Magnetic semiconductorsMagnetic semiconductors

• Electronic structure of diluted magnetic semiconductors

• Ferromagnetic semiconductors– Global electronic structure– Electronic structure near the chemical potential -

charge carriers• Soft x-ray magnetic circular dichroism studies

– Identification of ferromagnetic component– Local magnetic susceptibility

Magnetic semiconductorsMagnetic semiconductors

• Electronic structure of diluted magnetic semiconductors

Diluted magnetic semiconductors ADiluted magnetic semiconductors A11--xxMnMnxxX X based on IIbased on II--VI compoundsVI compounds

random Mn substitution

Cd2+1-xMn2+

xTe： Giant Faraday rot.Optical isolators

Zn2+1-xMn2+

xS：Electro-luminescence

InsulatingAF/spin glass

Cd2+Te

Zn2+S

Resonant photoemission spectroscopyResonant photoemission spectroscopyto extract to extract MnMn 33dd partial DOSpartial DOS

Mn

Mn

Mn

Mn

Resonant photoemission from CdResonant photoemission from Cd11--xxMnMnxxTeTe

CI Cluster-model calc.

Band theory

PES BIS

Cd0.35Mn0.65Te Mn 3d PDOS

Unoccupied Mn 3d

satellite

satellite

on-resonance

off-resonance

e↑

t2↑

e↓t2 ↓

PES: L. Ley et al., PRB ’87, BIS: A. Franciosi et al, PRB ’89Band-structure calc: S. H. Wei and A. Zunger, PRB ‘87 Cluster-model calc: T. Mizokawa and A. Fujimori, PRB ‘96Anerson-model calc: O. Gunnarsson et al., PRB ‘88

Electronic structure of CdElectronic structure of Cd11--xxMnMnxxTeTe

e↑

e↓

t2↑

t2 ↓

µ

M. Taniguchi et al., PRB ’86

ConfigurationConfiguration--interaction (CI) cluster model interaction (CI) cluster model for for MnMn impurity in semiconductorimpurity in semiconductor

p hole

d electronNβ

Transfer

integrals: T pd

Charge

-tran

energ

y:

∆

Coulombrepulsion energy: U

Mn

sfer

U, ∆, Tpd : adjustable parameters

-p-d exchange coupling Nβ ~ - Tpd2/(U - ∆ )

Giant magnetoGiant magneto--optical effect through optical effect through pp--dd exchange mechanismexchange mechanism

Mn 3d band(LHB)

Mn 3d band(UHB)

p valence band

conduction bandCd1-xMnxTe∆= 2 eVU = 4 eV

-

p-d exchange couplingNβ ~ - Tpd

2/(U - ∆ )

∆

U-∆

U

p-dexchangeNβ ~ - 1 eV

pp--dd exchange constant exchange constant NNββ for TM impurities in for TM impurities in IIII--VI semiconductors VI semiconductors

-

T. Mizokawa and A.Fujimori, PRB ‘97

Donor and acceptor levels for TM impurities Donor and acceptor levels for TM impurities in IIin II--VI semiconductors VI semiconductors

T. Mizokawa and A.Fujimori, PRB ‘93

Magnetic semiconductorsMagnetic semiconductors

• Ferromagnetic semiconductors

Ferromagnetism in MBEFerromagnetism in MBE--grown grown MnMn--dpoeddpoedIIIIII--V semiconductorsV semiconductors

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

H. Ohno et al., JMMM, ‘99T. Hayashi, M. Tanaka, J. Cryst. Growth, ‘97

Ferromagnetic semiconductors for Ferromagnetic semiconductors for spintronics applications spintronics applications

• Non-volatile memories• GMR devices• New devices utilizing spin injection

Si, Ge GaAs, InAs, ...Y. Ohno et al., Nature ‘99

Transport and magnetoTransport and magneto--transport properties transport properties of Gaof Ga11--xxMnMnxxAs As

Magneto-resistance

~ magnetization

Tc

Resistivity

F. Matsukura et al. PRB ‘98A. Oiwa et al., Solid State Commun. ’97

Magnetic semiconductorsMagnetic semiconductors

• Ferromagnetic semiconductors– Global electronic structure

Resonant photoemission from GaResonant photoemission from Ga11--xxMnMnxxAsAs

On resonance

Off resonance

Mn 3d DOS

J. Okabayashi et al. PRB ‘99

satellite

Semi-insulating GaAs(001)

