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Spintronics and Magnetic Semiconductors

Spintronics and Magnetic Semiconductors. Joaquín Fernández-Rossier , Department of Applied Physics, University of Alicante (SPAIN). Spintronics Magnetic Semiconductors Ferromagnetism in GaAsMn. Alicante, June 18 2003. Spintronics. Gate. Source. Drain. Semiconductor.

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Spintronics and Magnetic Semiconductors

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

  2. Spintronics

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

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

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

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

  7. 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

  8. Magnetic Semiconductors (materials for spintronics)

  9. “History” of magnetic semiconductors

  10. Ferromagnetism vs Paramagnetism

  11. Paramagnetic DMS V V VI VI IV IV II II III III B B C C N N O O EG EG Zn Zn Al Al Si Si P P S S Cd Cd Ga Ga Ge Ge As As Se Se Mn Hg Hg EF EF II-VI Zn-Se Zn-S Cd-Te (II,Mn)-VI (Zn,Mn)-Se (Zn,Mn)-S (Cd,Mn)-Te In In Sn Sn Sb Sb Te Te

  12. (diluted Ferromagnetic semiconductor) V VI IV III B C N O EG EG Zn Zn Al Si P S Cd Cd II Ga Ge As Se Mn Hg Hg (III,Mn)-V (Ga,Mn)-As (In,Mn)-As (Ga,Mn)-Sb EF EF In Sn Sb Te V VI IV III B C N O Al Si P S II Ga Ge As Se III-V Ga-As In-As Ga-Sb In Sn Sb Te

  13. “Chemistry” of II-VI:Mn and III-V:Mn Electronic configuration of Mn: 4s23d54p0 Electronic configuration of Ga (III): 4s23d104p1 Electronic configuration of Cd (II): 4s23d104p0 • Mn in III-V: gives magnetic moment and holes • Mn in II-VI: gives magnetic moment

  14. Why DMS? Y. OHNO et al., Nature402, 790 (1999) Tanaka, Higo, PRL 2001 H. Ohno, Nature (2000) • Magnetic Light emitting diode • Spin injection • Compatible with GaAs • ‘Electric Control of Ferromagnetism’ • First electrically tunable ferromagnet • Reversible change of Tc • ‘All semiconductor’ Magnetic Tunnel Junction • Large TMR (at 4 Kelvin) • Spin Injection • Heterostructures III-V + (III,Mn)-V • BUT: • Working at low temperature • Small effects • Curie Temperature < 150 Kelvin • Magnetic control of transport • Electric control of Magnetism

  15. Magnetic Semiconductors Summary • 2 types (ferro and para) • Compatible with semiconductor technology • Issue: Increase Tc

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

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

  18. PARA FERRO 0.03 0.05 0.08 x Material: Ga(1-x) AsMnx • Ferromagnetic below 160 kelvin • Homogeneous alloy for x<0.10 • Transport: p-doped semiconductor (p<cMn)

  19. jsdcMn<M> B <M>=0 jpdcMn<M> Giant Zeeman Splitting in (II,Mn)-VI

  20. p,s d d d d d d p,s d6 d6 d5 d5 As Mn As Mn 4 Exchange Interactions • Coulomb Exchange: ferromagnetic. (Reduction of Coulomb repulsion ) • Kinetic Exchange: Antiferromagnetic

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

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

  23. EF Model Hamiltonian “chemistry” s EG d p p d Holes in Valence Band No fluctuationin d levels

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

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

  26. 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

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

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

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