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 DIETL Institute of Physics, Polish Academy of Sciences Institute of Theoretical Physics, Warsaw University

Introduction

Ga 1-x Mn x As: 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 Zn 1-x Mn x Te: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)Te Ferrand et al. (Grenoble, Warsaw) PRB’01 Sawicki et al. (Warsaw) pss’02 ferromagnetism on both sides of metal-insulator transition

1/  Where are we? Wang/ Sawicki (Nottingham, Warsaw)ICPS’04 remanent magnetisation and 1/  vs. T hysteresis loops M REM T C = 173 K T C   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)O 2, (Ti,V)O 2, (Ti,Cr)O 2, (Sn,Co)O 2, (Sn,Fe)O 2, (Hf,Co)O 2 (Cd,Ge,Mn)P 2, (Zn,Ge,Mn)P 2, (Cd,Ge,Mn)As 2, (Zn,Sn,Mn)As 2 (Ge,Mn), (Ge,Cr), (Ge,Mn,Fe) (La,Ca)B 6, C, C 60, HfO 2, (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” T C ferro DMS -- metallic side -- insulator side – electronic phase separation „high” T C ferro DMS – chemical phase separation cf. A. Moreno

Metallic side of metal-to-insulator transition

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 EFEF

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 T C ~  2  (s) DOS Essential ingredient: Complexity of the valence band structure has to be taken into account M k EFEF

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, … x Mn = 5% p = 3.5x10 20 cm -3 T C   CW T C (p,x) consistent with p-d Zener model not double exchange

Insulator side of metal-to-insulator transition Anderson-Mott localization Small hole concentration r s > 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

Temperature (K) Resistivity (Ohm cm) B = 0 B = 11 T (Zn,Mn)Te:N x = 3.8% p = 3x10 19 cm -3 Collosal negative magnetoresistance on insulator side of MIT Ferrand et al. (Grenoble, Warsaw) PRB’02

Temperature (K) Resistivity (Ohm cm) B = 0 B = 11 T (Zn,Mn)Te:N x = 3.8% p = 3x10 19 cm -3 Collosal negative magnetoresistance on insulator side of MIT Ferrand et al. (Grenoble, Warsaw) PRB’02 Katsumoto et al. (Tokyo) pss’98 Reminiscent 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 Kępa et al. (Warsaw, NIST) PRL’03 inelastic neutron scattering of n.n. Mn pairs large single crystals of Zn 0.95 Mn 0.05 Te:P p = 5x10 18 cm -3, T CW = 2 K Insulator side of the MIT Zn 0.95 Mn 0.05 Te H int = -2(J AF + J h )S i S j J AF < 0 super-exchange J h > 0 hole-induced

Hole induced contribution empty dots - no holes, full dots – with holes  E = 2J h = 0.03  meV  E RKKY = meV  E BMP = 0.12 meV

Resistivity vs. carrier density at various T in (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 T in (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 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

Localization length  >> r s Ferromagnetic 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

High T C ferro DMS

Experimental indications of room temperature ferromagnetism in (Zn,Cr)Te K. Ando et al., PRL’03

Effect of doping Ando et al.. (Tsukuba) PRL’03 Ozaki et al. (Tsukuba) APL’05

Ferromagnetism of (Ga,Mn)N – effect of doping Reed et al. (NCSU) APL’05 (Ga,Mn)N x = 0.2% T C >> 300 K (Ga,Mn)N, x = 0.2% T C  0 for Si doping (Ga,Mn)N:Si

GaAs + 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 HRXRD Moreno et al. (Berlin) JAP’02 Heimbrodt et al. (Marburg) PRB’04 spinodal decomposition hex MnAs GaAs T C  320 K H (Oe) zb MnAs GaAs T C  350 K

Model for high T C 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 T C ferromagnetic metal or ferrimagnetic insulator, which accounts for spontaneous magnetisation at RT

Model for high T C 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 T C 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 +3 EFEF GaN EFEF Mn +2 GaN:Si Cr +2 EFEF ZnTe EFEF 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

(La,Ca)MnO 3 DMS: interactions determine spatial distribution of both carriers and localized spins

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