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