Expecting the unexpected in the spin Hall effect:
from fundamental to practical
JAIRO SINOVA
Vivek Amin
Texas A&M University
Institute of Physics ASCR
Institute of Physics ASCR
Tomas Jungwirth, Vít Novák, Karel Vyborny, et al
Hitachi Cambridge
Joerg Wünderlich, A. Irvine, et al
Frontiers in Materials: Spintronics
Strasboug, France
May 13th, 2012
Research fueled by:
Expecting the unexpected in the spin Hall effect: from fundamental to practical!
The spin Hall effects is a relativistic spin-orbit coupling phenomenon which can be used to electrically
generate or detect spin currents in magnetic and non-magnetic systems. In spite of the short time
ince its discovery it has now become ubiquitous in the field of spintronics and its development has
also sparked a renewed interest in the full understanding of all the relating effects, such as the
anomalous Hall effect. Exploiting such effects fully requires a better understanding of the effects and
heir intricate connections. SHE has been used to create one of the first spin FETs, to measure spin-currents
enerated by magnetization dynamics, and even to generate spin-currents large enough to produce spin-torque
ffects! In this talk I attempt to give a short, brief, and possibly oversimplified overview of some of the
ritical developments of this vibrant subfield of spintronics, its present understanding, its present
applications, and some of its future challenges.
Expecting the unexpected
in the spin Hall effect:
from fundamental to practical
I. Introduction:
•Basic AHE and SHE phenomenology
•Mechanism
II. Spin Hall effect: the early days
•First proposals: from theory to experiment
•First observations of the extrinsic and intrinsic (optical)
• Inverse spin Hall effect
•Direct iSHE in metals
•Spin pumping and FMR
•SHE-FET
•Spin Hall injection and spin precession manipulation
•iSHE device with spin-accumulation modulation
• SHE as a spin current generator and detector
•Spin based FET: old and new paradigm in charge-spin transport
•Theory expectations and modeling
•Experimental results
Conclusion
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
3
The electron:
the key character with dual personalities
SPIN 1/2
Makes the electron
antisocial: a fermion
CHARGE
Easy to manipulate:
Coulomb interaction
quantum mechanics
E=p2/2m
E→ iħ d/dt
p→ -iħ d/dr
+
special relativity
E2/c2=p2+m2c2
(E=mc2 for p=0)
& spin
= particles/antiparticles
Dirac equation
“Classical” external manipulation of charge & spin
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
4
Using charge and spin in information technology
Using charge to create a field effect transistor:
work horse of information processing
Vg >0
S
gate
insulator
semiconductor
Using spin: Pauli exclusion principle and
Coulomb repulsion →ferromagnetism
work horse of information storage
total wf antisymmetric = orbital wf
antisymmetric × spin wf symmetric (aligned)
D
substrate
HIGH tunablity of electronic transport
properties the key to FET success in
processing technology
What about the internal communication
between charge & spin? (spintronics)
• Robust (can be as strong as bonding in solids)
• Strong coupling to magnetic field
(weak fields = anisotropy fields needed
only to reorient macroscopic moment)
e-
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
5
Internal communication between spin and charge:
spin-orbit coupling interaction
(one of the few echoes of relativistic physics in the solid state)
e-
Classical explanation (in reality it arises from a second order expansion of
Dirac equation around the non-relativistic limit)
• “Impurity” potential
•
V(r)
Produces
an electric field
In the rest frame of an electron
the electric field generates an
Motion of an electron
effective magnetic field
This gives an effective interaction with the electron’s magnetic moment
s
p
V
Beff
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
6
Internal communication between spin and charge:spinorbit coupling interaction
(one of the few echoes of relativistic physics in the solid state)
e-
Classical explanation (in reality it arises from a second order expansion of
Dirac equation around the non-relativistic limit)
• “Impurity” potential
•
V(r)
Produces
an electric field
In the rest frame of an electron
the electric field generates an
Motion of an electron
effective magnetic field
This gives an effective interaction with the electron’s magnetic moment
V
s
p
Beff
Consequence #1
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
7
Internal communication between spin and charge:spinorbit coupling interaction
(one of the few echoes of relativistic physics in the solid state)
e-
Classical explanation (in reality it arises from a second order expansion of
Dirac equation around the non-relativistic limit)
• “Impurity” potential
•
V(r)
Produces
an electric field
In the rest frame of an electron
the electric field generates an
Motion of an electron
effective magnetic field
This gives an effective interaction with the electron’s magnetic moment
V
s
p
Beff
Consequence #2
Mott scattering
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
8
How spintronics has impacted your life: Metallic spintronics
1992 - dawn of (metallic) spintronics
•Anisotropic magnetoresistance (AMR): In
ferromagnets the current is sensitive to the
relative direction of magnetization and current
direction
magnetization
e-
current
Appreciable sensitivity, simple design, cheap BUT only a 2-8 % effect
Giant magnetoresistance (GMR) read head - 1997
Fert, Grünberg et al. 1998
e-
×
Nobel Price 2007
Fert and Grünberg
High sensitivity, very large effect 30-100%
 and  are almost on and off states:
“1” and “0” & magnetic  memory bit
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
9
What next? The need for basic research
Industry has been successful in doubling of transistor numbers on a chip
approximately every 18 months (Moore’s law). Although expected to continue for
several decades several major challenges will need to be faced.
