Antiferromagnetic semiconductors: A new half-metallic system

1
Antiferromagnetic semiconductors A new
half-metallic system

• Hisazumi Akai
• Masako Ogura
• Department of Physics
• Osaka University

2
What is half-metallicity?
Half-metallic antiferromagnetic semiconductors

• metallic only in one spin channel, say up.
• semi-conducting or insulating as for the other
  spin channel, i.e., down.
• normally possible for alloys, typically 2 - 4
  components.

EF
metal
E
insulator
3
Typical half-metallic alloys
All ferromagnetic

• half-heusler NiMnSb
• full-heusler Co2MnAl
• perovskites (CaLa)MnO3
• double perovskites Sr2FeReO6
• oxides CrO2
• zinc-blendes CrAs
• diluted magnetic semiconductors (GaMn)As

4
Electronic structure

• in most cases, band structure calculations are
  available.
• half and full heuslers
• I. Galanakis and P.H. Dederichs, Phys. Rev. B66,
  174429 (2002), B66, 134428 (2002).
• zincblende
• M. Shirai, J. Appl. Phys. 93, 6844 (2003).
• I. Galanakis and P. Mavropoulos, Phys. Rev. B67,
  104417 (2003).
• double perovskites
• K.I. Kobayashi et al., Nature 395, 677 (1998),
  Phys. Rev. B 59, 11159 (1999).
• W.E. Pickett, Phys. Rev. B57 10613 (1998).

5
Full heusler alloys
I. Galanaskis et al., Phys. Rev B 66, 174429
(2002)
6
Magnetization of full heusler alloys

• Slator-Pauling like behevior

n 3 t1u 3 t2g 2 eg 1 s 3 p 12
I. Galanaskis et al., Phys. Rev B 66, 174429
(2002)
M Z-24
7
Half heusler alloys
M Z-18
I. Galanaskis et al., Phys. Rev B 66, 134428
(2002)
8
Zinc-blendes
M Z-8
9
Double perovskites
B-site ordering
Sr2FeMoO6 half-metallic ferro Sr2FeWO6
antiferro Sr2FeReO6 half-metallic ferro
K.I. Kobayashi et al., Phys. Rev. B 59, 11159
(1999).
10
New class of half-metallic systems DMS

• DMS diluted magnetic semiconductors
• (In, Mn)As
• (Ga, Mn)As
• (Zn, Co)O, (Zn, V)O
• (Ga, Mn)N, (Ga, Cr)N
• (Cd, Mn)GeP2, (Zn, Mn)GeP2
• (Ti, Cr)O2
• (Zn, Cr)Se
• .

11
Theoretical Approach todiluted magnetic
semiconductors
(In100-x-y-z Mn?x Mn?y Asz) As
H. Akai, Phys. Rev. Lett. 81, 3002 (1998).
12
Carrier induced ferromagnetism and the doping
effects

(In
Mn
Mn
?
A
)As
94-x
6-y
y
x
E(mRy)
Concentration of A ()
Carrier concentration
13
Mechanism of ferromagnetism

• Doble exchange vs. superexchan
• Impurity bands
• Symmetry of orbitals

Mn2
Mn3
14
Zinc-blende CrAs
ferro E0Ry
non-magnetic E0.0734Ry
spin-glass E0.0205Ry
Tc1880 C
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Double exchange (many electron picture)
down
Ferro
DE -Vdp2/D
up
D
Cr
Cr
As
Vdp
Vdp
Superexchange
Antiferro
DE -Vdp4/D2/JH
JH
Cr
Cr
As
16
Zinc-blende MnSb
ferro
non-magnetic
spin-glass
Tc970 C
Does double exchange explain ferromagnetism?
17
Local DOS of MnSb
Zenners pd-hybridization mechanism also seems to
work.
18
Pd hybridization (many electron picture)
Ferro
DE -4Vdp2/D
Sb
Vdp
Vdp
Mn
Mn
Antiferro
DE -2Vsp2/D
Sb
Mn
Mn
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What is the half-metallic antiferromagnet ?
From symmetry, it is impossible to realize
half-metallic bulk antiferromagnetism.
Yet, it is possible to realize ferrimagnetism
with zero magnetization.
Could be even more useful than half-metallic
ferromagnets. High TN, insensitive to external
fields,

• van Leuken and de Groot, Phys. Rev. Lett. 74,
  1171 (1995).

20
Theoretical predictions

• W.E. Pickett, Phys. Rev. B57, 10613 (1998).

