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Ferromagnetic semiconductors for spintronics

Ferromagnetic semiconductors for spintronics. Kevin Edmonds, Kaiyou Wang, Richard Campion, Devin Giddings, Nicola Farley, Tom Foxon, Bryan Gallagher, Tomas Jungwirth School of Physics & Astronomy, University of Nottingham Mike Sawicki, Tomasz Dietl IFPAN, Warsaw, Poland

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Ferromagnetic semiconductors for spintronics

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  1. Ferromagnetic semiconductors for spintronics Kevin Edmonds, Kaiyou Wang, Richard Campion, Devin Giddings, Nicola Farley, Tom Foxon, Bryan Gallagher, Tomas Jungwirth School of Physics & Astronomy, University of Nottingham Mike Sawicki, Tomasz Dietl IFPAN, Warsaw, Poland Tarnjit Johal, Gerrit van der Laan Daresbury Laboratory

  2. Semiconductor spintronics Electron Spin Electron Charge Semiconductor Spintronics Photon Polarisation Benefits: Fast, small, low dissipation devices Quantum computation? New physics

  3. (Ga,Mn)As H. Ohno et al. (1996): ferromagnetism in GaAs thin films doped ~5% with Mn Growth by low temperature MBE to beat equilibrium solubility limit

  4. Carrier-mediated ferromagnetism Substitutional Mn is an acceptor and a J=5/2 magnetic moment. Mn: [Ar] 3d5 4s2 Ga: [Ar] 3d10 4s2 4p1 Ferromagnetism driven by antiferromagnetic exchange coupling Jp-d S.s between Mn moments and spin-polarised GaAs valence electrons Mn Carrier density determines the key magnetic properties of (Ga,Mn)As (e.g. TC, HC,...)

  5. Carrier-mediated ferromagnetism Photogenerated magnetism Koshihara PRL (1997) Spin-FET H. Ohno et al., Nature (2000) Vgate ħw InMnAs GaSb InMnAs B (mT)

  6. Curie temperatures Max. TC=172K (so far...) Wang et al., JAP ‘04

  7. Interstitial Mn: a magnetism killer Mn Yu et al., PRB ’02: ~10-20% of total Mn concentration is incorporated as interstitials Increased TC on annealing corresponds to removal of these defects. As Negative effects on magnetic order: compensating donor – reduces hole density antiferromagnetic coupling between interstitial and substitutional Mn predicted Blinowski PRB ‘03

  8. 1D diffusion process T=190oC Diffusion to free surface - activation energy  0.7eV Edmonds, Bogusławski et al., PRL 92, 037201 (2004)

  9. Magnetic moment and antiferromagnetic coupling X-ray absorption measurements, ALS line 4.0.2 and ESRF line ID8: XMCD asymmetry  30% Magnetic moment  2.3mB XMCD asymmetry  55% Magnetic moment  4.5mB

  10. Ferromagnetic moment vs. field in unannealed film at 6K: annealed B5/2(6K) B=2T B=5T as-grown B5/2(28K) AF coupling described by Teff = T + TAF = (6+22)K

  11. Ferromagnetic semiconductor heterostructures Protocols for growth of semiconductor heterostructures are well-established Addition of spin gives a new degree of freedom e.g. tunnelling structure (Ga,Mn)As AlAs (Ga,Mn)As Tanaka et al. (2001) 70% TMR Chiba et al. (2003) 400% Rüster et al. (2004) >100,000% !!

  12. Tunnelling Anisotropic Magnetoresistance [100] [100] AlOx Au (Ga,Mn)As [110] Gould et al., PRL (2004) TMR-like signal with in control sample with only one ferromagnetic contact Tunnelling probability depends on magnetisation direction of single layer (two step reversal process)

  13. Anisotropic magnetoresistance M I q Magnetoresistance on rotating M away from ‘x’ direction - strong function of Mn concentration, well described by mean-field model Jungwirth et al. APL ‘03

  14. TAMR in Nanoconstrictions 5nm (Ga,Mn)As film with 30nm wide constrictions Giant anisotropic magnetoresistance ~100% in tunnelling regime Giddings et al., cond-mat/0409209

  15. Ge 300K! GaAs GaSb InAs CB VB Mn 3d GaSb GaAs GaP GaN Prospects for room temperature ferromagnetism Predicted TC in (III,Mn)V semiconductors, if Mn is a shallow acceptor T. Dietl, Science ’00; JVSTB ‘03

  16. Ga1-xMnxN Small RT ferromagnetic signal superimposed on larger paramagnetic part (Sonoda ’01; Reed ’01; Thaler ’02; Biquard ’03 etc.) Most are n-type  results are inconsistent with carrier-mediated ferromagnetism Dietl Science ‘00 Several MnxNy magnetic phases exist Zajac et al. ‘03 Phase segregation?

  17. Cubic (Ga,Mn)N: a key to p-type conductivity Wurtzite (Ga,Mn)N is usually n-type; Mn ionisation energy ~1.4eV (Graf et al APL (2002)) But in zincblende (Ga,Mn)N/GaAs we observe robust p-type behaviour DEa~50meV Evidence for collective magnetic effects at low T: Novikov et al. Semicond. Sci. Tech. (2004)

  18. Conclusions  GaAs doped with ~% Mn is ferromagnetic – a model system for investigating magnetic phenomena in semiconductors- gate controlled magnetism - tunnelling magnetoresistance - new tunnelling effects prospects for semiconductors with room temperature ferromagnetism – but phase segregation may be an issue

  19. Magnetic anisotropy Strong cubic anisotropy with <100> easy axes, reduced to biaxial (in-plane) or uniaxial (perpendicular) due to strain. Weaker uniaxial anisotropy between in-plane [110] and [110] orientations, origin unknown. B// B

  20. Magnetic anisotropy rotation In-plane uniaxial easy axis rotates from [110] to [110] on increasing the carrier density above ~6 x 1020 cm-3 by annealing. Sawicki et al., PRB (submitted) easy axis [110 ] easy axis [110]

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