phase separation effects in diluted magnetic semiconductors collaborators: t. andrearczyk, p....
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Phase separation effects in diluted magnetic semiconductors
Phase separation effects in diluted magnetic semiconductors
collaborators:
T. Andrearczyk, P. Kossacki, J. Jaroszyński, M. Sawicki – Warsaw
F. Matsukura, H. Ohno – Sendai
K. Edmonds, C.T. Foxon, B.L. Gallagher, K.Y. Wang – Nottingham
J. Cibert, D. Ferrand – Grenoble
G. Bauer, A. Bonanni, W. Jantsch – Linz
D. Kechrakos, N. Papanikolaou, K. N. Trohidou -- Athens
support: Ohno Semiconductor Spintronics ERATO Project of JST
NANOSPIN -- EC projects
Humboldt Foundation
Tomasz DIETLInstitute of Physics, Polish Academy of Sciences
Institute of Theoretical Physics, Warsaw University
Ga1-xMnxAs: resistance vs. temperature and Curie temperature vs. x
Ga1-xMnxAs: resistance vs. temperature and Curie temperature vs. x
• ferromagnetism on both sides of metal-insulator transitions• ferromagnetism disappears in the absence of holes
Matsukura et al. (Tohoku) PRB’98
III-V DMS
Effect of acceptor doping on magnetic susceptibility in Zn1-xMnxTe:P
Sawicki et al. (Warsaw) pss’02
-1 vs. T
• ferromagnetism driven by hole doping • competition between intrinsic short-range AFM and hole-induced long-range FM
II-VI DMS
Ferromagnetic temperature in p-(Zn,Mn)TeFerromagnetic temperature in p-(Zn,Mn)Te
Ferrand et al. (Grenoble, Warsaw) PRB’01Sawicki et al. (Warsaw) pss’02
1
10
1
10
3030
Fer
rom
agn
etic
Tem
p.
T F /
x eff
(K) 10
1710
1810
1910
205x1020
Hole concentration (cm-3)
(Zn,Mn)Te:P
(Zn,Mn)Te:N
InsulatingMetallic
• ferromagnetism on both sides of metal-insulator transition
1/
Where are we?
-1 0 1 2 3
-0.05
0.00
0.05
0.10
T = 175 K
T = 172 K8% (Ga,Mn)As
M[1
10](T
) / M
Sat
(5K
) [
r.u
. ]
Magnetic Field [ Oe ]
Wang/ Sawicki (Nottingham, Warsaw)ICPS’04
remanent magnetisation and 1/ vs. Thysteresis loops
MREM
TC = 173 K
TC CW
Semiconductor materials showing hysteresis and spontaneous magnetisation at 300 K
wz-c-(Ga,Mn)N, (In,Mn)N, (Al,Mn)N, (Ga,Cr)N, (Al,Cr)N (Ga,Fe)N (Ga,Gd)N, (Ga,Eu)N (Ga,Mn)As, (In,Mn)As, (Ga,Mn)Sb, (Ga,Mn)P:C
(Zn,Mn)O, (Zn,Ni)O, (Zn,Co)O, (Zn,V)O, (Zn,Fe,Cu)O, (Zn,Cu)O
(Zn,Cr)Te
(Ti,Co)O2, (Ti,V)O2, (Ti,Cr)O2, (Sn,Co)O2, (Sn,Fe)O2, (Hf,Co)O2
(Cd,Ge,Mn)P2, (Zn,Ge,Mn)P2, (Cd,Ge,Mn)As2, (Zn,Sn,Mn)As2
(Ge,Mn), (Ge,Cr), (Ge,Mn,Fe) (La,Ca)B6, C, C60, HfO2, (Ga,Gd)N – materials in which magnetic
moment is claimed to do not come from 3d or 4f shell will not be discussed cf. G. Bouzerar
SQUID studies of DMS in Warsaw
M. Sawicki et al.:
wz-c-(Ga,Mn)N, (Ga,Fe)N
(Ga,Mn)As
(Zn,Mn)Te:N, P
(Cd,Mn)Te, (Cd,Mn)Se
(Cd,Cr)Te, (Zn,Cr)Se
(Zn,Mn)O, (Zn,Co)O, (Zn,Cr)O
Today’s talk
• „low” TC ferro DMS
-- metallic side -- insulator side – electronic phase separation
