Transcript Slide 1

Spin-orbit coupling and spintronics in ferromagnetic
semiconductors (and metals)
Tomas Jungwirth
Institute of Physics ASCR
Alexander Shick, Jan Mašek, Josef Kudrnovský,
František Máca, Karel Výborný, Jan Zemen,
Vít Novák, Kamil Olejník, Jairo Sinova et al.
University of Nottingham
Bryan Gallagher, Tom Foxon,
Richard Campion, Kevin Edmonds,
Andrew Rushforth, Chris King et al.
Hitachi Labs., UK & Japan
Jorg Wunderlich, Byong-Guk Park,
Andrew Irvine, Elisa De Ranieri, Samuel Owen,
David Williams, Akira, Sugawara, et al.
Outline
1. Intro – spin-orbit coupling in spintronics
2. GaMnAs based spintronic devices
3. GaMnAs and other spin-orbit coupled ferromagnetic materials
e-
Spintronics
Spin-orbit couping
nucleus rest frame
I  Qv
&
E
electron rest frame
Q
4 0 r
… it’s all about spin and charge of
electron communicating
3
r
&
0 I  r
B

3
4 r
1
B   0 0 v  E  2 v  E
c
Lorentz transformation  Thomas precession
SO-couping = E&M and postulated electron spin
H SO
g B
e
e  1 dV (r ) 

SB 
S vE 
S  l
2
2 
2m c
2
2m c  er dr 
Ferromagnetism = Pauli exclusion principle & Coulomb repulsion
etotal wf antisymmetric
e-
= orbital wf antisymmetric * spin wf symmetric
(aligned)
DOS
e… collective
communication
DOS
macroscopic moment  large effects
GMR
~ 1% MR effect
~ 10% MR effect
<
AMR
FM & SO-coupling
(M )
FM only (  )
+ larger MR
+ linear sensing, low-noise
- low MR, low-resistance
TAMR
AlOx
Au
TDOS
-
CBAMR
TMR
~ 100% MR effect
Au
(M )
low-resistance, non-linear, spin-coherence,
exchange biasing or interlayer coupling,
higher noise
TDOS  TDOS
chem. pot.
Combining “+” and eliminating “-” of
AMR and TMR(GMR)
& SET gating  spintronic transistor
+ very large MR, high resistance,
bistable  memory
-
non-linear, spin-coherence, exchange
biasing, higher noise
SO-coupling  magnetocrystalline anisotropies  sensitivity to lattice distortions
Ferromagnetic/magnetostrictive
magneto-sensors, transducors,
memory, storage
piezo/FM
hybrids
FM semiconductors
Semicondicting/gatable
Ferroelectric/piezoelectric
electro-sensors, transducors,
memory
FeFET
transistors, processors
Systems integrating all three basic elements of current microelectronics
Outline
1. Intro – spin-orbit coupling in spintronics
2. GaMnAs based spintronic devices
3. GaMnAs and other spin-orbit coupled ferromagnetic materials
(Ga,Mn)As: archetypical system for SO-coupling based spintronics research
Ga
SW-transf.  Jpd SMn . shole
As-p-like holes
Mn
As
Mn
Mn-d-like local
moments
Dilute Mn-doped SC:
sensitive to doping; 100smaller Ms than in conventional metal FMs  weak dipolar fields
Mn-Mn coupling mediated by holes in SO-coupled SC valence bands:
sensitive to gating, comparable magnetocrystalline anisotropy energy and stiffness to metal FMs
Model sp-d ferromagnet:
kinetic-exchange (Jpd) & host SC bands provides simple yet often semiquantitative description
Coulomb blockade AMR – anisotropic chemical potential
Source
Q VD
Drain
Gate
VG
M || <110>

Q( M )
U   dQ'VD ( Q' ) 
e
0
[110]
M || <100>
[010] M
F
Q
[100]
[110]
[010]


( Q  Q0 )2
( M ) C
U
& Q0  CG [ VG  VM ( M )] &VM 
2C
e
CG
electric
& magnetic
control of Coulomb blockade oscillations
(M)
Tunneling AMR – anisotropic TDOS
TAMR in GaMnAs
Au
GaMnAs
Au
Anisotropc tunneling amplitudes
M perp.
Resistance
AlOx
Magnetisation in plane
~ 1-10% in metallic GaMnAs
M in-plane
Huge when approaching MIT in GaMnAs
One
Strain controlled micromagnetics
(b)
0.1-1 m
DW structure and dynamics directly reflecting
e.g. (strain dependent) competition between
uniaxial and cubic anisotropies
500 nm
strain ~ 10-4
… plus 100-10x smaller currents for DW switching
and 100-10x weaker dipolar crosslinks  prospect
for dense integration of magnetic microelements
switchable by low currents
One
Sensitivity of AMR to lattice distortions
bulk
GaMnAs
GaAs
~100nm - 1m wide bars
Outline
1. Intro – spin-orbit coupling in spintronics
2. GaMnAs based spintronic devices
3. GaMnAs and other spin-orbit coupled ferromagnetic materials
coupling strength / Fermi energy
Magnetism in systems with coupled dilute moments
and delocalized band electrons
band-electron density / local-moment density
(Ga,Mn)As
GaAs VB
GaAs:Mn extrinsic semiconductor
Mn-acceptor level (IB)
GaMnAs disordered VB
2.2x1020 cm-3
VB-IB
VB-CB

