Ferromagnetic semiconductors for spintronics

Kevin Edmonds, Kaiyou Wang, Richard Campion, Devin Giddings, Nicola Farley, Tom Foxon, Bryan Gallagher, Tomas JungwirthSchool of Physics & Astronomy, University of Nottingham

Mike Sawicki, Tomasz DietlIFPAN, Warsaw, Poland

Tarnjit Johal, Gerrit van der LaanDaresbury Laboratory

ElectronCharge

Photon Polarisation

ElectronSpin

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

Magnetic Polarisation

Magnetic Field Strength

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

Jp-d S.sbetween Mn moments and spin-polarised GaAs valence electrons

Carrier density determines the key magnetic properties of (Ga,Mn)As (e.g. TC, HC,...)

Mn: [Ar] 3d5 4s2

Ga: [Ar] 3d10 4s2 4p1

Mn

Carrier-mediated ferromagnetism

Spin-FETH. Ohno et al., Nature (2000)

Vgate

InMnAs

Photogenerated magnetism Koshihara PRL (1997)

InMnAs

GaSb

B (mT)

ħ

Curie temperatures

0 2 4 6 80

100

200 Zener mean-fieldprediction(parameter- free)

Curie temp.

(K)

Mn concentration (%)0 2 4 6 8

0

5

10

15

as-grown

annealed

Carrier density

(x10

20

/cc)

Mn concentration (%)

Max. TC=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 TC 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

1 10 1001.6

2.0

2.4

2.8

3.2

0.00 0.02 0.040

20

40

60

t / L2 (hours / nm2) d(1/ρ)/dt (mΩ-1 cm

-1 hours

-1)

Resistivity (m

Ω cm)

Time (hours)

L=100 nm L=50 nm L=25 nm L=10 nm

AsGaMnON

dΦ / dE

( )Kinetic energy eV

1D diffusion process

Diffusion to free surface

- activation energy 0.7eV

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

T=190oC

Magnetic moment and antiferromagnetic coupling

XMCD asymmetry 55%

Magnetic moment 4.5

B

640 650 660

0

10

20

summedXAS

(Ga,Mn)As as-grown(Ga,Mn)As annealed

summedXAS

XMLDXMCD

absorption (a.u.)

Photon energy (eV)640 650 660

0

10

20

XMCD

Photon energy (eV)

XMCD asymmetry 30%

Magnetic moment 2.3

B

X-ray absorption measurements, ALS line 4.0.2 and ESRF line ID8:

640 650

X-ray energy (eV)

0 1 2 3 4 50

1

2

3

4

5

S+l (B/ )Mn

( )B T

B=2T

B=5T

annealed

as-grown

B5/2(6K)

B5/2(28K)

Ferromagnetic moment vs. field in unannealed film at 6K:

AF coupling described by

Teff = T + TAF = (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)AsAlAs(Ga,Mn)As

Tanaka et al. (2001) 70% TMR

Chiba et al. (2003) 400%

Rüster et al. (2004) >100,000% !!

-0.1 0.0 0.1

R (ΜΩ)

( )Field T

Tunnelling Anisotropic Magnetoresistance

(Ga,Mn)As

Au AlOx

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] [100]

0 5 10

0

4

8

exper. AMR⊥ . exper AMR// theory AMR⊥ theory AMR//

(-%)AMR

(%)Mn

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 ‘03-0.5 0.0 0.5

315

320

R (Ω)

( )B T

10K 4.2K 1.5K

V [mV]

I [nA]

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

GaAs

InAs

GaSb

Ge

300K!

T. Dietl, Science ’00; JVSTB ‘03

GaSb GaAs GaP GaN

CB

VB

Mn 3d

Predicted TC in (III,Mn)V semiconductors,

if Mn is a shallow acceptor

-50 -40 -30 -20 -10 0-5

-4

-3

-2

-1

0

1Ga

1-xMn

xN x=0.3%

M (emu/cm

3)

H (kOe)

T = 400 K T = 10 K T = 8 K T = 6 K T = 4 K

Ga1-xMnxN

Small RT ferromagnetic signal superimposed on larger paramagnetic part(Sonoda ’01; Reed ’01; Thaler ’02; Biquard ’03 etc.)

Several MnxNy magnetic phases

existZajac 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

0.000 0.005 0.010 0.015 0.020 0.025

1E16

1E17

1E18

0 2 4

40

50

60

70

0.2%

2.5%

4.2%

ΔEa ( )meV

(%)Mn concentration

pHall

(cm

-3)

1/ (T K-1)

ΔEa~50meV

Evidence for collective magnetic effects at low T:

0 1000 2000 3000 4000

-1

0

1

2

3

4

T = 5 / 15 / 50 K

.OPJ 25/03/04 12:20:16

Graph26

Moment [ emu/cm

3 ]

Magnetic Field [ Oe ]

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

-0.6 -0.3 0.0 0.3 0.6

Magnetisation (a.u.)

Field (T)

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

0 5 100.0

0.5

1.0

1.5

Carrier density (nm

-3)

Mn concentration (%)

easy axis [110]

easy axis [110 ]

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)