry of the transition-metal atomic site is high and is neglectedhere with respect to Mspin.

Chapter 3

Experiment

3.1 Experimental setup

3.1.1 NSRRC BL11A

A Dragon beam line 11A at National Synchrotron Radiation Research Center(NSRRC) has been constructed for photoemission, x-ray absorption, MCDand magnetic linear dichroism (MLD) measurements. A schematic diagramof the beamline is shown in Fig. 3.1. The light source is a bending magnet.Synchrotron radiation with horizontal acceptance of 12 mrad was reflected byhorizontally and vertically focusing spherical mirrors (HFM and VFM) to agrating (6m-GSM). The monochromatic light was reflected by toroidal refo-cusing mirror (RFM), and introduced to the end station. Two vertical planemirrors (VPM) between the gratings and the exit slit to extend the lowestphoton energy to 10 eV. In this beamline, six spherical gratings are used tocover an energy range from 10 eV to 1700 eV. In practical measurements,the photon energy was scanned using a grating which have 1200 lines/mmand covers the photon energy range 400 − 1200 eV. Photon flux is 1 × 1010

with the energy resolution E/∆E = 10, 000. The measurement chamber islocated at the end station of the beamline.

Spectra were measured both in the total-electron-yield (TEY) mode andthe total-fluorescence-yield (TFY) mode. In the TEY mode, the sample wasconnected to the ground through a current measurement system and the neu-tralization current was monitored. The detection depth of the TEY mode isabout 5 nn. Therefore, one can investigate the surface state of the sample byusing the TEY mode. In the TFY mode, emitted photons which are created

19

20 Chapter 3. Experiment

when soft x-ray hit the sample were monitored. The detection depth of theTFY mode is about 100 nn. Therefore, one can obtain information aboutthe bulk properties of the sample by using the TFY mode. In the XMCDmeasurements, the circular polarization of the incident photons was fixedand the direction of the applied magnetic field was changed. The XAS andXMCD measurements were made at a temperature of 20 K in an ultrahighvacuum below ∼ 10−10 Torr.

Figure 3.1: Schematic optical layout of the BL11A at NSRRC.

3.2. samples 21

3.2 samples

We performed XMCD measurements to investigate the electronic structureof ZnTe-capped Zn1−xCrxTe films. However, the effect of oxidation was ob-served. Then, we prepared Al-capped samples and succeeded to protect thesurface region from oxidation. Here, details of ZnTe-capped and Al-cappedZn1−xCrxTe films are described.

3.2.1 ZnTe-capped samples

ZnTe-capped sample used in this study was undoped Zn1−xCrxTe with x =0.031. The TC is 70K.

The sample was grown on insulating GaAs (001) substrates by molecularbeam epitaxy. After depositing a 600 nm thickness of ZnTe buffer layer,the Zn1−xCrxTe thin films of 300 nm thickness were grown. During thedeposition, the substrate was kept at a temperature of 330℃.

3.2.2 Al-capped samples

Al-capped samples used in this study were undoped Zn1−xCrxTe with x =0.053, I-doped Zn1−xCrxTe with x = 0.039, lightly N-doped Zn1−xCrxTe withx = 0.043, and heavily N-doped Zn1−xCrxTe with x = 0.047. The TC ofthe undoped, I-doped, and lightly N-doped samples are 90K, 210K and 60K,respectively. According to secondary ion mass spectroscopy (SIMS) anal-ysis, the N concentration in the lightly and heavily N-doped samples was1.8× 1018cm−3 and 1.0× 1020cm−3, respectively. These values correspond tothe ratios of substituting N for Te of 0.1 % and 0.56 %, respectively, whenwe assume all the N atoms substitute for Te. These samples were grown oninsulating GaAs (001) substrates by molecular beam epitaxy. After deposit-ing a 600 nm thickness of ZnTe buffer layer, the Zn1−xCrxTe thin films of300 nm thickness were grown. During the deposition, the substrate was keptat a temperature of 330-360℃.

Chapter 4

Influences of capping layers onZn1−xCrxTe

4.1 Introduction

We performed XMCD measurements to investigate the electronic structureof ZnTe-capped Zn1−xCrxTe films. However, the effect of oxidation was ob-served. Then, we prepared Al-capped samples and succeeded to protect thesurface region from oxidation. In this Chapter, we compare the results of theAl-capped sample and the ZnTe-capped sample.