GaMnAs

As cap

Photon Factory surface interface beamline BL-18A

Comparison with bandComparison with band--structure calculation structure calculation and clusterand cluster--model calculationmodel calculation

p-d exchange Nβ = -1 eV

Mn3+ (substituting Ga3+)Mn2+ + hole

satellite

CI cluster-model calc.assuming Mn2+

Mn is divalent: Mn2+

∆ = 1.5 eVU = 3.5 eV

L. Ley et al., PRB ‘87

Band theory

satellitesatellite

CarrierCarrier--induced ferromagnetism throughinduced ferromagnetism throughpp--dd exchange mechanismexchange mechanism

hole

Ga3+ → Mn3+ → Mn2+ ＋ holesubstitution

p valence band

conduction band Nβ ∼ -1 eV

p hole

Mn 3d band(LHB)

Mn 3d band(UHB)

∆

U-∆ p-d exchange couplingNβ ~ - Tpd

2/(U - ∆ ) ~ -1 eV

large ∆ large Nβdeep p valence band Wide-gap semiconductor

U

Curie temperatures for Curie temperatures for MnMn--doped doped pp--type semiconductors type semiconductors

if hole-doped

Wide-gap semiconductors are predicted to have high TC’sT. Dietl et al, Science (2000)

MnMn 33dd DOS in various DOS in various DMSDMS’’ss and and their CI clustertheir CI cluster--model analysesmodel analyses

satellite

pp--dd exchange coupling from CI clusterexchange coupling from CI cluster--model analysesmodel analyses

∆ U (pdσ) Nβ Ref. .

In1-xMnxAs 1.0 3.5 -0.8 -0.7 1Ga1-xMnxAs 1.5 3.5 -1.0 -1.0 2

Ga1-xMnxN 4.0 5.0 -1.5 -1.6 3

Cd1-xMnxTe 2.0 4.0 -1.1 4

Zn1-xMnxO 6.5 5.2 -1.6 -2.7 5Zn1-xMnxS 3.0 4.0 -1.3 -1.3 4Zn1-xMnxSe 2.0 4.0 -1.1 -1.0 4Zn1-xMnxTe 1.5 4.0 -1.0 -0.9 4

Ref.1 J. Okabayashi et al., PRB ‘02Ref.2 J. Okabayashi et al., PRB ‘99Ref.3 J.-I. Hwang et al., PRB, in pressRef.4 T. Mizokawa et. al., PRB ‘93Ref.5 T. Mizokawa et. al., PRB ‘02

Wide-gap semiconductors indeed have large Nβ.However, it is difficult to dope them with holes.

Magnetic semiconductorsMagnetic semiconductors

• Ferromagnetic semiconductors– Electronic structure near the chemical potential –

charge carriers

Band structure of GaAs and GaBand structure of GaAs and Ga11--xxMnMnxxAsAs

8

6

4

2

0

GaAs

∆ ΓX

Bin

ding

Ene

rgy

(eV

)

8

6

4

2

0

Ga0.965Mn0.035As

∆ ΓX

Bin

ding

Ene

rgy

(eV

)J. Okabayashi et al. PRB 2001

GaAs Ga0.965Mn0.035As

““ImpurityImpurity”” band near Eband near EFF in Gain Ga11--xxMnMnxxAsAs

Ga0.931Mn0.069As - GaAs difference

GaMnAsGaAs

“impurity” band

“impurity” band

J. Okabayashi et al. PRB 2001

Fermi edge in Fermi edge in GaGa11--xxMnMnxxAs ?As ?