• „high” TC ferro DMS
– chemical phase separation
cf. A. Moreno
p-d Zener/RKKY model of hole-controlled ferromagnetism in DMS
Driving force: lowering of the hole energy due to redistribution between hole spin subbands split by p-d exchange interaction
T.D. et al.,’97-Jungwirth et al. (Austin/Prague) ’99-
k
EF
p-d Zener/RKKY model of hole-controlled ferromagnetism in DMS
Driving force: lowering of the hole energy due to redistribution between hole spin subbands split by p-d exchange interaction, ~ M
T.D. et al.,’97-MacDonald et al. (Austin/Prague) ’99-
No adjustable parameters
TC ~ 2(s)DOS
Essential ingredient: Complexity of the valence band structurehas to be taken into account
M
k
EF
Mn-based p-type DMS to which p-d Zener model has been found to apply
Expl.: Tohoku, Tokyo, Grenoble, Wuerzburg, PSU, Notre Dame, UCSB, Nottingham, …
xMn = 5%p = 3.5x1020 cm-3
• TC CW
• TC (p,x) consistent with
p-d Zener model• not double exchange
Insulator side of metal-to-insulator transitionAnderson-Mott localization
Small hole concentration rs > 2.4 because of either:
-- small acceptor concentration
-- large compensation
-- depletion by gates
-- depletion at surfaces and interfaces
e.g. TAMR devices of (Ga,Mn)AS Ruster et al. (Wuerzburg) PRL’05
Giddings et al. (Hitachi, Nottingham) PRL’05
Insulator side of metal-to-insulator transition
Suggested model:
percolation of bound magnetic polarons
Bhatt et al. (Princeton) PRL’02; Das Sarma et al., PRL’02,’04, ....
p-type(II,Mn)VI
(III,Mn)V
Resistivity and magnetisation in (Ga,Mn)As
4 K
Co-existence of ferromagnetic and paramagnetic components in non-metallic samples
F. Matsukura et al..(Tohoku) PRB ’98, SSC’97
104
102
100
10-2
1.5 2 5 10Temperature (K)
Res
istiv
ity (
Ohm
cm
)
B = 0
B = 11 T
(Zn,Mn)Te:Nx = 3.8%
p = 3x1019 cm-3
Collosal negative magnetoresistance on insulator side of MIT
Ferrand et al. (Grenoble, Warsaw) PRB’02
104
102
100
10-2
1.5 2 5 10 Temperature (K)
Res
istiv
ity (
Ohm
cm
)
B = 0
B = 11 T
(Zn,Mn)Te:Nx = 3.8%
p = 3x1019 cm-3
Collosal negative magnetoresistance on insulator side of MIT
Ferrand et al. (Grenoble, Warsaw) PRB’02
Katsumoto et al. (Tokyo) pss’98Reminiscent to CMR oxides
Ferromagnetism on insulator side of MIT-- competing models
• Percolation of bound magnetic polarons
• Ferromagnetic metallic-like regions embeded in insulating paramagnetic matrix
electronic nanoscale phase separation
To tell the model:
• inelastic neutron scattering Kepa et al. (Warsaw, NIST) PRL’03
• search for collosal MR in modulation-doped quantum wells, where no BMP are expected Jaroszynski et al. (Warsaw, NHMFL) cond-mat/0509
• Monte Carlo + Schroedinger eq. with magnetic disorder
Dechrakos et al. (Athenes, Warsaw) PRL’05
cf. E.L. Nagaev, E. Dagotto et al.
