Post on 01-Jan-2016
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//