Transcript SPIN HALL EFFECT

USING SPIN IN (FUTURE) ELECTRONIC DEVICES
Tomas Jungwirth
IP ASCR, Prague
Jan Mašek,Alexander Shick
Jan Kučera, František Máca
University of Nottingham
Bryan Gallagher, Kevin Edmonds
Tom Foxon, Richard Campion, et al.
Texas A&M
Jairo Sinova, et al.
University of Wuerzburg
Laurens Molenkamp, Charles Gould et al.
University of Texas
Allan MaDonald, Qian Niu, et al.
Hitachi Cambridge
Jorg Wunderlich, Bernd Kaestner et al.
Electron has a charge (electronics) and
spin (spintronics)
Electrons do not actually “spin”,
they produce a magnetic moment that is
equivalent to an electron spinning clockwise
or anti-clockwise
OUTLINE
- Current and future (???) spintronic devices
- Challenges for spintronics  research topics
- Electrical manipulation of spin in normal semiconductors
(Spin Hall effect)
- Ferromagnetic semiconductors - materials and devices
CURRENT SPINTRONIC DEVICES
HARD DISKS
HARD DISK DRIVE READ HEADS
spintronic read heads
horse-shoe read/write heads
Anisotropic magnetoresistance (AMR) read head
1992 - dawn of spintronics
Ferromagnetism  large response (many spins) to small magnetic fields
Spin-orbit coupling  spin response detected electrically
Giant magnetoresistance (GMR) read head
1997
GMR
MEMORY CHIPS
.
DRAM (capacitor) - high density, cheep x slow,
high power, volatile
.
SRAM (transistors) - low power, fast x low density,
expensive, volatile
.
Flash (floating gate) - non-volatile x slow, limited life,
expensive
Operation through electron charge manipulation
MRAM – universal memory
(fast, small, non-volatile)
Tunneling magneto-resistance effect
RAM chip that won't forget
↓
instant on-and-off computers
MRAM – universal memory
(fast, small, non-volatile)
Tunneling magneto-resistance effect
RAM chip that won't forget
↓
instant on-and-off computers
FUTURE (? or ???) SPINTRONIC DEVICES
Where Does All the Power
Go?
PROCESSORS
United States Energy Consumption: An
Overview
Low-dissipation microelectronics
The power we use at home and
outside of work accounts for only
about a fifth of the total energy
consumed in the United States
every year, according to the
Department of Energy.
(ABCNEWS.com)
April 24 — We have electronic gizmos for just
about every part of our daily lives, from brushing
our teeth to staying in touch no matter where we
are. Our swollen houses are stuffed with TVs,
computers, and ever-larger and more complicated
appliances.
Long spin-coherence times → information carried by spin-currents
Instead of electrical currents. Functionality based on spin-dynamics,
e.g., domain wall motion
NOT gate
Allwood et al., Science ’02
QUANTUM COMPUTERS
1
0
Classical bit
Q-bit
a
+b
massive quantum
parallelism
CHALLENGES FOR SPINTRONICS
EXANGE-BIAS
FM
AFM
fails when scaled down to
~10 nm dimensions
Look for other MR concepts
EXTERNAL MAGNETIC FIELD
problems with integration - extra wires, addressing neighboring bits
Current (insted of magnetic field) induced switching
Buhrman & Ralph, NNUN ABSTRACTS '02
Slonczewski, JMMM '96; Berger, PRB '96
Angular momentum conservation  spin-torque
magnetic field
current
Myers et al., Science '99; PRL '02
local, reliable, but fairly
large currents needed
Likely the future of MRAMs
INTEGRATION WITH SEMICONDUCTOR ELECTRONICS
Spin-valve transistor
Metal ferromagnet to
semiconductor spin-injector
All-semiconductor spintronics
- electrical manipulation of spins (no external magnetic field)
- making semiconductors ferromagnetic
ELECTRICAL MANIPULATION OF SPINS IN NORMAL
SEMICONDUCTORS - SPIN HALL EFFECT
Ordinary Hall effect
Lorentz force deflect charged-particles towards the edge
B
_ _ _ _ _ _ _ _ _ _
_
FL
+++++++++++++
I
V
Detected by measuring transverse voltage
Spin Hall effect
Spin-orbit coupling “force” deflects like-spin particles
_
FSO
__
FSO
non-magnetic
I
V=0
Spin-current generation in non-magnetic systems
without applying external magnetic fields
Spin accumulation without charge accumulation
excludes simple electrical detection
Kato, Myars, Gossard, Awschalom, Science
Wunderlich, Kaestner, Sinova, Jungwirth, PRL '04
Spin-orbit coupling (relativistic effect)
Produces
an electric field
Ingredients: - potential V(r)
E


