物構研シンポジウム’09「放射光・中性子・ミュオンを用いた表面・界面科学の最前線」
平成21年11月17～ 18 日，つくば国際会議場エポカル

軟X線磁気円二色性によるスピントロニクス材料の研究
藤森 淳
東京大学理学系研究科
分光測定
片岡隆史，Vijay Raj Singh，山崎陽, 小林正起（東大理・新領域）
小出常晴，朝倉大輔， 間宮一敏（物構研PF）
竹田幸治，岡根哲夫，大河内拓雄，斎藤祐児 (原子力機構)
F.-H. Chang, C.-S. Yang, L. Lee, H.-J. Lin, D.-J. Huang, C.T. Chen （NSRRC, Taiwan）

試料
田中雅明，大矢 忍，ファムナムハイ（東大工）Ga1-xMnxAs
福村知昭，山田 良則, 上野 和紀，川崎雅司（東北大金研・WPI材料機構）Ti1-xCoxO2
黒田眞司，石川弘一郎, 張 珂（筑波大物質工） Zn1-xCrxTe
K.V. Rao, M. Kapilashrami, L. Belova (Royal Inst Tech, Swedem) Zn1-xMnxO薄膜
D. Karmakar (BARC), S.K. Mandal, T.K. Nath (IIT Kharagpur), I. Dasgupta (IACS)
Zn1-xFexO, Zn1-x(Fe, Co)xO , Zn1-x(Mn, Co)xO ナノ粒子理論
田中 新 （広大先端物質） 多重項計算，クラスター計算

Discovery of ferromagnetism in MBE-grown
Mn-doped III-V-based DMS

Growth “phase diagram” of Ga1-xMnxAs Curie temperature of Ga1-xMnxAs

T. Hayashi, M. Tanaka, J. Cryst. Growth, ‘97

H. Ohno et al., JMMM, ‘99

Diluted magnetic semiconductors (DMS):
Expected for future spintronics applications
GaAs

Ga1-xMnxAs• GMR devices• Non-volatile memories• New logic circuits• New devices utilizing spin injection- Circularly polarized LED- Magnetization control by spin current

Y. Ohno et al., Nature ‘99

Theoretical prediction of room-temperature
ferromagnetism in DMS
TC of Mn-doped semiconductors
in p-d exchange mechanism

Stability of magnetic states in DMS
in double-exchange mechanism
Ferromagnetic

RoomTemperature

Paramagnetic

300 K

T. Dietl et al. Science ‘00

K. Sato et al. JJAP ‘00.

Wide-gap semiconductors such as ZnO, GaN, and TiO2 are promising
host materials for room-temperature ferromagnetic DMS.

Rom-temperature ferromagnetic DMS
Ti1-xCoO2

CdGeP2:Mn

Ga1-xMnxN

Zn1-xCoxO

Zn1-xMnxO

Zn1-xCrxTe• Are they intrinsic DMS?
– No ferromagnetic contamination such as Fe metal?
– No precipitation of second phases, etc?

• To answer these questions:– SQUID measurements are not sufficient– Anomalous Hall effect is useful but controversial…..

X-ray magnetic circular dichroism (XMCD)
in core-level x-ray absorption spectra (XAS)
XAS

spin-orbit splitting

Mn 3dvalence levelspin-orbit
splitting

XMCD

spin sum rulehνMn 2p
core level

Element-specificChemical environment-specificContamination-insensitive
Magnetism-selective
Drawback: Surface sensitivity

orbital sum rule

B. T. Thole et al., PRL ’01, P. Carra et al., PRL ‘93

Ferromagnetic, paramagnetic and non-magnetic
components in XMCD vs SQUID signals
SQUID data of a DMS thin film sample
M-H curve

M-T curve

Magnetization (μB/Co)

5K300 K

Mferro0.4
0.2

Mpara0-0.2-0.4

Mdia

-0.6
-1.0-0.5

00.5

Magnetic Field (T)1.0Magnetization (μB/Co)

00.6

Mpara-0.2

1.0 T-0.4-0.6-0.8

Mferro
050

100

150

Mdia
200

Temperature (K)M = Mdia + Mferro + MparaÆXMCD signals,

250

chemically decomposed

300

XMCD experiment at BL11-A of Photon
Factory (previous)

ARPES装置

MCD装置

SC magnet H <5 T
T >10 K
Angle dependence
T. Koide et al. Rev．Rev. Sci. Instr. 63, 1462 (1992)