Short-range ~ M . 
s potential
- additional Mn-hole binding
- ferromagnetism
- scattering
MIT (and ferromagnetism) at relatively large
doping  suppressed gating effect
MIT in p-type GaAs:
- shallow acc. (30meV) ~ 1018 cm-3
- Mn (110meV) ~1020 cm-3
MIT in GaAs:Mn at order of magnitude
higher doping than quoted in text books
Weak hybrid.
Delocalized holes
long-range coupl.
optimal combination of large SO-cupling,
hole delocalization, hole-Mn coupling
SO-coupling strength, band-parabolicity
Search for optimal III-V host
InSb, InAs
d5
GaAs
GaP
Strong hybrid.
Impurity-band holes
short-range coupl.
AlAs
d5d4
no holes
d
GaN
d4
I(II,Mn)V dilute-moment ferromgantic semiconductors
III = I + II  Ga = Li + Zn
• GaAs and LiZnAs are twin
semiconductors
• Prediction that Mn-doped are also
twin ferromagnetic semiconductors
• No limit for Mn-Zn (II-II) substitution
within the same crystal structure
• Independent carrier (holes and electrons)
doping by Li-Zn stoichiometry adjustment
Zinc Blende – (III,Mn)V
I(II,Mn)V as a link between DMSs and
high-Tc half-metalic Heuslers,
all comaptible with III-V technology
I(II,Mn)V
+ interstitial
FCC
+ interstitial
+ interstitial
Half Heusler (NiMnSb)
Rock Salt
+ interstitial
+ interstitial
High Tc large SO-coupling TM thin films and ordered alloys
heavy TM
FM TM
heavy TM
FM TM
FM TM
heavy TM
spontaneous moment
spin-orbit coupling
magnetic susceptibility
Key: large induced moment on
strongly SO-coupled heavy TM
B. G. Park, J. Wunderlich, D. A. Williams, S. J. Joo, K. Y. Jung, K. H. Shin, K. Olejnik, A. B. Shick, and T.
Jungwirth: Tunneling anisotropic magnetoresistance in multilayer-(Co/Pt)/AlOx/Pt structures, submitted
to Phys. Rev. Lett. (2007)
Akira Sugawara, H. Kasai, A. Tonomura, P. D. Brown, R. P. Campion, K. W. Edmonds, B. L.
Gallagher, J. Zemen, and T. Jungwirth: Domain walls in (Ga,Mn)As diluted magnetic semiconductor,
Phys. Rev. Lett. in press (2007)
A. W. Rushforth, K. Výborný, C. S. King, K. W. Edmonds, R. P. Campion, C. T. Foxon, J. Wunderlich,
A. C. Irvine, P. Vašek, V. Novák, K. Olejník, Jairo Sinova, T. Jungwirth, B. L. Gallagher: Anisotropic
magnetoresistance components in (Ga,Mn)As, Phys. Rev. Lett. 99 (2007) 147207
J. Masek, J.Kudrnovsky, F. Maca, B. L. Gallagher, R. P. Campion, D. H. Gregory, and T. Jungwirth:
Dilute moment n-type ferromagnetic semiconductor Li(Zn,Mn)As, Phys. Rev. Lett. 98 (2007) 067202
J. Wunderlich, T. Jungwirth, B. Kaestner, A. C. Irvine, K.Y. Wang, N. Stone, U. Rana, A. D. Giddings, A. B.
Shick, C. T. Foxon, R. P. Campion, D. A. Williams, B. L Gallagher: Coulomb Blockade Anisotropic
Magnetoresistance Effect in a (Ga,Mn)As Single-Electron Transistor, Phys. Rev. Lett. 97 (2006) 077201
T. Jungwirth, Jairo Sinova, J. Mašek, J. Kučera, and A.H. MacDonald: Theory of ferromagnetic (III,Mn)V
semiconductors, Rev. Mod. Phys. 78 (2006) 809
C. Rüster, C. Gould, T. Jungwirth, J. Sinova, G.M. Schott, R. Giraud, K. Brunner, G. Schmidt, L.W.
Molenkamp: Very Large Tunneling Anisotropic Magnetoresistance of a (Ga,Mn)As/GaAs/(Ga,Mn)As
Stack, Phys. Rev. Lett. (2005) 027203