Half-metallic antiferromagnetism (ferri)
La2VCuO6 La2MnVO6 La2MnCoO6
Mn3d
V3d
(B-site ordering double provskites)

• New type of spintronics materials
• Single spin superconductors

V3d
Mn3d
EF
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Prediction by Pickett
Phys. Rev. B57, 10613 (1998)
EF.EA.F.
V d1 - Cu d9
EF.gtEA.F.
Mn d4 - V d2
EF.ltEA.F.
Mn d4 - Co d6
22
Experiments

• M. Uehara, M. Yamada and Y. Kimishima, Solid
  State Commun. (2003)

CaLaVMoO6 TN120K SrLaVMoO6 TN130K
(AB-site ordering double perovskites)
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AB-site ordering type
CaLaVMoO6
M. Uehara, M. Yamada and Y. Kimishima, Solid
State Commun. 129, 385 (2004).
experiment
CaLaVMoO6 TN120K SrLaVMoO6 TN130K
calculation Ferrimagnet m0.6mB
Very close to half-metallic antiferromagnet
24
Other candidates

• Band gap technology
• Semiconductors
• Chalcopyrites
• Many others

EF
metal
CB
VB
E
insulator
25
HM AF DMS

• HF ferromagnetic DMS
• double exchange and/or pd hybridization
• HF antiferromagnetic DMS
• due to mainly double exchange

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Traditional DMS (single electron picture)
E
E

• Consider the situation where the ttwo same
  magnetic ions whose d electron number more than
  or less than 5 exist
• Compare the parallel (upper) or anti-parallel
  (lower) coupling of the local magnetic moments.
• Anti-parallel coupling is stabilized by the
  super-exchange (2nd order perturbation.)
• Parallel coupling is stabilized by the double
  exchange (1st order perturbation.)
• Normally the 1st order perturbation prevails the
  2nd order perturbation and the ferromagnetic
  coupling is more stable than antiferromagnetic
  one
• However, near the half-filling, the double
  exchange that is proportional to the number of
  holes or electrons does not work anymore and the
  antiferromagnetic coupling is realized.

hybridization
ferro
hybridization
Cr
Cr
E
E
antiferro
hybridization
Cr
Cr
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HM ferromagnets

• From the figures it is clear that the
  ferromagnetic coupling may cause the
  half-metallic electronic structure.
• On the other hand the half-metallic electronic
  structure is impossible for the antiferromagnetic
  coupling.

E
E
E
Ferro
E
E
E
Cr
Cr
Cr
Cr
half metallic
Antiferro
metallic
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When two magnetic ions coexist
E
E
Ferro

• Consider two magnetic ions, one of them (e.g. V)
  being more than half and the other (e.g. Co)
  less than half.
• Compare two cases, parallel or anti-parallel
  magnetic coupling.
• For parallel (ferromagnetic) coupling the
  superexchange plays a role.
• On the other hand, for antiparallel
  (antiferromagnetic) coupling the double exchange
  works.
• In contrast to normal DMSs, the double exchange
  stabilizes antiferromagnetism while the
  superexchange does ferromagnetism.

hybridization
V
Co
E
E
Antiferro
hybridization
hybridization
V
Co
29
In electron-hole picture
E
E
Ferro
hybridization
V
Co
Electron state
hole state
E
E
Antiferro
hybridization
V
Co
30
HM antiferromagnets

• While the double exchange is the first order
  process with respect to the electron transfer,
  the superexchange is the second order process.
  For this reason the antiferromagnetic coupling is
  preferable in the present situation.
• Moreover, the antiferromagnetic state is
  halfmetallic in contrast to the normal DMSs.

E
E
E
Ferro
E
E
V
Co
metallic
E
V
Co
Antiferro
half metallic
31
(ZnCrFe)S
AF
F
SG
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Calculated TN
33
(ZnVCo)S
AF
VB
CB
half metallic
F
VB
CB
metallic
SG
VB
CB
34
(ZnCrCo)O
AF
VB
CB
half metallic
SG
VB
CB
metallic
35
(ZnCrFe)Se
AF
VB
CB
half metallic
SG
VB
CB
metallic
36
III-V based HM AF DMS?
Ferrimagnets but still metallic
(InMnFe)As
(GaCrCo)As
37
Calculated TN
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Other examples

• host semiconductors
• II-VI compound semiconductors
• ZnO, ZnSe, ZnS, ZnTe,CdTe, etc.
• III-V compounds semiconductors
• GaAs, GaN, InAs, AlAs, InSb, GaSb, GaP, InP, etc.
• magnetic ions substituted for II or III elements
  of the host semiconductors
• Ti-Ni, V-Co, Cr-Fe, Cr-Co, V-Fe, etc.
• Ti-Fe, Ti-Co, V-Fe, V-Ni, Cr-Co, Cr-Ni, etc.
• Ti-V-Ni, Ti-V-Co, Ti-Ni-Co, V-Ni-Co, etc.

39
And more
Chalcopyrites

• host semiconductors
• I-III-VI chalcopyrite type semiconductors
• CuAlS2, AgCaS2, etc.
• II-IV-V chalcopyrite type semiconductors
• CdGeP2, ZnGeP2, CdGeAs2, etc.
• magnetic ions substituted for I, II or III
  elements
• Ti-Ni, V-Co, Cr-Fe
• Ti-Fe, Ti-Co, V-Fe, V-Ni, Cr-Co, Cr-Ni, etc.
• Ti-V-Ni, Ti-V-Co, Ti-Ni-Co, V-Ni-Co, etc.

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Possible applications

• Magnetic RAM
• single-spin super conductors
• spin injection devices
• magneto optical devices
• spin-transistor
• spin FET

41
Summary

• Half-metallic antiferromagnets
• new spintronics materials
• single spin superconductors
• etc.
• Other possible candidates
• Semiconductors
• Promising new spintronics materials