XMCD endstation at BL-16A of Photon Factory(present)

Superconducting magnet, up to 5 T
Low temperature down to ~10 K
Angular-dependent XMCD (L, T)
T. Koide et al. Rev．Rev. Sci. Instr. 63, 1462 (1992)
Electron orbit
To Beamline
Undulator IKicker magnet

Undulator IISR light

Fast switching(future)XMCD endstation at JAEA beamline BL-23SU
of SPring-8

ARPES装置

MCD装置

Helical undulator with phase modulation
Superconducting magnet, up to 10 T
Low temperature, down to ~6 K
High energy resolution and brightness
monochromator
Y. Saitoh et al., Nucl. Instrum. Meth. A ‘01

XMCD endstation at the Dragon beamlineBL-11A at NSRRCNSRRC, Taiwan

Bending magnet
Electro-magnet, up to 1 T
Low temperature down to 20 K
Total-electron-yield (TEY) and totalfluorescence-yield (TFY) mode detections

C.T. Chen and F. Sette, Rev. Sci. Instr. 60, 1616 (1989)

Outline

Ga1-xMnxAs
Ti1-xCoxO2
Zn1-xCrxTe
Zn1-xTxO (T=Mn, Co, Fe)Ga1-xMnxAs

Effects of low-temperature post-annealing
in Ga1-xMnxAs
Change of TC by post-annealing

Molecular dynamic calculation@250oCAsGa

Mn first enters interstices then substitutes Ga.
S.J. Potashnik et al., APL ‘01
T. Hayashi et al., APL ‘01

S.C. Erwin and A.G. Petukhov PRL ‘02

Ga1-xMnxAs samples
Sample: as grown Ga1-xMnxAshνe
As capGaAs

GaMnAs

A

1 nm
20 nm

semi ins. GaAs(001)As-grown:x =0.042, 0.078
TC ~ 60 K

Post-annealed: x =0.04, @270oC, 2 h
TC ~ 120 K
SPring-8 BL23SU

XAS and XMCD spectraParamagnetic component of XMCD vs T
XMCD intensity vs H & T in as-grown Ga1-xMnxAs
in as-grown Ga1-xMnxAs
XMCD intensityResidual
magnetizatio
n20K

AFM coupling?

Slope

70K

100Kextrinsic

20K

150K extrinsic

150K

Y. Takeda et al., PRL ’08

Ferromagnetic
Paramagnetic component
componentofofXMCD
XMCDvsvs TT
in as-grown Ga11--xxMnxxAs
Distribution of Tc

XMCD intensity
Magnetic polarons?

Residualmagnetizatio
Reducedmagnetization
n20K

Short-range
FM order?
AFM coupling?

Slope

70K

FM

100Kextrinsic

20K

150K extrinsic

150K

A. Kaminski and S. Das Sarma, PRL ’02

Y. Takeda et al., PRL ’08

Effects of post annealing on XMCD intensities
vs T & H in Ga1-xMnxAs

Magnetization
increases

Magnetization Pos
increases

net-a naledPo

Susceptibility increases
As-grownastledeannnow-gr
AsLong/short-range
FM order at higher T

Post annealing diminishes AF interaction byreducing the number of interstitial Mn ions.

Y. Takeda et al.,Effects of low-temperature post-annealing
on XAS and XMCD of Ga1-xMnxAs
extrinsic(Mn2+ oxide)

Post annealing expels interstitial Mn ions to surface
Y. Takeda et al.,Zn1-xCrxTeDiluted ferromagnetic semiconductor Zn1-xCrxTeM-H curves
VIS-UV MCD

MCD vs H

H. Saito et al., PRL ‘03

Doping effects in Zn1-xCrxTe

I-doping: Electron-doped
N-doping: Hole-doped
N. Ozaki et al., PRL ‘06

Doping effects in Zn1-xCrxTe: XMCDXMCD intensity vs H

I-dopedundoped
FMN-doped
heavily N-doped

Y. Yamazaki et al.

Inhomogeneous distribution of Cr in Zn1-xCrxTe
Energy-dispersive X-ray spectroscopy image
N-doped

I-dopeduniform
TC low

non-uniform
TC high
S. Kuroda et al., Nat. Mater. ‘07

Spinodal decomposition in high-TC DMS’s

K. Sato et al., JJAP ’05
T. Fukushima et al., JJAP ‘06

Cr valence vs inhomogeneous distribution of Cr
in Zn1-xCrxTe
Cr 2p XAS of Zn1-xCrxTe
Heavily N-doped

Cr-Cr pair formation energy

I-dopedundoped
N-doped

Cr2+Cr2+Cr3+

Inhomogeneousdistribution
Y. Yamazaki et al.