J. Okabayashi et al.

-2.0 -1.5 -1.0 -0.5 0 0.5eVEnergy Relative to EF (eV)

Ga0.95Mn0.05As

11K 300 K GaAs (300 K)

Au

hν=50 eV

Inte

nsity

(arb

. uni

ts)

MBE-in situ PES exptPhoton Factory BL-1CWeak Fermi edge at low temperatures

Disappears at high temperatures

-1.0 -0.8 -0.6 -0.4 -0.2 0eV

11K 300 K GaAs

Ga0.95Mn0.05As; hν=80 eV

Energy Relative to E F (eV)In

tens

ity (a

rb. u

nits

)

Au

“impurity” band

““ImpurityImpurity”” band near Eband near EFF in Gain Ga11--xxMnMnxxAsAs

wide energy distribution

low DOS at EF

acceptor level

Undoped semiconductor

Doped semiconductor

Heavily doped, degenerate semiconductor

Mn-doped semiconductor

HightHight DOS at EDOS at EFF according to bandaccording to band--structure structure calculationscalculations

0-4-8 4

J.H. Park et al., Physica B ’00. K. Sato et al., Europhys. Lett. ‘03

NonNon--Drude behavior of optical conductivity Drude behavior of optical conductivity in Gain Ga11--xxMnMnxxAsAs

K. Hirakawa et al., PRB ’02.no Drude peakno Drude peak

Phonon peaks

Y. Nagai et al., JJAP ’01.

Magnetic Magnetic polaronspolarons in in GaGa11--xxMnMnxxAs ?As ?

Weak or no Fermi edge in photoemission spectraNo Drude weight in optical spectra

A. Kaminski and S. Das Sarma, PRL ‘02

--- (Magnetic) polaron ?Incoherent metal

Magnetic semiconductorsMagnetic semiconductors

• Soft x-ray magnetic circular dichroism studies– Identification of ferromagnetic component

Soft xSoft x--ray magnetic circular dichroism (XMCD)ray magnetic circular dichroism (XMCD)in corein core--level xlevel x--ray absorption (XAS) spectraray absorption (XAS) spectra

spin sum rule

orbital sum rule

Mn 2pcore level

Mn 3dvalence level

hν

+sensitive to local chemical environment

microscopic, element specific probe

Sensitivity of XAS and XMCD to chemical Sensitivity of XAS and XMCD to chemical environmentenvironment

Atomic Co in K Co metal

Atomic multiplet structure characteristic of * valence, * spin, * crystal field

P. Gambardella et al., PRL ‘02 C.T. Chen et al., PRL ‘95

MCD装置

ARPES装置

XMCD endstation at JAERI beamline BLXMCD endstation at JAERI beamline BL--23SU 23SU of Springof Spring--88

Helical undulator with phase modulation

Superconducting magnet, up to 10 TLow temperature, down to 10 KHigh energy resolution and brightness

10m VLS PGMY. Saitoh et al., Nucl. Instrum. Meth. A ‘01

RoomRoom--temperature ferromagnetism in Titemperature ferromagnetism in Ti11--xxCoCoxxＯＯ22

TEM SEMAnatase

Rutile

XRD, UV-VIS MCD --> no precipitation

(a) Co L2,3-edge XAS

Photon Energy (eV)

780 790 800

Intensity (Arb. U

nits)

0

1

2

3

4

5

6

L3 L2

As-grown

2-min.

10-min.

20-min.

Co-metal

(b) Co L2,3-edge XMCD

Photon Energy (eV)

780 790 800 8100

2

4

6

8

As-grown

2-min.

10-min.

20-min.

Co-metal

ρ+

ρ−

(c) ρ+ − ρ−

Photon Energy (eV)

780 790 800 810-1

0

1

2

3

4As-grown

2-min.

10-min.

20-min.

Co-metal

Y.J. Kim et al. PRL ‘03

Ferromagnetism is due to Co metal segregation.

Co 2Co 2pp corecore--level XAS and XMCD of Tilevel XAS and XMCD of Ti11--xxCoCoxxＯＯ22: : Effect of annealingEffect of annealing

~400oC ~400oC

Anatase

RoomRoom--temperature ferromagnetism temperature ferromagnetism in Znin Zn11--xxCoCoxxOO

carrier concentration

Magnetization

H. Saeki et al., J. Phys. Cond. Mat. ‘04

O CZn o

M. Venkatesan et al., PRL ‘04

Co 2Co 2pp corecore--level XAS and XMCD of Znlevel XAS and XMCD of Zn11--xxCoCoxxO:O:Comparison with Co metalComparison with Co metal

M. Kobayashi et al., cond-mat/05, to PRB

Co metal MCD : C. T. Chen et al, PRL ‘95

2.0

1.5

1.0

0.5

0

Abso

rptio

n (a

rb. u

nits)

