Research fueled by:
Forschungszentrum JülichNovember 11th, 2009
JAIRO SINOVATexas A&M University
Institute of Physics ASCR
Hitachi CambridgeJoerg Wünderlich, A. Irvine, et al
Institute of Physics ASCRTomas Jungwirth, Vít Novák, et al
A road to next generation technologies through basic research:Nanoelectronics, spintronics, and materials control in multiband complex systems
University of Würzburg Laurens Molenkamp, E. Hankiewiecz, et al
University of Nottingham Bryan Gallagher, Richard Campion, et al.
2Nanoelectronics, spintronics, and materials control by spin-orbit coupling
I. Role of basic research in technology development
• Control of material and transport properties through spin-orbit coupling:II.Ferromagnetic semiconductorsIII.Tunneling anisotropic magnetoresistanceIV.Anomalous Hall effect and spin-dependent Hall effects
• Spin-injection Hall effect device concept•Spin based FET: old and new paradigm in charge-spin transport•Theory expectations and modeling•Experimental results
• Future challenges and perspectives
Nanoelectronics, spintronics, and materials control in multiband complex
systems through spin-orbit coupling
Nanoelectronics, spintronics, and materials control in multiband complex
systems through spin-orbit coupling
3Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Circuit heat generation is one key limiting factor for scaling device speed
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.
The need for basic research in technology development
4Nanoelectronics, spintronics, and materials control by spin-orbit coupling
International Technology Roadmap for Semiconductors
Basic Research Inc.
1D systems
Single electron systems (FETs)
Spin dependent physics
Ferromagnetic transport
Molecular systems
New materials
Strongly correlated
systems
Nanoelectronics
The need for basic research in technology development
Nanoelectronics Research Initiative
SWANMIND
INDEX WIN
• Advanced Micro Devices, Inc.• IBM Corporation• Intel Corporation• MICRON Technology, Inc.• Texas Instruments Incorporated
• Advanced Micro Devices, Inc.• IBM Corporation• Intel Corporation• MICRON Technology, Inc.• Texas Instruments Incorporated
SinovaTexas A&M
5Nanoelectronics, spintronics, and materials control by spin-orbit coupling
I. Role of basic research in technology development
• Control of material and transport properties through spin-orbit coupling:•Ferromagnetic semiconductors•Tunneling anisotropic magnetoresistance•Anomalous Hall effect and spin-dependent Hall effects
• Spin-injection Hall effect device concept•Spin based FET: old and new paradigm in charge-spin transport•Theory expectations and modeling•Experimental results
• Future challenges and perspectives
Nanoelectronics, spintronics, and materials control in multiband complex
systems through spin-orbit coupling
Nanoelectronics, spintronics, and materials control in multiband complex
systems through spin-orbit coupling
6Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Spin-orbit coupling interaction
(one of the few echoes of relativistic physics in the solid state)
This gives an effective interaction with the electron’s magnetic moment
Consequences•Effective quantization axis of the spin depends on the momentum of the electron. Band structure (group velocities, scattering rates, etc.) mixed strongly in multi-band systems
•If treated as scattering the electron gets asymmetrically scattered to the left or to the right depending on its “spin”
Classical explanation (in reality it is quantum mechanics + relativity )
• “Impurity” potential V(r) Producesan electric field
∇V
BBeffeff
pss
In the rest frame of an electronthe electric field generates and effective magnetic field
• Motion of an electron
7Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Control of materials and transport properties via spin-orbit coupling
AsAsGaGaMnMn
New magnetic materials
Nano-transport
Spintronic Hall effects
Magneto-transport
Caloritronics
Topological transport effects
Effects of spin-orbit coupling in
multiband systems
8Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Nano-transport
Spintronic Hall effects
Magneto-transport
Caloritronics
Topological transport effects
Effects of spin-orbit coupling in
multiband systems