S. Kuroda, T. Dietl et al., Nat. Mater. ‘07

Ti1-xCoxO2

Room-temperature ferromagnetism in Ti1-xCoxＯ2
SQUID

UV-VIS MCD
RutileAnatase“Phase” diagram

TEM

PO2 = 10-8 Torr

Anatase
Rutile10-4 TorrNo precipitation
Y. Matsumoto et al., Science ’01

T. Fukumura, New J. Phys. ‘08

Co 2p core-level XAS and XMCD of Ti1-xCoxＯ2:
Effect of high-T annealing in vacuum
XAS
6XAS (σ+ - σ-)(a) Co L2,3-edge XAS

8Anatase(b) Co L2,3-edge XMCD

As-grown

Intensity (Arb. Units)

As-grown 6~400oC
2-min.

~400oC
2-min.

3410-min.

10-min.
220-min.
10L3L2780790

Photon Energy (eV)As-grown

42-min.

310-min.

220-min.

120-min.

2Co-metal

0Co-metal
800(c) ρ+ − ρ−ρ+ρ−

54XMCD

Co-metal
0780

790

800810

Photon Energy (eV)-1780790

800810

Photon Energy (eV)

“Ferromagnetism is due to segregated Co metal.”
Y.J. Kim et al. PRL ‘03

Co 2p core-level XMCD of Ti1-xCoxＯ2
without high-T annealingμ+(↑↑)μ−(↑↓)∫(μ++μ−)dω

scale

40
20

A2r = 781

F GE0XMCD
∫Δμdω

0600
400200-2p = -5.9

scale-4-4

scale

780-6790

800

Photon energy (eV)201T = 300 K
D B = ±1 TCo L3402-2BC6000q = -2.180

0Integral of XMCDXMCD (Δμ = μ+−μ−)scale
T = 300 K
B = ±1 T

DTi0.97Co0.03O2-δ

XAS (μ+, μ-)

XAS (μ+, μ−)60

BIntegral of XAS

80

Rutile type

100

800

XMCD (Δμ = μ+−μ−)C Ti0.97Co0.03O2-δ

100Aμ+(↑↑)μ−(↑↓)

E

Ti0.97Co0.03O2-δ
Co metal (×0.06)

0-1

Ferromagnetism isdue to Co2+ ion
However, moment
is very small:~ 0.13 μB/Co
cf. SQUID ~ 1 μB/Co-2-3-4778

780

782784

Photon energy (eV)786

K. Mamiya et al. APL ‘06

Surface- and bulk-sensitive modes of XAS andXMCD measurements
Total electron yield (TEY ) mode
probing depth ~ 3-5nm

Auger electron
Dragon beamline @NSSRC

Transmitted light
Fluorescence
Total fluorescence yield (TFY) mode
probing depth ~ 100nm ~ sample thickness

Transmission mode
Bulk
Prohibited by ~mm thik substrate

Co 2p XAS and XMCD of Ti1-xCoxO2
measured by bulk-sensitive TFY mode

RutileV.R. Singh et al. submitted to PRLXMCD intensity and magnetization vs H for
Ti1-xCoxO2
T = 300 K

TFY: bulk-sensitiveTEY: surface sensitive

Large
FM component

V.R. Singh et al. submitted to PRLSurface dead layer on Ti1-xCoxO2 film
Total fluorescence yield (TFY) mode (probing depth ~ 100nm)

Æ Strong XMCD intensity

Total electron yield (TEY ) mode (probing depth ~ 3-5nm)

Æ Weak XMCD intensity

Suppression of AHE for Ti1-xCoxO2-δ films

thinner

T. Fukumura et al., submitted to APL

Surface layer of ~5 nm is magnetically dead due to carrier depletion.

