using spin in (future) electronic devices tomas jungwirth ip ascr, prague jan mašek,alexander shick...
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USING SPIN IN (FUTURE) ELECTRONIC DEVICESUSING SPIN IN (FUTURE) ELECTRONIC DEVICES
Tomas Jungwirth
IP ASCR, PragueJan Mašek,Alexander ShickJan Kučera, František Máca
University of Texas Allan MaDonald, Qian Niu, et
al.
University of WuerzburgLaurens Molenkamp, Charles Gould et al.
University of NottinghamBryan Gallagher, Kevin EdmondsTom Foxon, Richard Campion, et al.
Hitachi CambridgeJorg Wunderlich, Bernd Kaestner et al.
Texas A&MJairo Sinova, et al.
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OUTLINEOUTLINE
- Current and future (???) spintronic devices- Current and future (???) spintronic devices
- Challenges for spintronics - Challenges for spintronics research topics research topics
- Electrical manipulation of spin in normal semiconductors- Electrical manipulation of spin in normal semiconductors (Spin Hall effect)(Spin Hall effect)
- Ferromagnetic semiconductors - materials and devices- Ferromagnetic semiconductors - materials and devices
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
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CURRENT SPINTRONIC DEVICESCURRENT SPINTRONIC DEVICES
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HARD DISKSHARD DISKS
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HARD DISK DRIVE READ HEADSHARD DISK DRIVE READ HEADS
horse-shoe read/write heads
spintronic read heads
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Anisotropic magnetoresistance (AMR) read headAnisotropic magnetoresistance (AMR) read head
1992 - dawn of spintronics1992 - dawn of spintronics
Ferromagnetism large response (many spins) to small magnetic fields
Spin-orbit coupling spin response detected electrically
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Giant magnetoresistance (GMR) read headGiant magnetoresistance (GMR) read head
19971997
GMR
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MEMORY CHIPSMEMORY CHIPS
.DRAMDRAM (capacitor) - high density, cheephigh density, cheep x slow,
high power, volatile
.SRAMSRAM (transistors) - low power, fastlow power, fast x low density,
expensive, volatile
.Flash (floating gate) - non-volatilenon-volatile x slow, limited life,
expensive
Operation through electron chargecharge manipulation
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MRAM – universal memoryMRAM – universal memory (fast, small, non-volatile)
RAM chip that won't forget
↓
instant on-and-off computers
Tunneling magneto-resistance effect
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MRAM – universal memoryMRAM – universal memory (fast, small, non-volatile)
RAM chip that won't forget
↓
instant on-and-off computers
Tunneling magneto-resistance effect
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FUTURE (? or ???) SPINTRONIC DEVICESFUTURE (? or ???) SPINTRONIC DEVICES
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Low-dissipation microelectronics
Where Does All the Power Go?United States Energy Consumption: An
Overview
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.
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)
Long spin-coherence times → information carried by spin-currents Instead of electrical currents. Functionality based on spin-dynamics,e.g., domain wall motion
PROCESSORS PROCESSORS
Allwood et al., Science ’02
NOT gate
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QUANTUM COMPUTERSQUANTUM COMPUTERS
1 0
a + b
Classical bit
Q-bit massive quantummassive quantumparallelismparallelism
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CHALLENGES FOR SPINTRONICSCHALLENGES FOR SPINTRONICS
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FM
AFM
EXANGE-BIASEXANGE-BIAS
fails when scaled down to ~10 nm dimensions Look for other MR concepts
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EXTERNAL MAGNETIC FIELDEXTERNAL MAGNETIC FIELD
problems with integration - extra wires, addressing neighboring bits
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Current (insted of magnetic field) induced switching
Angular momentum conservation spin-torque
Buhrman & Ralph, NNUN ABSTRACTS '02Slonczewski, JMMM '96; Berger, PRB '96
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current
magnetic field
local, reliable, but fairlylarge currents needed
Myers et al., Science '99; PRL '02
Likely the future of MRAMsLikely the future of MRAMs
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INTEGRATION WITH SEMICONDUCTOR ELECTRONICSINTEGRATION WITH SEMICONDUCTOR ELECTRONICS
Spin-valve transistor
Metal ferromagnet to semiconductor spin-injector
All-semiconductor spintronicsAll-semiconductor spintronics