µ+

µ−

Back Ground

Zn1-xCoxO (x=0.05) Co L2,3 edge

Magnetic Field : 4.5 TTemperature : 20 K

2.0

1.5

1.0

0.5

0

XAS

(arb

. uni

ts)

(µ+ + µ−)/2

-0.10

-0.08

-0.06

-0.04

-0.02

0

0.02

XMCD

(arb

. uni

ts)

800795790785780775770

Photon Energy (eV)

µ+ − µ

−

-0.10

-0.08

-0.06

-0.04

-0.02

0

0.02

XM

CD (a

rb. u

nits)

800795790785780775770

Photon Energy (eV)

Zn0.95Co0.05O Co metal

Co L2,3 edge

-0.08

-0.04

0

782780778776

Co L3

Co metal

Zn1-xCoxOXAS

MCD

Comparison with Co metal

Co 2Co 2pp corecore--level XAS and XMCD of Znlevel XAS and XMCD of Zn11--xxCoCoxxO:O:Comparison with atomic Comparison with atomic multipletmultiplet calc.calc.

784782780778776Photon Energy (eV)

Co L3

798796794792790Photon Energy (eV)

Co L2

XA

S (a

rb. u

nits

)

800795790785780775770Photon Energy (eV)

Co2+, 10Dq=-0.7 eV

Co2+, 10Dq=+0.5 eV

Co3+, 10Dq=-0.5 eV

Co3+, 10Dq=+0.5 eV

Zn0.95Co0.05O Co L2,3 XAS at H=7.0T, T=20K

XM

CD

(arb

. uni

ts)

800795790785780775770Photon Energy (eV)

Zn0.95Co0.05O Co L2,3 XMCD at H=7.0T, T=20K

Co2+, 10Dq=-0.7 eV

Co2+, 10Dq=+0.5 eV

Co3+, 10Dq=-0.5 eV

Co3+, 10Dq=+0.5 eV

784782780778776Photon Energy (eV)

Co L3

798796794792790Photon Energy (eV)

Co L2

・10Dq > 0Oh symmetry

・10Dq < 0Td symmetry

Ferromagnetismis due to Co2+

substituting Zn

M. Kobayashi et al., cond-mat/05, to PRB

Room temperature ferromagnetism in Room temperature ferromagnetism in ZZnn11--xxCrCrxxTeTe

H. Saito et al., PRL ‘03

Cr 2Cr 2pp corecore--level MCD of Znlevel MCD of Zn11--xxCrCrxxTe (x=0.045)Te (x=0.045)

590580570

XA

S or

XM

CD

(arb

. uni

ts)

Photon energy (eV)

Te M4,5 µ+

µ-

(µ++ µ-)/2

(µ+- µ-)/2

XAS

XMCD×5

Zn1-xCrxTe (x = 0.045)B = 2 TT = 20 K

Cr L3 Cr L2

0.20

0.15

0.10

0.05

0.0086420

Field (T)Cr L

3 XM

CD

inte

nsity

(a.u

.)

T = 20 K

0.20

0.15

0.10

0.05

0.00250200150100500

×5

Temperature (K)Cr L

3 XM

CD

inte

nsity

(a.u

.)

B = 2 T B = ~0.1 T TC ~ 70 K

Y. Ishida et al.

Comparison with atomic multiplet calculationComparison with atomic multiplet calculation

600590580570

10 Dq = 3.0 eV

2.5

2.0

1.5

1.0

0.5

0

Cr2+(d4) Tetrahedral

Hig

h sp

in

Low

spin

600590580570

Cr2+(d4) Octahedral10 Dq = 3.0 eV

2.5

2.0

1.0

1.5

0.5

0

Photon energy (eV)

600590580570

Cr3+(d3) Tetrahedral

10 Dq = 3.0 eV

2.5

2.0

1.5

1.0

0.5

0

Expt

G. van der Laan and I.W. Kirkman, ‘92

t2

e

Magnetic semiconductorsMagnetic semiconductors

• Soft x-ray magnetic circular dichroism studies– Local magnetic susceptibility

Soft xSoft x--ray magnetic circular dichroism (XMCD)ray magnetic circular dichroism (XMCD)in corein core--level xlevel x--ray absorption (XAS) spectraray absorption (XAS) spectra

A1 - A2

spin sum rule

orbital sum rule

Mn 2pcore level

Mn 3dvalence level

hν

microscopic, element specific probe+

magnetism specific probeParamagnetic ? Ferromagnetic ?