ZnO-based DMSTheoretical prediction of room-temperature
ferromagnetism in DMS
TC of Mn-doped semiconductors
in p-d exchange mechanism

Stability of magnetic states in DMS
in double-exchange mechanism
Ferromagnetic

RoomTemperature

Paramagnetic

300 K

T. Dietl et al. Science ‘00

K. Sato et al. JJAP ‘00.

Wide-gap semiconductors such as ZnO, GaN, and TiO2 are promising
host materials for room-temperature ferromagnetic DMS.

Zn0.98Mn0.02O films prepared under N2atmosphereMagnetization

O 1s XAS spectra

T = 300 K

Holes on oxygen

ZnO
Sample A: PN2 = 1.5 x 10-4 mbar
Sample B: PN2 = 4.0 x 10-4 mbarcf) P. Sharma et al., Nature Mater ‘03.

T. Kataoka et al.

XMCD intensity and magnetization vs H
in N-doped Zn0.98Mn0.02O films
Magnetization

Comparison of magnetization
and XMCD intensity

T = 300 K

Sample A: PN2 = 1.5 x 10-4 mbar
Sample B: PN2 = 4.0 x 10-4 mbar

Small but finiteFM signalcf) P. Sharma et al., Nature Mater ‘03.

T. Kataoka et al.

Mn 2p XAS and XMCD of N-doped
Zn0.98Mn0.02O films
Mn 2p XAS spectra

Mn 2p XMCD spectra

Mn 2p Æ 3d

Mn2+

Mn3+,4+

T. Kataoka et al.

Mn and Co 2p core-level XAS and XMCD of
Zn1-2xMnxCoxO nano-particles

Mn3+

Paramagnetic, unfortunately.

T. Kataoka et al.

Mn and Co 2p core-level XAS and XMCD of
Zn1-2xFexCoxO nano-particles
1.2

ZnO:(Fe, Co)1Fe 2p-3d XAS

0.8

Surface sensitive(~2-3 nm)
Fe3+

0.6

Co3+

Fe2+, Fe3+710

720730Co 2p-3d XAS
Surface sensitive
Co2+, Co3+

0.5

Bulk sensitive(~100 nm)

0.4
700

1ZnO:(Fe, Co)

Bulk sensitiveCo2+0770 780 790 800 810

Photon energy (eV)T. Kataoka et al.

Normalized intensity (%)

XMCD intensity vs H of Zn1-2xFexCoxO
nano-particles
3ZnO:(Fe, Co)

2FeCo
Fe 2p XMCD

1

FM from Fe
Co 2p XMCD

00 Co
not from0.5
Magnetic field (T)1
T. Kataoka et al.

Conclusion••

XMCD of measurements as a function of T and H provide uniqueand important information about dilute magnetism in semiconductors.
Ga1-xMnxAs:– Interstitial Mn are AFM-coupled to substitutional Mn.
– Low-temperature annealing expels interstitial Mn toward surface andincrease the FM moment in bulk.

•Ti1-xCoxO2:– Co2+ ions are responsible for the large FM moment.
– There exists a magnetically dead layer of ~5 nm thickness.

••

Zn1-xCrxTe– Cr2+ drives inhomogeneous distribution of Cr ions and thereby increases
TC.

ZnO-based DMS:

– Films: Hole doping creates FM component.
– Nano-particles: Valence of TM ions increases in the surface region.

XMCD endstation at BL-16A of Photon Factory(present)

Superconducting magnet, up to 5 T
Low temperature down to ~10 K
Angular-dependent XMCD (L, T)
T. Koide et al. Rev．Rev. Sci. Instr. 63, 1462 (1992)
Electron orbit
To Beamline
Undulator I
Kicker magnet

Undulator IISR light

Fast switching(future)Co(4ML)/Pt(10ML)多層膜に対する予備実験結果
軟X共鳴散乱の測定配置
Co 2p edge
1st order reflect2θ scan at Co 2p edge

XMCD

小出，朝倉

Inhomogeneous chemical and electronic
states in high-TC DMS
Small angle neutron scattering Energy-dispersive X-ray spectroscopy image
from La2/3Ca1/3MnO3N-doped
I-dopeduniform
TC low

non-uniform
TC highS. Kuroda et al., Nat. Mater. ‘07

Spinodal decomposition in high-TC DMS’s

Ferromagnetic nano-particles
J.M. De Teresa et al., Nature ‘97

K. Sato et al., JJAP ’05, T. Fukushima et al., JJAP ‘06→Cr2+とCr3+を分離した空間分布, ナノ粒子の電子構造分布