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

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

(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

Carrier-mediated ferromagnetism Substitutional Mn is an acceptor and a J=5/2 magnetic moment. Ferromagnetism driven by antiferromagnetic exchange coupling J p-d S.s between Mn moments and spin- polarised GaAs valence electrons  Carrier density determines the key magnetic properties of (Ga,Mn)As (e.g. T C, H C,...) Mn: [Ar] 3d 5 4s 2 Ga: [Ar] 3d 10 4s 2 4p 1 Mn

Carrier-mediated ferromagnetism Spin-FET H. Ohno et al., Nature (2000) V gate InMnAs Photogenerated magnetism Koshihara PRL (1997) InMnAs GaSb B (mT) ħħ

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

Interstitial Mn: a magnetism killer Yu et al., PRB ’02: ~10-20% of total Mn concentration is incorporated as interstitials Increased T C on annealing corresponds to removal of these defects. Mn As Negative effects on magnetic order:  compensating donor – reduces hole density  antiferromagnetic coupling between interstitial and substitutional Mn predicted Blinowski PRB ‘03

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

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

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

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% R ü ster et al. (2004) >100,000% !!

Tunnelling Anisotropic Magnetoresistance (Ga,Mn)As Au AlO x 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) [110] [100]

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

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

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

Ga 1-x Mn x N Small RT ferromagnetic signal superimposed on larger paramagnetic part (Sonoda ’01; Reed ’01; Thaler ’02; Biquard ’03 etc.) Several Mn x N y magnetic phases exist Zajac et al. ‘03 Most are n-type  results are inconsistent with carrier-mediated ferromagnetism Dietl Science ‘00 Phase segregation?

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  E a ~50meV Evidence for collective magnetic effects at low T: Novikov et al. Semicond. Sci. Tech. (2004)

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

Magnetic anisotropy Strong cubic anisotropy with 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 B //

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