4.2 Experimental

The samples used in this study were ZnTe-capped Zn1−xCrxTe with x = 0.031(Tc = 70 K) and Al-capped Zn1−xCrxTe with x = 0.053 (Tc = 90 K). Thesample surfaces were capped with a 3 nm thick ZnTe or Al layer to avoidoxidation for the Zn1−xCrxTe films. XAS and XMCD measurements wereperformed at the Dragon Beamline BL11-A of National Synchrotron Radi-ation Research Center (NSRRC), Taiwan. Spectra were measured both inthe total-electron-yield (TEY) mode and the total-fluorescence-yield (TFY)mode. Because the probing depth of the TEY and TFY mode are about 5nm and 100 nm, respectively, one can consider that the TEY mode is surfacesensitive and the TFY mode is bulk sensitive.

23

24 Chapter 4. Influences of capping layers on Zn1−xCrxTe

4.3 Results and Discussion

4.3.1 ZnTe capped sample

Figure 4.1 shows the Cr 2p XAS spectra of the ZnTe-capped Zn1−xCrxTetaken in the TEY mode and the TFY mode. For comparison, the Cr 2pXAS spectrum of Cr2O3 (Cr3+ state) taken in the TEY mode is also shown.The Cr ions in the Cr2O3 are Cr3+, as described elsewhere [31]. The majortwo peaks in each spectrum are due to the 2p3/2 − 2p1/2 spin-orbit doublet ofthe Cr 2p core level. Due to self-absorption effect of the TFY method, theintensity of the 2p3/2 peak relative to the 2p1/2 peak is reduced compared toone obtained by the TEY method.

One can see in Fig. 4.1 that the spectral line shapes and the peak positionsof Zn1−xCrxTe taken in the TEY mode and Cr2O3 are almost the same.Considering that the TEY mode is surface sensitive, this result indicatesthat the Cr ions in the surface region of the ZnTe-capped sample are Cr3+.This indicates that the Cr ions in the Zn1−xCrxTe layer were diffused to theZnTe-capping layer, and that Cr ions were oxidized.

On the other hand, the spectral line shapes and the peak positions of theZnTe-capped Zn1−xCrxTe taken in the TFY mode and Cr2O3 are different.One can find two peaks in the 2p3/2 edge of Cr in Zn1−xCrxTe taken in theTFY mode. The peak which is positioned at higher energy corresponds toCr3+ because the peak position is almost same as that of Cr2O3. Consideringthat the other peak, which is positioned at lower energy, is stronger than thatof the Cr3+ spectra, Cr2+ ions were dominant in the bulk region, however,the oxidized Cr3+ ions which exist at ZnTe-capped region were also detectedin the TFY mode.

To avoid the oxidation in surface region, we prepared Al-capped Zn1−xCrxTecompounds. The results of Al-capped sample are shown in the next section.

4.3. Results and Discussion 25

Inte

nsity

(ar

b.un

it)

590585580575570

Photon energy (eV)

Cr2O3

TFY

TEY

ZnTe-capped Zn1-xCrxTe

Figure 4.1: Comparison of the XAS spectra of the ZnTe-capped Zn1−xCrxTesample taken in the TEY mode, the ZnTe-capped Zn1−xCrxTe sample takenin the TFY mode and Cr2O3 taken in the TEY mode.


4.3.2 Al capped sample

To avoid the oxidation in the surface region, we then prepared Al-cappedZn1−xCrxTe samples.

Figure 4.2 show the Cr 2p XAS spectra of the Al-capped Zn1−xCrxTetaken in the TEY mode and the TFY mode. For comparison, the Cr 2p XASspectrum of Cr2O3 (Cr3+ state) taken in the TEY mode is also shown. Crions in Cr2O3 are Cr3+, as described elsewhere [31]. The major two peaks ineach spectrum are due to the Cr 2p3/2 − 2p1/2 spin-orbit doublet of the Cr2p core level. Due to self-absorption effect of the TFY method, the intensityof 2p3/2 peak relative to the 2p1/2 peak is reduced compared to one obtainedby the TEY method.

