Phase separation effects
in diluted magnetic semiconductors
Tomasz DIETL
Institute of Physics, Polish Academy of Sciences
Institute of Theoretical Physics, Warsaw University
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
Introduction
(Ga,Mn)As/(Al0.9Ga0.1)As d=200 nm
Ga1-xMnxAs: resistance vs. temperature and Curie
temperature vs. x
Ga1-xMnxAs
1
10
80
0
Tc (K)
RESISTIVITY (cm)
120
10
40
INSULATOR
-1
0
10
0.00
0.04
x
METAL
-2
10
0
III-V DMS
100
200
x
0.08
0.015
0.022 Matsukura et al.
0.071 (Tohoku) PRB’98
0.035
0.043
300 0.053
TEMPERATURE (K)
• ferromagnetism on both sides of metal-insulator transitions
• ferromagnetism disappears in the absence of holes
Effect of acceptor doping on magnetic
susceptibility in Zn1-xMnxTe:P
5
p -Zn1-xMnxTe
-1 vs. T
3
-1
[ a.u. ]
-3
p
x = 0.023
4

17
p  10 cm
18
p  510 cm
2
-3
Sawicki et al.
(Warsaw) pss’02
1
TCW
0
0
5
10
15
Temperature [ K ]
II-VI DMS
• ferromagnetism driven by hole doping
• competition between intrinsic short-range AFM
and hole-induced long-range FM
Ferromagnetic temperature in p-(Zn,Mn)Te
Ferromagnetic Temp. TF / xeff (K)
-3
17
10
30
Hole concentration (cm )
18
19
20
10
10
10
20
5x10
( Zn ,Mn )Te:N
30
( Zn ,Mn )Te:P
10
10
Metallic
Insulating
1
1
• ferromagnetism on both sides of metal-insulator transition
Ferrand et al. (Grenoble, Warsaw) PRB’01
Sawicki et al. (Warsaw) pss’02
Where are we?
remanent magnetisation and 1/ vs. T
hysteresis loops
8% (Ga,Mn)As
TC  CW
8% (Ga,Mn)As
[ a.u. ]
T = 172 K
0.05
MREM
MREM (MSpontaneous )
M[110](T) / MSat(5K) [ r.u. ]
0.10
T = 175 K
0.00
-0.05
-1
0
1
2
3140
TC = 173 K
1/
150
Magnetic Field [ Oe ]
Wang/ Sawicki (Nottingham, Warsaw)ICPS’04
160
170
180
Temperature [ K ]
190
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 cf. A. Moreno
-- insulator side
– electronic phase separation
• „high” TC ferro DMS
– chemical phase separation
Metallic side of metal-to-insulator transition
p-d Zener/RKKY model of hole-controlled
ferromagnetism in DMS
EF
Driving force:
lowering of the hole energy due to redistribution between
hole spin subbands split by p-d exchange interaction
T.D. et al.,’97Jungwirth et al. (Austin/Prague) ’99-
p-d Zener/RKKY model of hole-controlled
ferromagnetism in DMS
EF
Driving force:
lowering of the hole energy due to redistribution between
hole spin subbands split by p-d exchange interaction,  ~ M
M
Essential ingredient:
Complexity of the valence band structure
has to be taken into account
No adjustable parameters
TC ~ 2(s)DOS
T.D. et al.,’97MacDonald et al. (Austin/Prague) ’99-
Mn-based p-type DMS to which p-d Zener model
has been found to apply
GaAs
• TC  CW
• TC (p,x) consistent with
p-d Zener model
• not double exchange
GaSb
InAs
InSb
ZnTe
CdTe
10
xMn = 5%
p = 3.5x1020 cm-3
100
300
Curie temperature (K)
Expl.: Tohoku, Tokyo, Grenoble, Wuerzburg, PSU, Notre Dame, UCSB, Nottingham, …
Insulator side of metal-to-insulator transition
Anderson-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
p-type
(II,Mn)VI
(III,Mn)V
Bhatt et al. (Princeton) PRL’02; Das Sarma et al., PRL’02,’04, ....
