SPINTRONICS AND FERROMAGNETIC SEMICONDUCTORS Tomasz Dietl, Warsaw.

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Information Storage and Spintronics 18

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SPINTRONICS AND FERROMAGNETIC SEMICONDUCTORS Tomasz Dietl, Warsaw

Discontinuous technologies

New information carrier - electron  photon, flux (SQUID loops), vortex (type II superconductors); - spin rather than charge, domain walls,... Discontinuous technologies

New information carrier - electron  photon, flux (SQUID loops), vortex (type II superconductors); - spin rather than charge, domain walls,... New principle of device operation quantum devices, spin transistors,... Discontinuous technologies

New information carrier - electron  photon, flux (SQUID loops), vortex (type II superconductors); - spin rather than charge, domain walls,... New principle of device operation quantum devices, spin transistors,... New architecture - reprogramable devices, SOC - physical, chemical and biological processes - quantum computing Discontinuous technologies

New information carrier - electron  photon, flux (SQUID loops), vortex (type II superconductors); - spin rather than charge, domain walls,... New principle of device operation quantum devices, spin transistors,... New architecture - reprogrammable devices, SOC - physical, chemical and biological processes - quantum computing => Spintronics Discontinuous technologies

Spintronics -- specific goals reading of magnetic information - magnetisation (field) sensors

Spintronics -- specific goals reading of magnetic information - magnetisation (field) sensors TMR

Spintronics -- specific goals reading of magnetic information - magnetisation (field) sensors writing of magnetic information - magnetisation manipulation other than magnetic field TMR

Spintronics -- specific goals reading of magnetic information - magnetisation (field) sensors writing of magnetic information - magnetisation manipulation other than magnetic field generation of spin polarised currents - ferromagnetic metallic electrodes (spin injection) - spin selective ferromagnetic barriers (spin filtering) spin detectors - spin/charge conversion TMR

Spintronics -- specific goals reading of magnetic information - magnetisation (field) sensors writing of magnetic information - magnetisation manipulation other than magnetic field generation of spin polarised currents - ferromagnetic metallic electrodes (spin injection) - spin selective ferromagnetic barriers (spin filtering) spin detectors - spin/charge conversion single spin manipulation - quantum information (low temperatures acceptable) TMR

Spintronics -- materials aspects Why do not combine complementary resources of ferromagnets and semiconductors? TopGaN

Spintronics -- materials aspects Why do not combine complementary resources of ferromagnets and semiconductors?  hybrid semiconductor/ferromagnetic metal structures TopGaN

Spintronics -- materials aspects Why do not combine complementary resources of ferromagnets and semiconductors?  hybrid semiconductor/ferromagnetic metal structures  ferromagnetic semiconductors – multifunctional materials TopGaN

SPINTRONICS AND FERROMAGNETIC SEMICONDUCTORS Tomasz DIETL – Warsaw collaborators: T. Andrearczyk, P. Kossacki, M. Sawicki – Warsaw F. Matsukura, H. Ohno – Sendai J. Cibert, D. Ferrand, S. Tatarenko – Grenoble C.T. Foxon, B.L. Gallagher, K. Edmonds, K.Y. Wang – Nottingham T. Jungwirth (Prague) J. Koenig, A.H. MacDonald, J. Sinova – Texas reviews: Semicond. Sci. Technol. 17 (2002) ; MRS Bulletin, October 2003, p. 714, Europhys. News 34 (2003) 216. support: FENIKS, AMORE -- EC projects, Polonium Project Ohno Semiconductor Spintronics ERATO Project of JST Humboldt Foundation

OUTLINE 1.Frromagnetic semiconductors – background 2.Understanding of carrier-controlled ferromagnetic semiconductors 3. Magnetisation manipulation 4. Have we a functional room temperature ferromagnetic semiconductor?

magnetic semiconductors short-range ferromagnetic super- or double exchange EuS, ZnCr 2 Se 4, La 1-x Sr x MnO 3,... Ferromagnetic semiconductors

magnetic semiconductors short-range ferromagnetic super- or double exchange EuS, ZnCr 2 Se 4, La 1-x Sr x MnO 3,... EuS/KCl,... diluted magnetic semiconductors long-range hole-mediated ferromagnetic exchange IV-VI: p-Pb 1-x-y Mn x Sn y Te Story et al. (Warsaw, MIT) PRL’86 III-V: In 1-x- Mn x As Ohno et al. (IBM) PRL’92 Ga 1-x- Mn x As Ohno et al. (Tohoku) APL’96 T C  100 K for x = 0.05 II-VI: Cd 1-x Mn x Te/Cd 1-x-y Zn x Mg y Te:N QW Haury et al. (Grenoble, Warsaw) PRL’97 Zn 1-x Mn x Te:N,P Ferrand et al. (Grenoble, Warsaw) PRB’01 Ferromagnetic semiconductors

magnetic semiconductors short-range ferromagnetic super- or double exchange EuS, ZnCr 2 Se 4, La 1-x Sr x MnO 3,... EuS/KCl,... diluted magnetic semiconductors long-range hole-mediated ferromagnetic exchange IV-VI: p-Pb 1-x-y Mn x Sn y Te Story et al. (Warsaw, MIT) PRL’86 III-V: In 1-x- Mn x As Ohno et al. (IBM) PRL’92 Ga 1-x- Mn x As Ohno et al. (Tohoku) APL’96 T C  100 K for x = 0.05 II-VI: Cd 1-x Mn x Te/Cd 1-x-y Zn x Mg y Te:N QW Haury et al. (Grenoble, Warsaw) PRL’97 Zn 1-x Mn x Te:N,P Ferrand et al. (Grenoble, Warsaw) PRB’01 III-V and II-VI DMS: quantum nanostructures and ferromagnetism combine Ferromagnetic semiconductors