One can find two peaks at the Cr 2p3/2 edge of the Al-capped Zn1−xCrxTesample taken in the TEY mode, however, the peak which corresponds to thatof Cr3+ is considerably suppressed compared to the Cr3+ peak observed inthe spectrum of the ZnTe-capped sample. This indicates that Al capping ef-fectively protects the sample from the oxidation than ZnTe capping. Indeed,in the spectrum which was taken in the TFY mode, there is no peak thatsuggests the existence of Cr3+, indicating that one can eliminate the effectof oxidation in the bulk region of the sample by Al-capping.

Therefore, in the next chapter, we performed detailed analysis of the dataof the Al-capped Zn1−xCrxTe samples taken in the TFY mode.


Inte

nsity

(ar

b.un

it)

590585580575570Photon energy (eV)

TEY

TFY

Cr2O3

Al-capped Zn1-xCrxTe

Figure 4.2: Comparison of the XAS spectra of the Al-capped Zn1−xCrxTesample taken in the TEY mode, and the TFY mode and Cr2O3 (Cr3+) takenin the TEY mode.


4.4 Conclusion

We have performed XAS measurements on ZnTe-capped and Al-capped Zn1−xCrxTesamples in the TEY and TFY modes. It was found that the influence of oxi-dation was observed in spectra which were taken in the TEY and TFY modesin the ZnTe-capped Zn1−xCrxTe samples. In the Al-capped Zn1−xCrxTe sam-ple, the influence of oxidation was observed in the spectrum which was takenin the TEY mode, however, it was not observed in the spectrum which wastaken in the TFY mode.

Chapter 5

Effect of co-doping of donorand acceptor impurities in theferromagnetic semiconductorZn1−xCrxTe studied by softx-ray magnetic circulardichroism

5.1 Introduction

The II-IV DMS Zn1−xCrxTe is known to show ferromagnetism at room tem-perature, as confirmed by magnetization measurements and magnetic circu-lar dichroism (MCD) measurements in the visible to ultra-violet region [16].Recently, the effects of additional doping of atoms with different valenciesin Zn1−xCrxTe were investigated, that is, iodine which is expected to act asan n-type dopant enhances the ferromagnetism [20], while nitrogen whichis expected to act as as a p-type dopant suppresses it [21]. On the otherhand, it has been predicted theoretically that spinodal decomposition causesthe apparent ferromagnetic behavior of Zn1−xCrxTe [25, 32]. Experimentally,indeed, spatially resolved energy-dispersive x-ray spectroscopy revealed thatthe Cr ions are distributed inhomogeneously in the I-doped samples whilethey are distributed homogeneously in the N-doped samples [22]. The re-

29

30 Chapter 5. Effect of co-doping of donor and acceptor impurities ...

sults of ab initio calculations of the total energies suggest that the valencestate of Cr seems to be important for distribution of the Cr ions, that is,Cr ions will be distributed inhomogeheously if Cr ions are in the neutral 2+charge state, while they are distributed homogeneously if Cr ions are in differ-ent +(2± δ) charge states [22, 33]. So far, x-ray magnetic circular dichroismstudies have been done on Zn1−xCrxTe to investigate the electronic structureof Cr ions and Cr ions have been found to be in the 2+ state [18, 19]. It isnecessary to know how the electronic state of the Cr ion is modified by the N-and I- doping to understand the mechanism of the spinodal decomposition.In order to investigate the effects of I- and N-doping, we have performedx-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism(XMCD) experiments at the Cr 2p absorption edge of the undoped, I-doped,lightly N-doped, and heavily N-doped Zn1−xCrxTe thin films.

5.2 Experimental

The samples used in this study were undoped Zn1−xCrxTe with x = 0.053(Tc = 90 K), I-doped Zn1−xCrxTe with x = 0.039 (Tc = 210 K), lightlyN-doped Zn1−xCrxTe with x = 0.043 (Tc = 60 K), and heavily N-dopedZn1−xCrxTe with x = 0.047 (no ferromagnetism). According to secondary ionmass spectroscopy (SIMS) analysis, the N concentration in the lightly andheavily N-doped samples was 1.8×1018cm−3 and 1.0×1020cm−3, respectively.These values correspond to the ratios of substituting N for Te of 0.1 % and0.56 %, respectively, when we assume all the N atoms substitute for Te.