Circuit heat generation is one key limiting factor for scaling device speed
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
10
Information and communication technology
power consumption HAS consequences
Relative electricity consumption of ICT equipment
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
11
The need for basic research in technology development
International Technology Roadmap for Semiconductors
Basic
Research Inc.
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
12
Control of materials and transport properties via spin-orbit coupling
Nanotransport
New
magnetic
materials
GaAs
Mn
Magnetotransport
Caloritronics
Effects of spin-orbit
coupling in
multiband systems
Spintronic
Hall effects
Topological
transport
effects
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
13
Control of materials and transport properties via spin-orbit coupling
Nanotransport
Magnetotransport
New
Anomalous
Hall effects
magnetic
materials
GaAs
majority Mn
Caloritronics
FSO
Effects of spin-orbit
FSO
coupling in
multiband systems
ISpintronic
Hall effects
minority
V
Topological
transport
effects
Nagaosa, Sinova, Onoda,
MacDonald, Ong, RMP 10
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
14
Anomalous Hall Effect: the basics
Spin dependent “force” deflects like-spin particles
M⊥
_
__
majority
FSO
FSO
I
minority
ρH=R0B ┴ +4π RsM┴
AHE is does NOT originate from any internal
magnetic field created by M⊥; the field would
have to be of the order of 100T!!!
V
Simple electrical measurement
of out of plane magnetization (or
spin polarization ~ n↑-n↓)
InMnAs
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
15
Cartoon of the mechanisms contributing to AHE
Skew scattering
A
~σ~1/ni
Vimp(r) (Δso>ħ/τ)  λ*Vimp(r) (Δso<ħ/τ)
Asymmetric scattering due to the spin-orbit
coupling of the electron or the impurity.
Known as Mott scattering.
Intrinsic deflection B
independent of
impurity density
E
Electrons deflect to the right or to the left as
they are accelerated by an electric field ONLY
because of the spin-orbit coupling in the
periodic potential (electronics structure)
SO coupled quasiparticles
Electrons have an “anomalous” velocity perpendicular to the electric field
related to their Berry’s phase curvature which is nonzero when they have
spin-orbit coupling.
Side jump scattering B
independent of impurity density
Vimp(r) (Δso>ħ/τ)
 λ*Vimp(r) (Δso<ħ/τ)
Electrons deflect first to one side due to the field created by the impurity and deflect back when they leave the impurity
since the field is opposite resulting in a side step. They however come out in a different band so this gives rise to an
anomalous velocity through scattering rates times side jump.
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
16
Anomalous Hall effect: more than meets the eye
Spin Hall Effect
Anomalous Hall Effect
_
_
_
_
O
FS
majority
FS
I
O
FS
O
FS
I
O
minority
V
Topological
Insulators
V
Wunderlich, Kaestner, Sinova,
Jungwirth PRL 04
Inverse SHE
Kato et al
Science 03
Extrinsic
Intrinsic
Mesoscopic Spin Hall Effect
Spin-injection Hall Effect
V
Intrinsic
Kane and Mele
PRL 05
Valenzuela et al
Nature 06
Brune,Roth, Hankiewicz,
Sinova, Molenkamp, et al
Nature Physics 2010
Wunderlich, Irvine, Sinova,
Jungwirth, et al, Nature Physics 09
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
17
New twists in spintronics:
anomalous Hall effect, spin-helix transistors, and
topological thermoelectrics
I. Introduction: using the dual personality of the electron
•Internal coupling of charge and spin: origin and present use
•Control of material and transport properties through spin-orbit coupling
• Anomalous Hall effect: from the metallic to the insulating regime
•Anomalous Hall effect basics, history, progress in the metallic regime
•Spin injection Hall effect: a new paradigm in exploiting SO coupling
•Spin based FET: old and new paradigm in charge-spin transport
•Theory expectations and modeling
•Experimental results
•Topological thermoelectrics:Thermoelectric figure of merit
•Increase of ZT in topological insulators.
Conclusion
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
18
Towards a realistic spin-based non-magnetic FET device
Can we achieve direct spin polarization injection, detection, and manipulation by
electrical means in an all paramagnetic semiconductor system?