Zener/RKKY model of carrier-controlled ferromagnetism in DMS

Mn state and its coupling to carriers in DMS Mn: 3d 5 4s 2 II-VI: Mn electrically neutral (3d 5, S = 5/2) –doping by acceptors necessary III-V, IV: Mn acts as source of spins and holes sp-d exchange interaction => -- large p-d hybridisation and intra-site Hubbard U => Kondo hamiltonian H = -  N o Ss => strong Mn hole p-d exchange -- (Cd,Mn)Te:  N o  eV Gaj et al. (Warsaw, Paris) SSC’79 -- (Ga,Mn)As:  N o  eV Okabayashi et al. (Tokyo) PRB’98, Szczytko et al. (Warsaw)PRB’01 -- no s-d hybridisation => weaker Mn electron s-d exchange  N o  0.2 eV Gaj et al. (Warsaw, Paris) SSC’79 giant spin splitting of bands proportional to Mn magnetization

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- MacDonald et al. (Austin) ’99- k EFEF

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- MacDonald et al. (Austin) ’99- k EFEF No adjustable parameters T C ~  2  (s) DOS Essential ingredient: Complexity of the valence band structure has to be taken into account

Mn-based p-type DMS to which p-d Zener model has been found to apply Theory: T. D et al. (Warsaw, Tohoku, Grenoble) Science’00, PRB’01 Jungwirth et al. (Austin, Prague, PRB’02), also UCSD, NRL, … Expl.: Tohoku, Kanagawa, Tokyo, Grenoble, PSU, NRL, 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

Indication of ferromagnetism in n-Zn 1-x Mn x O:Al Temperature (K)  (mT) T C  160 mK consistent with s-d Zener model   R xx (  ) Magnetic field (T) 50mK 60mK 75mK 100mK 125mK 150mK 200mK Andrearczyk et al. (Warsaw, Yokohama) ICPS’00 n = 1.4x10 20 cm -3 x = 0.03

p-d Zener model for p-type DMS the model explains/predicted: -- T C (x, p, n), spin polarization, M(T,H), -- magnetic stiffness (domain width, spin wave spectrum) -- anomalous Hall effect -- magnetoresistance (WLR) and anisotropic magnetoresistance -- a.c. conductivity and magnetic circular dichroism -- … T.D. et al.,’97- A.H. MacDonald et al. ’99-

Spintronic functionalities of ferromagnetic DMS

Spintronic functionalities of ferro DMS As metal ferromagnets: spin injectors GMR, TMR, AMR, PHE Kerr effect current-induced magnetisation switching

Spintronic functionalities of ferro DMS Magnetisation manipulation unique to magnetic semiconductor: electric field light epitaxial strain

Electric field

Tuning of magnetic ordering by electric field (ferro-FET) (In,Mn)As Ohno et al. (Tohoku, Warsaw) Nature ‘00 M I VHVH

Tuning magnetic ordering by electric field (ferro-FET) (In,Mn)As Ohno et al. (Tohoku, Warsaw) Nature ‘00 M I VHVH

V QW barriers p doped n doped undoped Hole-induced ferromagnetism in a pin diode – ferro-LED (Cd,Mn)Te Hole liquidDepleted EvEv EcEc EFEF V Boukari et al. (Grenoble, Warsaw) PRL’02 Photoluminescence

Light

Effect of illumination in (Cd,Mn)Te p-i-n diode V QW illumination EvEv EcEc EFEF Boukari et al. (Grenoble, Warsaw) PRL’02 0 V 1.5 K Enhancement of ferromagnetism n+n+ p+p+

Effect of illumination in (Cd,Mn)Te p-i-p diode V QW illumination Boukari et al. (Grenoble, Warsaw) PRL’02 Destruction of ferromagnetism p+p+ p+p+ EFEvEFEv EcEc Temperature Hole concentration p = constT = const

Epitaxial strain

Magnetic anisotropy -- epitaxial strain engineering Tensile strain e.g. (Ga,Mn)As/(In,Ga)As Compressive strain e.g. (Ga,Mn)As/GaAs

Epitaxial-strain-induced magnetic anisotropy Sawicki et al.’(Warsaw, Wuerzburg)’04 after T.D. et al. (Warsaw, Tohoku) PRB ‘01 Ga 1-x Mn x As/GaAs  compressive strain j z =  3/2 x = 0.053

Epitaxial-strain-induced magnetic anisotropy Sawicki et al. (Warsaw, Wuerzburg)’04 after T.D. et al. (Warsaw, Tohoku) PRB ‘01 Ga 1-x Mn x As/GaAs  compressive strain j z =  3/2 j z =  1/2 theory x = 0.053

Reorientation transition – theory and expt. annealing Sawicki et al. (Warsaw, Wuerzburg)’04 after T.D. et al. (Warsaw, Tohoku) PRB ‘01 Ga 1-x Mn x As/GaAs  compressive strain j z =  3/2 j z =  1/2 theory x = 0.053

Controlling locally magnetisation direction to be demonstrated Ferromagnetic quantum dot array M M s /4

Can we push T C higher?