5.3 Results and Discussion

Figure 5.1 shows the Cr 2p XAS spectra of the undoped, I-doped, lightly N-doped, and heavily N-doped Zn1−xCrxTe thin films taken in the TFY mode.For comparison, those of Cr2O3 and those of Zn1−xCrxTe taken in the TEYmode are also shown. Cr ions in Cr2O3 and Zn1−xCrxTe are Cr3+ and Cr2+,respectively, as described elsewhere [31, 34]. The major two peaks in eachspectrum are due to the 2p3/2 − 2p1/2 spin-orbit doublet of the Cr 2p corelevel. Due to self-absorption effect of the TFY method, the intensity of2p3/2 peak relative to the 2p1/2 peak is reduced compared to one obtainedby the TEY method. One can see in Fig.5.1 that the spectral line shapes


and the peak positions of the undoped, I-doped and lightly N-doped samplesare similar to the spectra of Cr2+. Considering that the TFY mode is bulksensitive, one can conclude that Cr2+ is dominant in the undoped, I-dopedand lightly N-doped samples in the bulk region. On the other hand, theXAS spectra of the heavily N-doped sample showed clear peak shifts towardshigher energies compared to the ohter samples and located between Cr2+ andCr3+. This observation indicates that N replacing Te atoms in the heavily N-doped sample supplied holes and works as an acceptor and that Cr3+ statesare created.

Figure 5.2 shows the Cr 2p XAS and XMCD spectra of the undoped,I-doped, lightly N-doped, and heavily N-doped Zn1−xCrxTe thin films takenat T = 20 K. Magnetic fields were applied perpendicular to the sample sur-faces. We observed clear XMCD signals in the undoped, I-doped and lightlyN-doped samples, while the heavily N-doped sample showed no clear XMCDsignals, consistent with the magnetization measurements [22]. Figure 5.3(a)shows the XMCD spectra of the undoped, I-doped, lightly N-doped, andheavily N-doped samples taken under 1, 0.5 and 0.1 T. The line shapes ofthe XMCD spectra are same for all the samples, indicating Cr2+ ions con-tribute to the magnetism in these samples. Although applying of XMCDsum rules [29, 30] to TFY data does not give accurate results because of theself-absorption effect, we have attempted to deduce the magnetic moment ofCr ions by applying the XMCD sum rules to the TFY data in order to seerelative changes with N or I doping. Figure 5.3(b) shows the magnetic fielddependence of the approximate Cr magnetic moments of these samples esti-mated using the XMCD sum rules. If the XMCD intensities are extrapolatedto H = 0 T, one can see finite magnetization at H = 0 T in the undoped,I-doped, and lightly N-doped samples, indicating that the ferromagnetismin these samples are of intrinsic bulk origin. In addition, a higher magneticmoment was observed in the I-doped sample than in the undoped sample.

The recent atomic-scale analysis using energy-dispersive x-ray spectroscopyhas shown a close correlation between the spatial homogeneity of Cr dis-tribution with ferromagnetic properties [22]; the inhomogeneous distribu-tion, possibly due to spinodal decomposition, enhances the ferromagnetismwhile the homogeneous distribution suppresses it. According to a theoreticalmodel [33], the valence state of Cr ions is important for the spinodal decompo-sition. In the intrinsic situation, the valence state of Cr ions substituting forZn is 2+, being electrically neutral with the same charge state as Zn2+, andstrong attractive interaction between the electrically neutral Cr ions makes


the Cr distribution inhomogeneous. When the Cr charge state deviates from2+, the repulsive force of the electrostatic origin created between the Cr ionsmakes a homogeneous distribution. In the actual samples grown by MBE, itis considered that the Cr valence state in the undoped Zn1−xCrxTe slightlyincreases from 2+ due to the formation of Zn vacancies, which act as shal-low acceptors, while the Cr valence returns to 2+ in the I-doped Zn1−xCrxTedue to the compensation of these Zn vacancies by the donor impurities. Thismodel explains the experimentally observed inhomogeneous/homogeneous Crdistribution and the resultant enhancement/suppression of ferromagnetismin the I-doped/undoped Zn1−xCrxTe. In either case, the dominant Cr ionsare in the valence state of 2+, which is consistent with the result of the XASmeasurements in the I-doped and undoped samples. On the other hand, theco-doping of nitrogen, which acts as an acceptor impurity when substitutingfor Te, converts the Cr valence state from 2+ to 3+. As a result, in theheavily N-doped sample, the Cr valence state becomes the mixed valenceor an intermediate valence between Cr3+ and Cr2+, as demonstrated in theXAS spectra. The disappearance of ferromagnetism in the heavily N-dopedsample can be attributed to this conversion of the Cr valence state in addi-tion to the homogeneous Cr distribution. In the lightly N-doped sample, theconversion of the Cr valence is not sufficient to induce an apparent shift ofthe XAS spectra. Our experiment thus gives the first microscopic support ofthe model that acceptor doping changes the valence of Cr and this changeinfluences the ferromagnetism in Zn1−xCrxTe.