Ferromagnetic Semiconductors
Need true FSs not FM inclusions in SCs
Mn
Ga
As
MnGaAs - standard III-V
semiconductor+
Group-II Mn - dilute magnetic moments & holes
(Ga,Mn)As - ferromagnetic semiconductor
Control of materials and transport properties via spin-orbit coupling
9Nanoelectronics, spintronics, and materials control by spin-orbit coupling
AsAsGaGaMnMn
New magnetic materials
Nano-transport
Spintronic Hall effects
Magneto-transport
Caloritronics
Topological transport effects
Effects of spin-orbit coupling in
multiband systems
Transition to a ferromagnet when Mn concentration > 1.5-2 %
DO
S
spin ↓
spin ↑
valence band As-p-like holes
Mn
Ga
AsMn
>2% Mn
EF
ferromagnetism onset near MIT when
localization length is longer than Mn-Mn spacing. Zener type
model
Control of materials and transport properties via spin-orbit coupling
Jungwirth, Sinova, et al RMP 06
10Nanoelectronics, spintronics, and materials control by spin-orbit coupling
AsAsGaGaMnMn
New magnetic materials
Nano-transport
Spintronic Hall effects
Magneto-transport
Caloritronics
Topological transport effects
Effects of spin-orbit coupling in
multiband systems
Transition to a ferromagnet when Mn concentration increases
DO
S
spin ↓
spin ↑
valence band As-p-like holes
Mn
Ga
AsMn
>2% Mn
EF
ferromagnetism onset near MIT when
localization length is longer than Mn-Mn spacing. Zener type
model
Mn
Ga
As Mn
Ferromagnetic Ga1-xMnxAs x>1.5%
Ferromagnetism mediated by delocalized band states: •polarized carriers with large spin-orbit coupling
px
py
∇V
HHsoso
pssMany useful properties
•FM dependence on doping•Low saturation magnetization
What are the consequences of the strong spin-orbit coupling of the carriers “gluing” the localized Mn moments ?
Control of materials and transport properties via spin-orbit coupling
11
Effects of spin-orbit coupling in multiband
systems
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
AsAsGaGaMnMn
New magnetic materials
Nano-transport
Spintronic Hall effects
Magneto-transport
Caloritronics
Topological transport effects
Effects of spin-orbit coupling in
multiband systems
Mn
Ga
As Mn
Ferromagnetic Ga1-xMnxAs x>1.5%
Ferromagnetism mediated by delocalized band states: •polarized carriers with large spin-orbit coupling
px
py
∇V
HHsoso
pss
What are the consequences of the strong spin-orbit coupling of the carriers “gluing” the localized Mn moments ?
Many useful properties•FM dependence on doping•Low saturation magnetization
Control of magnetic anisotropy
Strain & SO ↓
Strain induces changes in the band structure and, in turn, change the ferromagnetic easy axis. Piezoelectric devices: fast magnetization switching Wunderlich, Sinova, et al PRB 06
Tensile strain Compressive strain
M→
M→
Control of materials and transport properties via spin-orbit coupling
12
Effects of spin-orbit coupling in multiband
systems
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
AsAsGaGaMnMn
New magnetic materials
Nano-transport
Spintronic Hall effects
Magneto-transport
Caloritronics
Topological transport effects
Magneto-transport in GaMnAs
G(T)MR~ 100% MR effect
Control of materials and transport properties via spin-orbit coupling
Fert, Grunberg et al. 1988
13Nanoelectronics, spintronics, and materials control by spin-orbit coupling
AsAsGaGaMnMn
New magnetic materials
Nano-transport
Spintronic Hall effects
Magneto-transport
Caloritronics
Topological transport effects
Effects of spin-orbit coupling in
multiband systems
Magneto-transport in GaMnAs
G(T)MR~ 100% MR effect
Control of materials and transport properties via spin-orbit coupling
Exchange split bands:σ~ TDOS(↑↓) < TDOS(↑↑)
Fert, Grunberg et al. 1988
14Nanoelectronics, spintronics, and materials control by spin-orbit coupling
AsAsGaGaMnMn
New magnetic materials
Nano-transport
Spintronic Hall effects
Magneto-transport
Caloritronics
Topological transport effects
Effects of spin-orbit coupling in
multiband systems
Magneto-transport in GaMnAs
TMR ~ 100% MR effect
Control of materials and transport properties via spin-orbit coupling
Exchange split bands:σ~ TDOS(↑↓) < TDOS(↑↑)
TAMR
Tunneling Anisotropic Magnetoresistance
discovered in (Ga,Mn)As
Gold et al. PRL’04
Au
σ ~ TDOS (M)→
TAMR can be enormous depending on doping
Now discovered in FM metals !!