Strategies Two strategies for pushing T C higher -- increasing p and/or x in existing ferromagnetic DMS -- searching for DMS with greater coupling constant  2  (E F ) T.D. et al. (Warsaw, Tohoku, Grenoble) Science’00

Strategies Two strategies for pushing T C higher -- increasing p and/or x in existing ferromagnetic DMS -- searching for DMS with greater coupling constant  2  (E F ) Obstacles -- self-compensation -- solubility limits -- tight binding of holes by TM ions (Zhang-Rice polaron) T.D. et al. (Warsaw, Tohoku, Grenoble) Science’00

1/  Where are we? Wang/ Sawicki (Nottingham, Warsaw) remanent magnetisation and 1/  vs. T hysteresis loops M REM T C = 173 K T C   CW

Record Curie temperature (K) T C in (III,Mn)As The progress due to increase of p by low temperature annealing  out diffusion of Mn I : Wojtowicz et al. (Notre Dame, Warsaw, Berkeley) PRB’02; APL’04 Edmonds et al. (Nottingham, Warsaw) PRL’04 IBM, Tohoku, Tokyo, Notre Dame, PSU, Tohoku, Nottingham, …

(Ga,Mn)As – growth phase diagram phase separation GaAs + MnAs LT Ga 1-x Mn x As:Mn I roughening polycrystal after Matsukura and Ohno

SUMMARY Mn-based arsenides, also antymonides, tellurides -- basic thermodynamic, magnetoelastic, optical, and also optical properties understood (p-d Zener model)

SUMMARY Mn-based arsenides, also antymonides, tellurides -- basic thermodynamic, magnetoelastic, optical, and also optical properties understood (p-d Zener model) -- magnetisation manipulations by electric field, light, and epitaxial strain demonstrated

SUMMARY Mn-based arsenides, also antymonides, tellurides -- basic thermodynamic, magnetoelastic, optical, and also optical properties understood (p-d Zener model) -- magnetisation manipulations by electric field, light, and epitaxial strain demonstrated -- role of point defects (interstitials) and also extended defects (e.g., MnAs precipitates) elucidated

OTHER SYSTEMS

Zener model prediction of T C for semiconductors containing 5% Mn d 5, p = 3.5  cm -3 T. D. et al. (Warsaw, Tohoku, Grenoble) Science’00, PRB’01 Light elements: strong p-d hybridisation weak spin-orbit interaction

Materials showing hysteresis and spontaneous magnetization at 300 K wz-c- (Ga,Mn)N, (In,Mn)N, (Ga,Cr)N, (Al,Cr)N, (Ga,Gd)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,Cr)Te (Ti,Co)O 2, (Sn,Co)O 2, (Sn,Fe)O 2, (Hf,Co)O 2 (Cd,Ge,Mn)P 2, (Zn,Ge,Mn)P 2, (Zn,Sn,Mn)As 2 (Ge,Mn) (La,Ca)B 6, C 60, C, …

Materials showing hysteresis and spontaneous magnetization at 300 K wz-c- (Ga,Mn)N, (In,Mn)N, (Ga,Cr)N, (Al,Cr)N, (Ga,Gd)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,Cr)Te (Ti,Co)O 2, (Sn,Co)O 2, (Sn,Fe)O 2, (Hf,Co)O 2 (Cd,Ge,Mn)P 2, (Zn,Ge,Mn)P 2, (Zn,Sn,Mn)As 2 (Ge,Mn) (La,Ca)B 6, C 60, C, … In many cases high T C consistent with ab initio computations within DFT

Materials showing hysteresis and spontaneous magnetization at 300 K wz-c- (Ga,Mn)N, (In,Mn)N, (Ga,Cr)N, (Al,Cr)N, (Ga,Gd)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,Cr)Te (Ti,Co)O 2, (Sn,Co)O 2, (Sn,Fe)O 2, (Hf,Co)O 2 (Cd,Ge,Mn)P 2, (Zn,Ge,Mn)P 2, (Zn,Sn,Mn)As 2 (Ge,Mn) (La,Ca)B 6, C 60, C, … In many cases high T C consistent with ab initio computations within DFT  None proven to be 300 K ferromagnetic semiconductor  Phase diagrams unknown  Each system brings new challenges

CONCLUSIONS  ( Ga,Mn)As, p-(Cd,Mn)Te, … emerges as the best understood ferromagnet  Beginning of the road for magnetically doped nitrides and oxides

END

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