Inte

nsity

(ar

b.un

its)

590585580575570Photon energy (eV)

I-doped Undoped

Zn1-xCrxTe (Cr2+

)

Cr2O3

T = 20 K2p 3/2 2p 1/2

Lightly N-doped Heavily N-doped

TFY

TEY

(Cr3+

) TEY

Figure 5.1: Comparison of the XAS spectra of the undoped, I-doped, lightlyN-doped, and heavily N-doped Zn1−xCrxTe thin films taken in the TFY mode.The XAS spectra of undoped Zn1−xCrxTe and Cr2O3 taken in the TEY modeare shown as references of Cr2+ and Cr3+, respectively.


Cr 2p XASI-doped +1T

-1T XMCD

200

100

0

+1T -1T XMCD

UndopedCr 2p XAS

200

100

0

590585580575570

Lightly N-doped

Cr 2p XAS

+1T -1T XMCD

590585580575570

Heavily N-doped

Cr 2p XAS

+1T -1T XMCD

Photon energy (eV)

XM

CD

Int

ensi

ty (

arb.

uni

ts)

Figure 5.2: Cr 2p XAS and XMCD spectra of the undoped, I-doped, lightlyN-doped, and heavily N-doped Zn1−xCrxTe thin films taken by the totalfluorescence yield mode at H = 1T and T = 20 K. The XMCD spectra havebeen normalized to the XAS [(µ+ + µ−)/2] peak height at around 576 eV.


1.5

1.0

0.5

0.0

Mag

netic

mom

ent (

µ B/C

r)

1.00.50.0Magnetic field (T)

I-doped Undoped Lightly N-doped (N: 0.1 %) Heavily N-doped (N: 0.56 %)

T = 20 K

1TI-doped

0.5T 0.1T

590585580575570

Heavily N-doped 1T 0.5T 0.1T

50

0

-50

590585580575570

Lightly N-doped 1T 0.1T 0.5T

50

0

-50

Undoped Cr 2p XMCD 1T 0.1T 0.5T

Photon energy (eV)

XM

CD

Int

ensi

ty (

arb.

uni

ts)

(a)

(b)

Figure 5.3: Magnetic field dependence of XMCD spectra (a) and magneticmoments (b) of the I-doped, undoped, lightly N-doped, and heavily N-dopedZn1−xCrxTe thin films at T = 20 K.


5.4 Conclusion

We have performed XAS and XMCD measurements on undoped, I-doped,lightly N-doped, and heavily N-doped Zn1−xCrxTe. From the XAS measure-ments, Cr ions were found to be in the Cr2+ state in the undoped, I-dopedand lightly N-doped samples, while Cr3+ and Cr2+ states were formed in theheavily N-doped sample. In the XMCD measurements, it was found thatthe magnetic moment of the Cr2+ state contributes to the ferromagnetism inthe undoped, I-doped and lightly N-doped samples, while the XMCD signalsat the Cr 2p edge were not observed in the heavily N-doped sample. Themagnitude of magnetic moment was found to be larger in the I-doped samplethan in the undoped one, consistent with the results of the magnetizationmeasurements.

Chapter 6

Summary

In the preceding Chapters, we have performed x-ray absorption spectroscopy(XAS) and x-ray magnetic circular dichroism (XMCD) studies of Zn1−xCrxTethin films. Here, we summarize the results.