- motion of an electron
In the rest frame of an electron
the electric field generates and
effective magnetic field
- gives an effective interaction with the electron’s
magnetic moment
k
E
H SO
Beff

1
E   V (r )
e
 
  μ  Beff


 k  
 E
Beff  

cm


Skew scattering off impurity potential
H SO

skew
scattering

 2s   
  2 2   k  Vimp(r)
m c 


SO-coupling from host atoms in a perfect crystal
H SO

E






 
 es   k  1 dV (r ) 
    Beff  
 r
   s  l
 mc   mc  er dr 
l=0 for electrons  weak SO
l=1 for holes  strong SO

E
Enhanced in asymmetric QW

v
z-component of spin due to precession in effective "Zeeman" field
 dk

Classical dynamics in k-dependent (Rashba) field:    ( z  k ), x  eE
x
dt

LLG equations for small drift  adiabatic solution:
 dy
x nz  

dt
x dt
dn y

nz  

x
2
eEx
y ( t )
ny ( t ) 
x
Conventional
vertical spin-LED
Novel co-planar spin-LED
Y. Ohno et al.: Nature 402, 790 (1999)
R. Fiederling et al.: Nature 402, 787
(1999)
B. T. Jonker et al.: PRB 62, 8180 (2000)
X. Jiang et al.: PRL 90, 256603 (2003)
R. Wang et al.: APL 86, 052901 (2005)
Top Emission
● Spin detection directly in the 2DHG
● Light emission near edge of the 2DHG
● 2DHG with strong and tunable SO
…
Electrod
e
QW
● No hetero-interface along the LED current
I
p-AlGaAs
etched
2DHG
Side Emission
i-GaAs
2DEG
n--doped AlGaAs
Spin polarization detected through circular polarization of emitted light
EXPERIMENT
10 µm
p n
Spin Hall Effect
2DEG
2DHG
VD
VT
Spin Hall Effect Device
IP
LED 1
p
1.5m
Experiment “A”
channel
Ip
-Ip
zI
LED 1
n
n
y
z
x
Experiment “B”
LED 2
Ip
y
x
x
z ILED 1
ILED 2
y
Experiment “A”
Ip
-Ip
zI
LED 1
y
1
x
CP [%]
0
-1
Experiment “B”
Ip
z ILED 1
ILED 2
y
1
0
-1
1.505
1.510
1.515
Opposite perpendicular polarization for opposite Ip currents
or opposite edges  SPIN HALL EFFECT
1.520
CP [%]
x
FERROMAGNETIC SEMICONDUCTORS
(Ga,Mn)As diluted magnetic semiconductor
Low-T MBE - random but uniform Mn distribution
up to ~ 10% doping
MnGa
As
5 d-electrons with
L=0, S=5/2
Ga
moderately shallow
acceptor
Theoretical descriptions
Microscopic: atomic orbitals & Coulomb correlation of d-electrons & hopping
Jpd = + 0.6 meV nm3
Jpd SMn.shole
Effective magnetic:
Coulomb correlation of d-electrons & hopping AF kinetic-exchange coupling
Mn Mn
Jungwirth, Wang, et al.
cond-mat/0505215
As
Mn
Ga
Intrinsic properties of (Ga,Mn)As: Tc linear in MnGa local moment
concentration; falls rapidly with decreasing hole density in more than
50% compensated samples; nearly independent of hole density
for compensation < 50%.
Extrinsic effects:
Interstitial Mn - a magnetism killer
Interstitial Mn is detrimental to magnetic order:
compensating double-donor – reduces carrier density
couples antiferromagnetically to substitutional Mn even in
low compensation samples  smaller effective number of Mn moments
Blinowski PRB ‘03, Mašek,
Máca PRB '03
Mn
As
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.
Tc as grown and annealed samples
Open symbols as grown. Closed symbols annealed
180
140
0.1
M[110](T) / MSat(5K)
160
8% (Ga,Mn)As
T = 172 K
0.0
120
-0.1
-1
0
1
Magnetic Field [ Oe ]
180
180
160
80
40
160
20
140
0
120