Probing competing AF and FM interactions by inelastic neutron scattering in p-(Zn,Mn)Te
Probing competing AF and FM interactions by inelastic neutron scattering in p-(Zn,Mn)Te
Kępa et al. (Warsaw, NIST) PRL’03
inelastic neutron scatteringof n.n. Mn pairs
large single crystals of Zn0.95Mn0.05Te:P
p = 5x1018 cm-3, TCW = 2 K
Insulator side of the MIT
Zn0.95Mn0.05Te
Hint = -2(JAF + Jh)SiSj
JAF < 0 super-exchange Jh > 0 hole-induced
Hole induced contributionHole induced contribution
empty dots - no holes, full dots – with holes
E = 2Jh = 0.03 0.006 meV
E RKKY = 0.020 meV
E BMP = 0.12 meV
Resistivity vs. carrier density at various Tin (Cd,Mn)Te/(Cd,Mg)Te:I quantum well
Jaroszynski et al. (Warsaw, NHMFL) cond-mat/0509 submitted to PRL
Electron density (cm-2)
Resistivity vs. carrier density at various Tin (Cd,Mn)Te/(Cd,Mg)Te:I quantum well
Jaroszynski et al. (Warsaw, NHMFL) cond-mat/0509 submitted to PRL
Electron density (cm-2)
Resistivity vs. carrier density at various Tin (Cd,Mn)Te/(Cd,Mg)Te:I quantum well
Electron density (cm-2)
Interpretation: nanoscale electronic phase separation into metallic ferromagnetic regions embeded in isolating paramagnetic matrix
Localization length >> rs
Ferromagnetic coupling via weakly-localised holesFerromagnetic coupling via weakly-localised holes
•At the distance between Mn ions wave function can be regarded as
extended =>only part of the spins contribute to the ferromagnetic signal
Random distribution of acceptors and spins Metallic and ferromagnetic lakes embedded in insulating matrix
Ferromagnetism of (Ga,Mn)N – effect of doping
Reed et al. (NCSU) APL’05
(Ga,Mn)Nx = 0.2%
TC >> 300 K
(Ga,Mn)N, x = 0.2%TC 0 for Si doping
(Ga,Mn)N:Si
GaAs + MnAs precipitatesGaAs + MnAs precipitates
depending on growth conditions precipitates or spinodal decomposition
Moreno et al. (Berlin) JAP’02
control magnetic properties De Boeck et al. (IMEC) APL’96 enhance magnetooptical effects (MCD) Akinaga et al. (Tsukuba) APL’00; Shimizu et al. (Tokyo) APL’01
affect conductance and Hall effect
not seen in HRXRDMoreno et al. (Berlin) JAP’02
Heimbrodt et al. (Marburg) PRB’04
spinodal decomposition
hexMnAs
GaAs
TC 320 K
H (Oe)
zbMnAs
GaAs
TC 350 K
Model for high TC DMS1. DMS in question undergo spinodal decomposition into TM reach
and TM poor phases that conserve the structure of host crystal [(Ga,Mn)As (Ge,Mn) — TEM; (Ga,Mn)N -- synchrotron radiation microprobe
Martinez-Criado et al.. (ESR, Schottky) APL’05]
2. TM reach phase is a high TC ferromagnetic metal or ferrimagnetic
insulator, which accounts for spontaneous magnetisation at RT
Model for high TC DMS1. DMS in question undergo spinodal decomposition into TM reach
and TM poor phases that conserve the structure of host crystal [(Ga,Mn)As (Ge,Mn) — TEM; (Ga,Mn)N -- synchrotron radiation microprobe Martinez-Criado et al.. (ESR, Schottky) APL’05
2. TM reach phase is a high TC ferromagnetic metal or ferrimagnetic
insulator, which accounts for spontaneous magnetisation at RT
3. Because of Coulomb repulsion spinodal decomposition is blocked if TM is charged – TM charge state is controlled by co-doping with shallow impurities T.D., submitted to Nature Mat.
Mn+3EF
GaN
EF
Mn+2
GaN:Si
Cr+2
EF
ZnTe
EF
Cr+3
ZnTe:N
SUMMARY
Three classes of DMS showing ferromagnetic properties:
1. Magnetically uniform hole-controlled ferromagnetic DMS p-d Zener model + real v.b. structure
2. Magnetically non-uniform ferro DMS exhibiting electronic nanoscale phase separation driven by:
-- quenched disorder: carrier density fluctuations on insulating side of MIT -- competition between FM and AFM interactions
Griffiths phase (?)
Monte Carlo simulations with random acceptor and spin distributions
3. Magnetically non-uniform ferromagnetic DMS exhibiting chemical nanoscale phase separation: -- annealed disorder (at growth temperature) -- controlled by magnetic ion charge state
new method of self-organised growth of nanostructures