In Chapter 4, we have performed XAS measurements on ZnTe-cappedand Al-capped Zn1−xCrxTe in the TEY and TFY mode. The influence ofoxidation was observed in the spectra which were taken in the TEY andTFY modes in the ZnTe-capped Zn1−xCrxTe samples. In the Al-cappedZn1−xCrxTe samples, the influence of oxidation was observed in the spec-trum which was taken in the TEY mode, however, it was not observed in thespectrum taken in the TFY mode.

In Chapter 5, we have performed XAS and XMCD measurements on Al-capped undoped, I-doped, lightly N-doped, and heavily N-doped Zn1−xCrxTein the bulk-sensitive TFY mode. From the XAS measurements, Cr ions werefound to be in the Cr2+ state in the undoped, I-doped and lightly N-dopedsamples, while Cr3+ and Cr2+ states were formed in the heavily N-dopedsample. In the XMCD measurements, it was found that the magnetic mo-ment of the Cr2+ state contributes to the ferromagnetism in the undoped,I-doped and lightly N-doped samples, while the XMCD signals at the Cr2p edge were not observed in the heavily N-doped sample. The magnitudeof magnetic moment was found to be larger in the I-doped sample than inthe undoped one, consistent with the results of the magnetization measure-ments.

37

Acknowledgment

I wish to express my special gratitude to the following people for my masterthesis.

First of all, I would like to express my sincere gratitude to Prof. AtsushiFujimori, who has given me the opportunity to be engaged in this research,for his lot of enlightening discussion based on his deep insight in physics. Hisclear advice always encouraged me. Let me also thank Dr. Teppei Yoshida.He has given me many valuable advice and comments based on his ampleexperience.

Zn1−xCrxTe thin films were provided by Prof. Shinji Kuroda’s group. Iam very thankful to Prof. Shinji Kuroda and the members of Prof. Kuroda’sgroup, Mr. Koichro Ishikawa, Mr. Ke Zhang. They provided us with highquality samples. Especially, I would like to express my great gratitude toProf. Kuroda, who has guided me and given me a lot of important advice.

The experiment at Taiwan Light Source were supported by Prof. C. T.Chen’s group. I would like to thank the members : Mr. Fan-Hsu Chang, Dr.Longlife Lee, Prof. Hong-Ji Lin, Prof. Di-Jing Huang. I have a lot to thankthem for their technical support and their kind advice.

I would like to express my gratitude to Prof. Tsuneharu Koide and Dr.Daisuke Asakura, who has taught me a lot of things about XMCD.

I would like to express special gratitude to Dr. Takashi Kataoka, Mr.Yuta Sakamoto, Mr. Vijay Raj Singh, Mr. Verma Virendra Kumar, Mr.Ishigami, Mr. Goro Shibata and Dr. Toshiharu Kadono, who helped andguided me during the measurements.

I would also like to thank the members of Fujimori group : Dr. MasakiIkeda, Dr. Walid Malaeb, Mr. Shinichiro Ideta, Mr, Shin-ichi Aizaki, Mr.Ichiro Nishi, Mr. Wataru Uemura, Mr. Ambolode II, Leo Cristobal C, Mr.Hakuto Suzuki and Ms. Emiko Murayama, for their cordial support.

Finally, I would like to thank my parents.

39

40 Chapter 6. Summary

January 2011Yo Yamazaki

References

[1] H. Ohno, Science 281, 951 (1998).

[2] H. Ohno, F. Matsukura, and Y. Ohno, JSAP International 5, 4 (2002).

[3] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science287, 1019 (2000).

[4] M. L. Reed, N. A. El-Masry, H. H. Stadelmaier, M. K. Ritums, M. J.Reed, C. A. Parker, J. C. Roberts, and S. M. Bedair, Appl. Phys. Lett.79, 3473 (2001).

[5] C. Zener, Phys. Rev. 81, 440 (1951).

[6] J. K. Furdyna, J. Appl. Phys. 64, R29 (1988).

[7] T. Mizokawa and A. Fujimori, Phys. Rev. B 56, 6669 (1997).

[8] C. Zener, Phys. Rev. 82, 403 (1951).

[9] A. H. MacDonald, P. Schiffer, and N. Samarth, Nat. Mater. 4, 195(2005).

[10] A. Kaminski and S. D. Sarma, Phys. Rev. Lett. 88, 247202 (2002).

[11] J. M. D. Coey, M. Venkatesan, and C. B. Fitzgerald, Nat. Mater. 4, 173(2005).

