Phase separation effects in diluted magnetic semiconductors

1
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. Jaroszynski, 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
2
Introduction
3
Ga1-xMnxAs resistance vs. temperature and Curie
temperature vs. x
Matsukura et al. (Tohoku) PRB98
III-V DMS

• ferromagnetism on both sides of metal-insulator
  transitions
• ferromagnetism disappears in the absence of holes

4

Effect of acceptor doping on magnetic
susceptibility in Zn1-xMnxTeP
?-1 vs. T

Sawicki et al. (Warsaw) pss02

• ferromagnetism driven by hole doping
• competition between intrinsic short-range AFM
• and hole-induced long-range FM

II-VI DMS
5
Ferromagnetic temperature in p-(Zn,Mn)Te

• ferromagnetism on both sides of metal-insulator
  transition

Ferrand et al. (Grenoble, Warsaw) PRB01 Sawicki
et al. (Warsaw) pss02
6
Where are we?
remanent magnetisation and 1/? vs. T
hysteresis loops
TC ? ?CW
MREM
TC 173 K
1/?
Wang/ Sawicki (Nottingham, Warsaw)ICPS04
7
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)PC
• (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
8
SQUID studies of DMS in Warsaw

• M. Sawicki et al.
• wz-c-(Ga,Mn)N, (Ga,Fe)N
• (Ga,Mn)As
• (Zn,Mn)TeN, P
• (Cd,Mn)Te, (Cd,Mn)Se
• (Cd,Cr)Te, (Zn,Cr)Se
• (Zn,Mn)O, (Zn,Co)O, (Zn,Cr)O

9
Todays talk

• low TC ferro DMS
• -- metallic side
• -- insulator side
• electronic phase separation
• high TC ferro DMS
• chemical phase separation

cf. A. Moreno
10
Metallic side of metal-to-insulator transition
11
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-
12
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
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.,97- MacDonald et al. (Austin/Prague)
99-
13
Mn-based p-type DMS to which p-d Zener model has
been found to apply

• TC ? ?CW
• TC (p,x) consistent with
• p-d Zener model
• not double exchange

xMn 5 p 3.5x1020 cm-3

Expl. Tohoku, Tokyo, Grenoble, Wuerzburg, PSU,
Notre Dame, UCSB, Nottingham,
14
Insulator side of metal-to-insulator
transitionAnderson-Mott localization
Small hole concentration rs gt 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)
PRL05
Giddings et al. (Hitachi, Nottingham)
PRL05
15
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) PRL02 Das Sarma et
al., PRL02,04, ....
16
Resistivity and magnetisation in (Ga,Mn)As
4 K
F. Matsukura et al..(Tohoku) PRB 98, SSC97
Co-existence of ferromagnetic and paramagnetic
components in non-metallic samples
17
Collosal negative magnetoresistance on insulator
side of MIT
Ferrand et al. (Grenoble, Warsaw) PRB02
18
Collosal negative magnetoresistance on insulator
side of MIT
Ferrand et al. (Grenoble, Warsaw) PRB02
Reminiscent to CMR oxides
Katsumoto et al. (Tokyo) pss98
19
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) PRL03
• 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) PRL05
20
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
Zn0.95Mn0.05TeP p 5x1018 cm-3, TCW 2
K Insulator side of the MIT
Hint -2(JAF Jh)SiSj JAF lt 0 super-exchange
Jh gt 0 hole-induced
Zn0.95Mn0.05Te
Kepa et al. (Warsaw, NIST) PRL03
21
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
22
Resistivity vs. carrier density at various Tin
(Cd,Mn)Te/(Cd,Mg)TeI quantum well
Jaroszynski et al. (Warsaw, NHMFL)
cond-mat/0509 submitted to PRL
Electron density (cm-2)
23
Resistivity vs. carrier density at various Tin
(Cd,Mn)Te/(Cd,Mg)TeI quantum well
Electron density (cm-2)
Jaroszynski et al. (Warsaw, NHMFL)
cond-mat/0509 submitted to PRL
24
Resistivity vs. carrier density at various Tin
(Cd,Mn)Te/(Cd,Mg)TeI quantum well
Electron density (cm-2)

Interpretation nanoscale electronic phase
separation into metallic ferromagnetic
regions embeded in isolating paramagnetic matrix
25
Ferromagnetic coupling via weakly-localised holes

• Random distribution of
• acceptors and spins
• Metallic and ferromagnetic
• lakes embedded in insulating
• matrix

Localization length ? gtgt rs

• At the distance between Mn ions wave function can
  be regarded as extended gtonly part of the spins
  contribute to the ferromagnetic signal

26
High TC ferro DMS
27
Experimental indications of room temperature
ferromagnetism in (Zn,Cr)Te
K. Ando et al., PRL03
28
Effect of doping

Ando et al.. (Tsukuba) PRL03 Ozaki et al.
(Tsukuba) APL05
29
Ferromagnetism of (Ga,Mn)N effect of doping

(Ga,Mn)N x 0.2 TC gtgt 300 K
(Ga,Mn)NSi
(Ga,Mn)N, x 0.2 TC ? 0 for Si doping
Reed et al. (NCSU) APL05
30
GaAs MnAs precipitates

• depending on growth conditions precipitates or
  spinodal decomposition

spinodal decomposition
Moreno et al. (Berlin) JAP02

• control magnetic properties De Boeck et al.
  (IMEC) APL96
• enhance magnetooptical effects (MCD)
• Akinaga et al. (Tsukuba)
  APL00 Shimizu et al. (Tokyo) APL01
• affect conductance and Hall effect
• not seen in HRXRD

Heimbrodt et al. (Marburg) PRB04
Moreno et al. (Berlin) JAP02
31
Model for high TC DMS

• 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)
  APL05
• TM reach phase is a high TC ferromagnetic metal
  or ferrimagnetic insulator, which accounts for
  spontaneous magnetisation at RT

32
Model for high TC DMS

• 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)
  APL05
• 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.

ZnTeN
Cr3
EF
33
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
34
(La,Ca)MnO3
DMS interactions determine spatial distribution
of both carriers and localized spins
35

END