- electrical manipulation of spins (no external magnetic field)- electrical manipulation of spins (no external magnetic field) - making semiconductors ferromagnetic- making semiconductors ferromagnetic
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ELECTRICAL MANIPULATION OF SPINS IN NORMAL ELECTRICAL MANIPULATION OF SPINS IN NORMAL SEMICONDUCTORS - SPIN HALL EFFECTSEMICONDUCTORS - SPIN HALL EFFECT
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B
V
I
_
+ + + + + + + + + + + + +
_ _ _ _ _ _ _ _ _ _ FL
Lorentz force deflect chargedcharged--particles towards the edge
Ordinary Hall effectOrdinary Hall effect
Detected by measuring transverse voltage
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Spin Hall effectSpin Hall effect
Spin-orbit coupling “force” deflects like-spinlike-spin particles
I
_ FSO
FSO
_ __
V=0
non-magnetic
Spin-current generation in non-magnetic systems Spin-current generation in non-magnetic systems without applying external magnetic fieldswithout applying external magnetic fields
Spin accumulation without charge accumulationexcludes simple electrical detection
Kato, Myars, Gossard, Awschalom, Science Wunderlich, Kaestner, Sinova, Jungwirth, PRL '04
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Ingredients: - potential V(r)
- motion of an electron
Producesan electric field
In the rest frame of an electronthe electric field generates and effective magnetic field
- gives an effective interaction with the electron’s magnetic moment
E
E
Beff
k
Spin-orbit coupling Spin-orbit coupling (relativistic effect)
effSO BμH
)(1
rVe
E
Ecm
kBeff
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(r)Vkcm
sH imp22
2
SO
Skew scattering off impurity potentialSkew scattering off impurity potential
skewscattering
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lsdr
rdV
err
mc
k
mc
seBH effSO
)(1
l=0 for electrons weak SO
l=1 for holes strong SO
SO-coupling from host atoms SO-coupling from host atoms in a perfect crystal
E
E
v
Enhanced in asymmetric QW
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Classical dynamics in k-dependent (Rashba) field:
z-component of spin due to precession in effective "Zeeman" fieldz-component of spin due to precession in effective "Zeeman" field
xx eEdt
dk),kz(
LLG equations for small drift adiabatic solution:
x
yy
)t()t(n
dt
d
dt
dnn y
x
yzx
x2
x
z eEn
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p -AlG a As
i-G a As
n- -d o p e d AlG a As
e tc he d
QW
I
Top Emission
Side Emission
Electrode
Spin polarization detected through circular polarization of emitted lightSpin polarization detected through circular polarization of emitted light
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)
…
● No hetero-interface along the LED current
● Spin detection directly in the 2DHG
● Light emission near edge of the 2DHG
● 2DHG with strong and tunable SO
2DHG2DHG
2DEG2DEG
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EXPERIMENT
Spin Hall Effect
2DHG
2DEG VT
VD
np
10 µ
m
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Spin Hall Effect Device
1 .5 mc h a n n e l
n
n
py
xz
L E D 1
L E D 2
I P
xy
zIp
-Ip
ILED 1
Experiment “A”
xy
zIpILED 1
ILED 2
Experiment “B”
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Experiment “B”
1.505 1.510 1.515 1.520
-1
0
1
xy
zIpILED 1
ILED 2
CP
[%]
1.505 1.510 1.515 1.520
-1
0
1
xy
zIpILED 1
ILED 2
xy
zIpILED 1
ILED 2
CP
[%]
Experiment “A”
-1
0
1
xy
zIp
-Ip
ILED 1
CP
[%]
-1
0
1
xy
zIp
-Ip
ILED 1
-1
0
1
xy
zIp
-Ip
ILED 1
xy
zIp
-Ip
ILED 1
CP
[%]
Opposite perpendicular polarization for opposite Opposite perpendicular polarization for opposite IIpp currents currents
or opposite edges or opposite edges SPIN HALL EFFECT SPIN HALL EFFECT
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FERROMAGNETIC SEMICONDUCTORSFERROMAGNETIC SEMICONDUCTORS
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(Ga,Mn)As diluted magnetic semiconductor(Ga,Mn)As diluted magnetic semiconductor
MnGa
As
Ga
Low-T MBE - random but uniform Mn distribution up to ~ 10% doping
5 d5 d-electrons with -electrons with L=0, S=5/2L=0, S=5/2
moderately shallow moderately shallow acceptor acceptor
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Effective magneticEffective magnetic: Coulomb correlation of d-electrons & hopping AF kinetic-exchange coupling
Jpd
= + 0.6 meV nm3
Theoretical descriptionsTheoretical descriptions
MicroscopicMicroscopic: atomic orbitals & Coulomb correlation of d-electrons & hopping
Jpd
SMn
.shole
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As
GaMn
Mn Mn
Intrinsic properties of (Ga,Mn)AsIntrinsic properties of (Ga,Mn)As: Tc linear in MnGa local momentconcentration; falls rapidly with decreasing hole density in more than50% compensated samples; nearly independent of hole density for compensation < 50%.
