Spintronics and Magnetic Semiconductors Joaquín Fernández-Rossier, Department of Applied Physics, University of Alicante (SPAIN) Alicante, June 18 2003.

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Presentation transcript:

Spintronics and Magnetic Semiconductors Joaquín Fernández-Rossier, Department of Applied Physics, University of Alicante (SPAIN) Alicante, June Spintronics Magnetic Semiconductors Ferromagnetism in GaAsMn

Spintronics

Semiconductors Source Gate Drain Semiconductor Semiconductors: Low carrier density (p, n) Heterostructures Electrical Control of p,n Volatile information Dimensional Reduction, L< B Mesoscopic Behavior, L<l s New exciting physics: Quantum Hall Effects, Conductance Quantization Single electron transistor Size reduction

Metallic Ferromagnets Ferromagnetic metals: Collective Coordinate Permanent information High carrier density Heterostructures Magnetic Control of current ‘Single atom’ magnet New Physics GMR L<l sr Size reduction Superparamagnetism

‘There is plenty of room at the bottom’ R. P. Feynman ‘There is plenty of room at the bottom’ R. P. Feynman

Alternative Approach: Spintronics BUT: Spin injection problem Spin scattering problem Source Gate Drain InAs Das and Datta spin transistor, APL 1990 Rashba spin-orbit controls spin orientation Spin dependent transmission controls resistance High Resistance Low Resistance SPINTRONICS: Merger of semiconductor based and ferromagnet based information technologies

Spintronics (from the table of contents of “Semiconductor Spintronics and Quantum Computation” Device concepts Interface Physics (spin injection) Spin dynamics, spin decoherence Optical Manipulation Materials

Magnetic Semiconductors (materials for spintronics)

“History” of magnetic semiconductors MaterialYearTcTc Transport EuO, EuS <65 KInsulating (II,Mn)VI Semic. PbSnMnTe CdTeMn:N ZnMnTe:X <1.5 KP-type semiconduc. InMnAs19927 KIns. (Ga,Mn)As KSemic, bad metal (Ga,Sb)Mn, GaP:Mn, GaN:Mn, ZnCrTe (DARPA time) 900 K??

Ferromagnetism vs Paramagnetism

BC AlSi NO PS GaGe InSn AsSe Sb II Zn Cd Hg IV V III VI Te EGEG EFEF II-VI Zn-Se Zn-S Cd-Te EGEG EFEF Paramagnetic DMS II BC AlSi NO PS GaGe InSn AsSe Sb IV V III VI Te Zn Cd Hg Mn (II,Mn)-VI (Zn,Mn)-Se (Zn,Mn)-S (Cd,Mn)-Te

BC AlSi NO PS GaGe InSn AsSe Sb II Zn Cd Hg IV V III VI Te EGEG EFEF III-V Ga-As In-As Ga-Sb EGEG (diluted Ferromagnetic semiconductor) BC AlSi NO PS GaGe InSn AsSe Sb Mn II Zn Cd Hg IV V III VI Te EFEF (III,Mn)-V (Ga, Mn)-As (In, Mn)-As (Ga, Mn)-Sb

“Chemistry” of II-VI:Mn and III-V:Mn Electronic configuration of Mn: 4s 2 3d 5 4p 0 Electronic configuration of Ga (III): 4s 2 3d 10 4p 1 Electronic configuration of Cd (II): 4s 2 3d 10 4p 0 Mn in III-V: gives magnetic moment and holes Mn in II-VI: gives magnetic moment

Why DMS? Y. OHNO et al., Nature 402, 790 (1999) Magnetic Light emitting diode Spin injection Compatible with GaAs ‘All semiconductor’ Magnetic Tunnel Junction Large TMR (at 4 Kelvin) Tanaka, Higo, PRL 2001 ‘Electric Control of Ferromagnetism’ First electrically tunable ferromagnet Reversible change of T c H. Ohno, Nature (2000) Spin Injection Heterostructures III-V + (III,Mn)-V Magnetic control of transport Electric control of Magnetism BUT: Working at low temperature Small effects Curie Temperature < 150 Kelvin

Magnetic Semiconductors Summary 2 types (ferro and para) Compatible with semiconductor technology Issue: Increase T c

Microscopic mechanism of ferromagnetism in (III,Mn)-V

The mechanism. Experimental evidence II-VI+Mn = PARAMAGNETIC II-VI+Mn+electrons, PARAMAGNETIC II-VI+Mn+ holes: FERROMAGNETIC III-V+Mn= FERROMAGNETIC

Material: Ga (1-x) AsMn x Ferromagnetic below 160 kelvin Homogeneous alloy for x<0.10 Transport: p-doped semiconductor (p<c Mn ) FERRO PARA x

Giant Zeeman Splitting in (II,Mn)-VI =0 j sd c Mn j pd c Mn B

4 Exchange Interactions Coulomb Exchange: ferromagnetic. (Reduction of Coulomb repulsion ) Kinetic Exchange: Antiferromagnetic d5d5 d6d6 AsMn d5d5 d6d6 AsMn p,s d d d dd d

The “model Hamiltonian” GaAs Hamiltonian Non Magnetic Scattering Exchange Sum over Impurities  SO

Carrier mediated ferromagnetism Entropic Penalty Paramagnetic gain Functional of carrier density matrix

Model Hamiltonian “chemistry ” EGEG EFEF dp p s d Holes in Valence Band No fluctuation in d levels

dp p s d Holes in d levels. Hund Exchange Half metal? “Ab initio” “chemistry ”

dp p s d Holes both in d levels and valence band. Both Hund and Kinetic exchange What about this “chemistry ”?

Microscopic Mechanism: Summary and questions Model Hamiltonian: no d-charge fluctuations, holes in valence band, kinetic exchange. Weak coupling “Double Exchange”: d charge fluctuations. Hund exchange. Strong coupling. Who is right? (from ab-initio) Interpolation from DE to KE

C. Piermarocchi Michigan State P.C. Chen UC Berkeley L. J. Sham UCSD JFR, in collaboration with A. H. MacDonald UT Austin M. Abolfath, UT Austin A. S. Núñez, UT Austin

Conclusions Spintronics: “make it happen” ideology Magnetic Semiconductors: bricks to build spintronics FM Mechanism: maybe an open problem.