Long standing paradigm: Datta-Das FET (1990)
Exploiting the large Rashba spin-orbit coupling in InAs
[010]
gate
⊗ ⊗⊗
ky [010]
[100]
Rashba effective
magnetic field
kx [100]
[001]
Electrons are confined in the z-direction in
the first quantum state of the asymmetric
trap and free to move in the x-y plane.
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
19
Towards a realistic spin-based non-magnetic FET device
Can we achieve direct spin polarization injection, detection, and manipulation by
electrical means in an all paramagnetic semiconductor system?
Long standing paradigm: Datta-Das FET (1990)
Exploiting the large Rashba spin-orbit coupling in InAs
High
resistance “1”
“0”
Low resistance
BUT lMF << LS-D at room temperature
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
20
Dephasing of the spin through the Dyakonov-Perel mechanism
LSD ~ μm
lMF ~ 10 nm
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
21
New paradigm using SO coupling: SO not so bad for dephasing
Problem: Rashba SO coupling in
the Datta-Das SFET is used for
manipulation of spin (precession)
BUT it dephases the spin too quickly
(DP mechanism).
1) Can we use SO coupling to manipulate spin AND increase spin-coherence?
• Can we detect the spin in a non-destructive way electrically?
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
22
Spin-dynamics in 2D electron gas with Rashba
and Dresselhauss spin-orbit coupling
1) Can we use SO coupling to manipulate spin AND increase spin-coherence?
a 2DEG is well described by the effective Hamiltonian:
Rashba: from the asymmetry of the
confinement in the z-direction
ky [010]
 > 0, = 0
[110]
Dresselhauss: from the broken inversion
symmetry of the material, a bulk property
ky [010]
 = 0, < 0
kx [100]
_
[110]
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
[110]
kx [100]
_
[110]
23
Effects of Rashba and Dresselhaus SO coupling
ky [010]
 > 0, = 0
 = -
[110]
ky [010]
[110]
kx [100]
_
[110]
 = 0, < 0
kx [100]
ky [010]
[110]
_
[110]
kx [100]
_
[110]
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
24
Spin-dynamics in 2D systems with Rashba and Dresselhauss SO coupling
For the same distance traveled along [1-10], the spin precesses by exactly the same angle.
[110]
_
[110]
_
[110]
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
25
Persistent state spin helix verified by pump-probe experiments
Similar wafer parameters to ours
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
26
Spin-helix state when α ≠ β
For Rashba or Dresselhaus by
themselves NO oscillations are
present; only and over damped
solution exists; i.e. the spin-orbit
coupling destroys the phase
coherence.
There must be TWO competing
spin-orbit interactions for the spin
to survive!!!
Wunderlich, Irvine, Sinova, Jungwirth, et al, Nature Physics 09
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
27
New paradigm using SO coupling: SO not so bad for dephasing
Problem: Rashba SO coupling in
the Datta-Das SFET is used for
manipulation of spin (precession)
BUT it dephases the spin too quickly
(DP mechanism).
1) Can we use SO coupling to manipulate spin AND increase spin-coherence?
Use the persistent spin-Helix state and control of SO coupling strength
(Bernevig et al 06, Weber et al 07, Wünderlich et al 09)
• Can we detect the spin in a non-destructive way electrically?
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
28
✓
AHE contribution to Spin-injection Hall effect in a 2D gas
Two types of contributions:
i)S.O. from band structure interacting with the field (external and internal)
•Bloch electrons interacting with S.O. part of the disorder
Type (i) contribution much smaller in the weak SO coupled regime where the SOcoupled bands are not resolved, dominant contribution from type (ii)
Crepieux et al PRB 01
Nozier et al J. Phys. 79
Lower bound
estimate of skew
scatt. contribution
Wunderlich, Irvine, Sinova, Jungwirth, et al, Nature Physics 09
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
29
Spin-injection Hall effect: theoretical expectations
Local spin-polarization → calculation of AHE signal
Weak SO coupling regime → extrinsic skew-scattering term is dominant
Lower bound
estimate
1) Can we use SO coupling to manipulate spin AND increase spin-coherence?
Use the persistent spin-Helix state and control of SO coupling strength
• Can we detect the spin in a non-destructive way electrically?