Separation of XMCD signals into ferromagnetic Separation of XMCD signals into ferromagnetic and paramagnetic componentsand paramagnetic components

SQUID data of DMS thin film sample

-150

-100

-50

0

50

100

150

Mag

netiz

atio

n x1

0-6 (e

mu)

-20 -10 0 10 20Magnetic Field (kG)

@5K @300K

M = Mdia + Mferro + Mpara

XMCD of ferromagnetic component

XMCD of paramagnetic component

300 K

5 K

or

XMCD signals

No contribution to XMCD signals

Indication of multiple Indication of multiple MnMn species in Gaspecies in Ga11--xxMnMnxxAs As

Magnetization curvesCurie temperature of Ga1-xMnxAs

T = 2 K

x = 0.071

0.053

0.043

0.035

A. Oiwa et al., Solid State Commun. ’97 H. Ohno et al., JMMM, ‘99

Interstitial Interstitial MnMn in Gain Ga11--xxMnMnxxAs ?As ?

Mn substituting Ga sites (MnGa): Mn3+ Mn2+ + holeMn at interstitial sites (MnI): Mn0 Mn2+ + electrons: compensates holes !

Change of TC by post annealing Molecular dynamics simulation of MBE

As Ga

Annealing converts MnI to MnGa ? First, Mn enters interstices then Ga siteS.J. Potashnik et al., APL (2001) S.C. Erwin and A.G. Petukhov PRL ‘02

Two signals in Two signals in MnMn 22pp XAS spectra of GaXAS spectra of Ga11--xxMnMnxxAsAsand LTand LT--annealing effectannealing effect

Interstitial Mn ?Substitutional Mn ?

Y. Ishiwata et al., PRB ‘02

Previous XMCD studies of GaPrevious XMCD studies of Ga11--xxMnMnxxAs As

x = 0.02T = 15 - 30 KH = 0.55 T

Mn 2p Mn 2p

As 2p3/2

Evidence for carrier-induced magnetism

Y. L. Soo et al., PRB ‘03

H. Ohldag et al., APL ‘00 D. J. Keavney et al., PRL ‘03

MnMn 22pp XMCD spectra of GaXMCD spectra of Ga11--xxMnMnxxAs As

Y. Takeda et al.

s.i. GaAs(001)

GaMnAs

As cap

20 nm1 nm

GaAs

Tc = 40 K

10Dq = -0.7 eV

A. Tanaka

Atomic multiplet calc. for Mn2+

MnMn 22pp XMCD spectra of GaXMCD spectra of Ga11--xxMnMnxxAs As

TC ~ 40 K

Y. Takeda et al.

MagneticMagnetic--field and temperature dependencesfield and temperature dependencesof XMCD intensity of XMCD intensity

20 K < Tc

100 K > Tc

4.2%

4.2%

7.8 %

7.8 %Ferromagnetic

component

Paramagnetic component

4.2%: TC ~ 40 K7.8%: TC ~ 60 K

Y. Takeda et al.

MnMn 22pp XMCD spectra of GaXMCD spectra of Ga11--xxMnMnxxAs As

TC ~ 40 K

PF

Y. Takeda et al.

Decomposition of XMCD signals intoDecomposition of XMCD signals intoferromagnetic and ferromagnetic and ““paramagneticparamagnetic”” componentscomponents

F

P

4.2%

4.2%

7.8 %

7.8 %

T = 20 K < Tc

PF

4.2%: TC ~ 40 K7.8%: TC ~ 60 K

Y. Takeda et al.

Two signals in Two signals in MnMn 22pp XAS spectra of GaXAS spectra of Ga11--xxMnMnxxAsAsand LTand LT--annealing effectannealing effect

Interstitial Mn ?Substitutional Mn ?

Substitutional MnOut-diffused Mn from interstitial sites

Fist-principles calc.In situ Auger, resistivity meas.K. W. Edmonds et al, PRL ‘04

Y. Ishiwata et al., PRB ‘02