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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
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

semiconductor spintronics
Semiconductor spintronics

Electron

Spin

Electron

Charge

Semiconductor Spintronics

Photon Polarisation

Benefits: Fast, small, low dissipation devices

Quantum computation?

New physics

ga mn as
(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
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,...)

slide5

Carrier-mediated ferromagnetism

Photogenerated magnetism Koshihara PRL (1997)

Spin-FET

H. Ohno et al., Nature (2000)

Vgate

ħw

InMnAs

GaSb

InMnAs

B (mT)

curie temperatures
Curie temperatures

Max. TC=172K (so far...)

Wang et al., JAP ‘04

interstitial mn a magnetism killer
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

1d diffusion process
1D diffusion process

T=190oC

Diffusion to free surface

- activation energy  0.7eV

Edmonds, Bogusławski et al., PRL 92, 037201 (2004)

magnetic moment and antiferromagnetic coupling
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

slide10

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

ferromagnetic semiconductor heterostructures
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% !!

tunnelling anisotropic magnetoresistance
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)

anisotropic magnetoresistance
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

tamr in nanoconstrictions
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

prospects for room temperature ferromagnetism

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

ga 1 x mn x n
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?

cubic ga mn n a key to p type conductivity
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)

conclusions
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
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

magnetic anisotropy rotation
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]