Use AHE to measure injected current polarization electrically
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
✓
✓
30
Spin-injection Hall effect device schematics
Vd
h h
h h h
h
Vs
e
e
VH e
e
e
e
2DHG
2DEG
For our 2DEG system:
Hence α ≈ -β
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
31
Spin-injection Hall device measurements
trans. signal
σ - σo σ + σo
VL
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
32
Spin-injection Hall device measurements
SIHE ↔ Anomalous Hall
trans. signal
σ - σo σ + σo
VL
Local Hall voltage changes sign and magnitude along a channel of 6 μm
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
33
Further experimental tests of the observed SIHE (preliminary)
T = 250K
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
34
SiHE: new results
SiHE
(a)
x
I
Vb
VH2 VH1
inverse SHE
(b)
x
VH2
VH1
Vb
Spin Hall effect transistor:
Wunderlich, Irvine, Sinova, Jungwirth,
et al, Science 2010
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
35
SiHE transistor
Vb
Vg
VH
I
x
VH
Vg
Vb
x=1 m
Spin Hall effect transitor:
Wunderlich, Irvine, Sinova, Jungwirth,
et al, Science 2010
+
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
36
SHE transistor AND gate
Vg2 [V]
+0.1
0
-0.1
0
RH1
[]
12
6
H1
-
-
RH2 0
[]
3
6
-
H2
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
37
New twists in spintronics:
anomalous Hall effect, spin-helix transistors, and
topological thermoelectrics
I. Introduction: using the dual personality of the electron
•Internal coupling of charge and spin: origin and present use
•Control of material and transport properties through spin-orbit coupling
• Anomalous Hall effect: from the metallic to the insulating regime
•Anomalous Hall effect basics, history, progress in the metallic regime
•Spin injection Hall effect: a new paradigm in exploiting SO coupling
•Spin based FET: old and new paradigm in charge-spin transport
•Theory expectations and modeling
•Experimental results
•Topological thermoelectrics:Thermoelectric figure of merit
•Increase of ZT in topological insulators.
Conclusion
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
38
Control of materials and transport properties via spin-orbit coupling
Nanotransport
New
magnetic
materials
GaAs
Mn
Magnetotransport
Effects of spin-orbit
coupling in
multiband systems
Caloritronics
Topological
thermoelectrics
Spintronic
Hall effects
Topological
transport
effects
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
39
From AHE to topological insulators to thermoelectrics
Topological Insulators: edge (2D) or surface
states (3D) survive disorder effects when the
bulk gap is produced by spin-orbit coupling
Kane, Zhang, Molenkamp, Moore, et al
Zhang, Physics 1, 6 (2008)
X
X
QSHE in HgTe
Dislocations have 1D channels
which also protected
Vishwanath et al Nature Physics 09
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
40
Thermoelectric generator
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
41
Courtesy of Saskia Fischer
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
42
Courtesy of Saskia Fischer
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
43
From AHE to topological insulators to thermoelectrics
Seebeck coefficient
Best thermoelectrics
electrical conductivity
electric thermal conductivity
phonon thermal conductivity
Dislocations have 1D channels
which also protected
?
Vishwanath et al 09
?
Can we obtain high ZT through the topological
protected states; are they related to the high ZT
of these materials?
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
44
Possible large ZT through dislocation engineering
Bi1−xSbx (0.07 < x < 0.22)
where the L’s are the linear Onsager dynamic coefficients
Localized bulk states
Tretiakov, Abanov, Murakami, Sinova APL 2010
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
45
Possible large ZT through dislocation engineering
Remains very speculative but simple theory
gives large ZT for reasonable parameters
Tretiakov, Abanov, Murakami, Sinova APL 2010
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
46
Beyond Bi1−xSbx (0.07 < x < 0.22)
So far only one material is believed to have protected 1D
states on dislocations: how to further exploit TI properties to
increase ZT?
Analogy to HolEy Silicon
Tang et al Nano Letters 2010
Also phononic nanomesh structures (Yu,
Mitrovic, et al Nature Nanotechnology 2010)
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
47
Extending the idea to the entire class of TI insulators
•The surface of the holes provide the
needed anisotropic transport
•Similar theory analysis as in 1D protected
states but not as robust
•Curvature of the holes can be critical for TI
to remain protected (Ostrovsky et al PRL
10, Zhang and Vishwanath PRL 10)
R=15 nm
d=60 nm
Tretiakov, Abanov, Sinova APL 2011
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
48
Control of materials and transport properties via spin-orbit coupling
Nanotransport
New
magnetic
materials
GaAs
Mn
Magnetotransport
Caloritronics
Effects of spin-orbit
coupling in
multiband systems
Spintronic
Hall effects
Topological
transport
effects
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
49
Sinova’s group
Xin Liu
Texas A&M U.
Vivek Amin
Texas A&M U.
Erin Vehstedt
Texas A&M U.
H. Gao
Texas A&M U.
Jacob Gyles
Texas A&M U.
Jan Jacob
U. Hamburg
Oleg Tretiakov
(main PI Abanov)
Texas A&M U.
Principal Outside Collaborators
Tomas Jungwirth
Texas A&M U.
Inst. of Phys. ASCR
U. of Nottingham
Joerg Wunderlich
Cambridge-Hitachi
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
50