Jungwirth, Wang, et al.cond-mat/0505215
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Extrinsic effects: 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
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 momentsBlinowski PRB ‘03, Mašek,
Máca PRB '03
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Tc as grown and annealed samplesTc as grown and annealed samples
0 1 2 3 4 5 6 7 8 9 100
20
40
60
80
100
120
140
160
180
T
C(K
)
Mntotal
(%)
-1 0 1-0.1
0.0
0.1
T = 172 K8% (Ga,Mn)As
M
[110](T
) / M S
at(5
K)
Magnetic Field [ Oe ]
Tc=173K
8% Mn
Open symbols as grown. Closed symbols annealed
Jungwirth, Wang, et al.cond-mat/0505215
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Number of holes per Mneff
Tc/xTc/xeffeff vs p/Mn vs p/Mneffeff
High (>40%) compensatio
n
Jungwirth, Wang, et al.cond-mat/0505215
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Theoretical linear dependence of Mnsub on total Mn confirmed experimentally
Generation of MnGeneration of Mnintint during growth during growth
Mnsub
MnInt
Jungwirth, Wang, et al.cond-mat/0505215
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Prospects of (Ga,Mn)As based materials with room TProspects of (Ga,Mn)As based materials with room Tcc
- 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
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Tunneling anisotropic magnetoresistance Tunneling anisotropic magnetoresistance
Single magnetic layerSingle magnetic layersensor or memory sensor or memory
Gould, Ruster, Jungwirth, et al., PRL '04
Giant magneto-resistance
[100]
[010]
[100]
[010]
[100]
[010]
(Ga,Mn)As(Ga,Mn)As
AuAu
no exchange-bias needed
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M || <111> M || <100>
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 energyDensity of states at the Fermi energy → → anisotropic tunnel magneto-resistanceanisotropic tunnel magneto-resistance
(Abolfath, Jungwirth et al., PRB '01
spin-split bands at M≠0
Dietl et al., Science '00
Spin-orbit coupling and anisotropiesSpin-orbit coupling and anisotropies
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GaMnAs Nanocontact TAMRGaMnAs Nanocontact TAMR
30nm constriction
Current [110]
5nm thick 2% Mn GaMnAs Hall bars & nanoconstrictions
Giddings, Khalid, Jungwirth, Sinova et al. PRL '05
-15 -10 -5 0 5 10 15-2.0
-1.0
0.0
1.0
2.0
10.0 K 4.2 K 1.5 K
V [mV]I [
nA]
Tunnelling conduction at low temperatures & voltages
30nm
Constriction
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Landauer-BLandauer-Büttiker üttiker tunnelling probabilitestunnelling probabilites
Magnetisation in plane
y
x
jt
z
Wavevector dependent tunnelling probabilityT (ky, kz) Red high T; blue low T.
xz
y
jt
strong z-confinement (ultra-thin film)
less strong y –confinement (constriction)
constriction:
Magnetization perpendicular to plane
Magnetization in plane
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30nm constriction
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6100
200
300
400
500
600
700
B || x
B || y
B || z
R (
MO
hm
s)
B (T)
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.60
1
2
3
4
5
6
R (
GO
hm
s)
B (T)
1400%
Very large TAMR in single nanocontacts
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-0.2 -0.1 0.0 0.1 0.2
0.17
0.18
0.19
0.20
0.21
R [M]
B [T]-0.2 -0.1 0.0 0.1 0.2
2
4
6
8
10
B [T]
3m bar 30nm constriction
B|| y
B || x
B || y
B || x
B || z B || z
AMRAMR in unstructured bar TAMRTAMR in constriction
MR response of constricted device and bar are very similar
in character but largely enhanced in the tunnel constriction
AMR & TAMRAMR & TAMR
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Spintronic nano-transistor Spintronic nano-transistor field-controlled MR device field-controlled MR device
Spintronic diode GMR, TMR, TAMR device
Spintronic wire AMR device
Final remark: spintronics in footsteps of electronicsFinal remark: spintronics in footsteps of electronics
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