0
1
2
3
4
1005
Mn80total(%)
60
1.7% Mn
2.2%
1.7% Mn
3.4%Mn
Mn
2.2%
4.5%Mn
Mn
3.4%
5.6%Mn
Mn
4.5%
6.7%Mn
Mn
5.6%
9%Mn
Mn
6.7%
Mn
8%
9% Mn
C
180
160
140
140
120
1.7% Mn
120
2.2%100
Mn
100
3.4%
Mn
80
4.5%80
Mn
5.6% 60
Mn
66.7%60
7
Mn
40
9% Mn
40
20
20
TC(K)
T (K)
60
TC(K)
TC(K)
100
Tc=173K
8
9
10
Jungwirth, Wang, et al.
cond-mat/0505215
A
A
A
A
Tc/xeff vs p/Mneff
High (>40%)
compensation
Jungwirth, Wang, et al.
cond-mat/0505215
Number of holes per Mneff
Generation of Mnint during growth
Theoretical linear dependence of Mnsub on total Mn confirmed
experimentally
Mnsub
MnInt
Jungwirth, Wang, et al.
cond-mat/0505215
Prospects of (Ga,Mn)As based materials with room Tc
- Concentration of uncompensated MnGa moments has to reach ~10%
only 6.2% in the current record Tc=173K sample
- Charge compensation not so important unless > 40%
- No indication from theory or experiment that the problem is other
than technological - better control of growth-T, stoichiometry;
new growth or chemical composition strategies to incorporate more
MnGa local moments or enhance p-d coupling
Tunneling anisotropic magnetoresistance
(Ga,Mn)As
Au
Au
no exchange-bias needed
[100]
[100]
[010]
[100]
[010]
[010]
Single magnetic layer
sensor or memory
Gould, Ruster, Jungwirth,
et al., PRL '04
Giant magneto-resistance
Spin-orbit coupling and anisotropies
M || <111>
M || <100>
spin-split bands at M≠0
Dietl et al., Science '00
(Abolfath, Jungwirth et al., PRB '01
Magnetization orientation dependences
- Hole total energy over Fermi volume
→ magnetic anisotropy
- Group velocities at the Fermi surface and density of
states for scattering
→ in plane magneto-resistance anisotropy
- Density of states at the Fermi energy
→ anisotropic tunnel magneto-resistance
GaMnAs Nanocontact TAMR
5nm thick 2% Mn GaMnAs Hall bars & nanoconstrictions
2.0
Current [110]
I [nA]
1.0
10.0 K
4.2 K
1.5 K
0.0
30nm
30nm
Constriction
constriction
-1.0
-2.0
-15
-10
-5
0
V [mV]
5
10
Tunnelling conduction at low
temperatures & voltages
Giddings, Khalid, Jungwirth, Sinova et al. PRL '05
15
Landauer-Büttiker tunnelling probabilites
Wavevector dependent tunnelling probabilityT (ky, kz)
Red high T; blue low T.
y
Magnetisation in plane
jt
z
x
y
Magnetization
perpendicular
to plane
constriction:
jt
Magnetization
in plane
z
x
strong z-confinement
(ultra-thin film)
less strong y –confinement
(constriction)
30nm
constriction
700
R (MOhms)
600
B || z
500
400
B || y
300
B || x
200
100
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.2
0.4
0.6
B (T)
6
Very large TAMR in
single nanocontacts
R (GOhms)
5
4
3
1400%
2
1
0
-0.6
-0.4
-0.2
0.0
B (T)
AMR & TAMR
3m bar
0.21
B || z
0.20
R [M]
30nm constriction
10
B|| y
0.19
B || z
8
0.18
6
B || y
4
B || x
0.17
-0.2
-0.1
0.0
0.1
B [T]
AMR in unstructured bar
0.2
B || x
2
-0.2
-0.1
0.0
0.1
B [T]
TAMR in constriction
MR response of constricted device and bar are very similar
in character but largely enhanced in the tunnel constriction
0.2
Final remark: spintronics in footsteps of electronics
Spintronic wire
AMR device
Spintronic diode
GMR, TMR, TAMR device
Spintronic nano-transistor
field-controlled MR device