[12] K. Sato, W. Schweika, P. Dederichs, and H. Katayama-Yoshida, Phys.Rev. B 70, 201202(R) (2004).

[13] J. T. Vallin, G. A. Slack, S. Roberts, and A. E. Hughes, Phys. Rev. B2, 4313 (1970).

41

42 References

[14] J. T. Vallin and G. D. Watkins, Phys. Rev. B 9, 2051 (1974).

[15] W. Mac, A. Twardowski, and M. Demianiuk, Phys. Rev. B 54, 5528(1996).

[16] H. Saito, V. Zayets, S. Yamagata, and K. Ando, Phys. Rev. Lett. 90,207202 (2003).

[17] H. Saito, V. Zayets, S. Yamagata, and K. Ando, J. Appl. Phys. 93, 6796(2003).

[18] Y. Ishida, M. Kobayashi, J. Hwang, Y. Takeda, S. i. Fujimori, T. Okane,K. Terai, Y. Saitoh, Y. Muramatsu, A. Fujimori, A. Tanaka, H. Saito,and K. Ando, Appl. Phys. Exp. 1, 041301 (2008).

[19] M. Kobayashi, Y. Ishida, J. Hwang, G. Song, A. Fujimori, C.-S. Yang, L.Lee, H.-J. Lin, D. Huang, C. Chen, Y. Takeda, K. Terai, S. I. Fujimori,T. Okane, Y. Saitoh, H. Yamagami, K. Kobayashi, A. Tanaka, H. Saito,and K. Ando, New J. Phys. 1, 0055011 (2008).

[20] N. Ozaki, N. Nishizawa, S. Marcet, S. Kuroda, O. Eryu, and K. Takita,Phys. Rev. Lett. 97, 037201 (2006).

[21] N. Ozaki, I. Okabayashi, T. Kumekawa, N. Nishizawa, S. Marcet, S.Kuroda, and K. Takita, Appl. Phys. Lett. 87, 192116 (2005).

[22] S. Kuroda, N. Nishizawa, K. Takita, M. Mitome, Y. Bando, K. Osuch,and T. Dietl, Nat. Mater. 6, 440 (2007).

[23] F. Matsukura, H. Ohno, A. Shen, and Y. Sugawara, Phys. Rev. B 57,R2037 (1998).

[24] N. Ozaki, N. Nishizawa, S. Kuroda, and K. Takita, J. Phys.: Condens.Matter 16, S5773 (2004).

[25] T. Fukushima, K. Sato, H. Katayama-Yoshida, and P. H. Dederichs,Jpn. J. Appl. Phys. 45, L416 (2006).

[26] H. Ofuchi, N. Ozaki, N. Nishizawa, H. Kinjyo, S. Kuroda, and K. Takita,AIP. Conf. Proc. 882, 517 (2007).

References 43

[27] S. Yamamoto, H. Kawata, H. Kitamura, M. Ando, N. Saki, and N.Shiotani, Phys. Rev. Lett. 62, 2672 (1989).

[28] H. Onuki, N. Saito, and T. Saito, Appl. Phys. Lett. 52, 173 (1988).

[29] B. T. Thole, P. Carra, F. Sette, and G. van der Laan, Phys. Rev. Lett.68, 1943 (1992).

[30] P. Carra, B. T. Thole, M. Altarelli, and X. Wang, Phys. Rev. Lett. 70,694 (1993).

[31] C. Theil, J. van Elp, and F. Folkmann, Phys. Rev. B 59, 7931 (1999).

[32] K. Sato, H. Katayama-Yoshida, and P. H. Dederichs, Jpn. J. Appl. Phys.44, L948 (2005).

[33] T. Dietl, Nat. Mater. 5, 673 (2006).

[34] M. Kobayashi, J. I. Hwang, G. S. Song, Y. Ooki, M. Takizawa, A.Fujimori, Y. Takeda, S.-I. Fujimori, K. Terai, T. Okane, Y. Saitoh,H.Yamagami, Y.-H. Lin, and C.-W. Nan, Phys. Rev. B 78, 155322(2008).

X-ray Magnetic Circular Dichroism Study of the Diluted...

Documents

Transcript of X-ray Magnetic Circular Dichroism Study of the Diluted...