Resistivity and magnetisation in (Ga,Mn)As
4K
F. Matsukura et al..(Tohoku) PRB ’98, SSC’97
Co-existence of ferromagnetic and
paramagnetic components in non-metallic samples
Collosal negative magnetoresistance on
insulator side of MIT
104
102
(Zn,Mn)Te:N
x = 3.8%
p = 3x1019 cm-3
100
B=0
B = 11 T
10-2
1.5
2
5
Temperature (K)
10
Ferrand et al.
(Grenoble, Warsaw) PRB’02
Collosal negative magnetoresistance on
insulator side of MIT
104
102
(Zn,Mn)Te:N
x = 3.8%
p = 3x1019 cm-3
100
B=0
Ferrand et al.
(Grenoble, Warsaw) PRB’02
B = 11 T
10-2
1.5
2
5
10
Temperature (K)
Reminiscent to CMR oxides
Katsumoto et al. (Tokyo) pss’98
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
cf. E.L. Nagaev, E. Dagotto et al.
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
Probing competing AF and FM interactions by
inelastic neutron scattering in p-(Zn,Mn)Te
inelastic neutron scattering
of n.n. Mn pairs
large single crystals of
Zn0.95Mn0.05Te:P
p = 5x1018 cm-3, TCW = 2 K
Insulator side of the MIT
Hint = -2(JAF + Jh)SiSj
Zn0.95Mn0.05Te
Kępa et al. (Warsaw, NIST) PRL’03
JAF < 0 super-exchange
Jh > 0 hole-induced
Hole 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 T
in (Cd,Mn)Te/(Cd,Mg)Te:I quantum well
Electron density
(cm-2)
Jaroszynski et al.
(Warsaw, NHMFL) cond-mat/0509
submitted to PRL
Resistivity vs. carrier density at various T
in (Cd,Mn)Te/(Cd,Mg)Te:I quantum well
Electron density (cm-2)
Jaroszynski et al.
(Warsaw, NHMFL) cond-mat/0509
submitted to PRL
Resistivity vs. carrier density at various T
in (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
Ferromagnetic coupling via weakly-localised holes
Random distribution of
acceptors and spins
 Metallic and ferromagnetic
lakes embedded in insulating
matrix
Localization length  >> rs
•At the distance between Mn ions wave function can be regarded as
extended =>only part of the spins contribute to the ferromagnetic signal
High TC ferro DMS
Experimental indications of room temperature
ferromagnetism in (Zn,Cr)Te
K. Ando et al., PRL’03
Curie temperature (K)
Effect of doping
300
Zn1-xCrxTe
Zn1-xCrxTe:N
200
100
0
0.00
0.10
0.20
Cr composition x
Ando et al.. (Tsukuba) PRL’03
Ozaki et al. (Tsukuba) APL’05
Ferromagnetism of (Ga,Mn)N – effect of doping
(Ga,Mn)N:Si
(Ga,Mn)N
x = 0.2%
TC >> 300 K
(Ga,Mn)N, x = 0.2%
TC  0 for Si doping
Reed et al. (NCSU) APL’05
GaAs + MnAs precipitates
 depending on growth conditions precipitates or spinodal decomposition
GaAs
GaAs
zb
MnAs
hex
MnAs
TC  320 K
Moreno et al. (Berlin) JAP’02
TC  350 K
spinodal
decomposition
H (Oe)
 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 HRXRD
Heimbrodt et al. (Marburg) PRB’04
Moreno et al. (Berlin) JAP’02
Model for high TC DMS
1. 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 DMS
1. 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 codoping with shallow impurities
T.D., submitted to Nature Mat.
GaN:Si
GaN
EF
Mn+3
EF
Mn+2
ZnTe:N
ZnTe
Cr+2
EF
Cr+3
EF
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
(La,Ca)MnO3
DMS: interactions determine spatial distribution
of both carriers and localized spins
END