Ruster, JS, et al PRL05
15Nanoelectronics, spintronics, and materials control by spin-orbit coupling
AsAsGaGaMnMn
New magnetic materials
Nano-transport
Spintronic Hall effects
Magneto-transport
Caloritronics
Topological transport effects
Effects of spin-orbit coupling in
multiband systems
Control of materials and transport properties via spin-orbit coupling
Magneto-transport in GaMnAs
TAMR
Tunneling Anisotropic Magnetoresistance
discovered in (Ga,Mn)As
Gold et al. PRL’04
Au
σ ~ TDOS (M)→
TAMR can be enormous depending on doping
Ruster, JS, et al PRL05
Now discovered in FM metals !!
16Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Effects of spin-orbit coupling in
multiband systems
AsAsGaGaMnMn
New magnetic materials
Nano-transport
Spintronic Hall effects
Magneto-transport
Caloritronics
Topological transport effects
Control of materials and transport properties via spin-orbit coupling
17Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Effects of spin-orbit coupling in
multiband systems
AsAsGaGaMnMn
New magnetic materials
Nano-transport
Spintronic Hall effects
Magneto-transport
Caloritronics
Topological transport effects
Control of materials and transport properties via spin-orbit coupling
Anomalous Hall effects
I
FSO
FSO
majority
minority
V
Nagaosa, Sinova, Onoda, MacDonald, Ong, RMP 10
18Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Simple electrical measurement of out of plane magnetization
InMnAs
Spin dependent “force” deflects like-spin particles
ρH=R0B ┴ +4π RsM┴
Anomalous Hall Effect: the basics
I
_ FSO
FSO
_ __
majority
minority
V
19Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Cartoon of the mechanisms contributing to AHEindependent of impurity density
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.
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)
E
SO coupled quasiparticles
Intrinsic deflection B
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.
independent of impurity density
Side jump scatteringVimp(r) (Δso>ħ/τ) ∝ λ*∇Vimp(r) (Δso<ħ/τ)
B
Skew scattering
Asymmetric scattering due to the spin-orbit coupling of the electron or the impurity. Known as Mott scattering.
~σ~1/niVimp(r) (Δso>ħ/τ) ∝ λ*∇Vimp(r) (Δso<ħ/τ) A
20Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Contributions understood in simple metallic 2D models
Semi-classical approach:Gauge invariant formulation
Sinitsyn, Sinvoa, et al PRB 05, PRL 06, PRB 07
Kubo microscopic approach:in agreement with semiclassical
Borunda, Sinova, et al PRL 07, Nunner, JS, et al PRB 08
Non-Equilibrium Green’s Function (NEGF) microscopic approach
Kovalev, Sinova et al PRB 08, Onoda PRL 06, PRB 08
21Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Review of AHE (to appear in RMP 2010), Nagaosa, Sinova, Onoda, MacDonald, Ong
Phenomenological scaling regimes of AHE
Scattering independent regime
Q: is the scattering independent regime dominated by the intrinsic AHE?
1. A high conductivity regime for σxx>106 (Ωcm)-1 in which AHE is skew dominated2. A good metal regime for σxx ~104-106 (Ωcm) -1 in which σxy
AH~ const3. A bad metal/hopping regime for σxx<104 (Ωcm) -1 for which σxy
AH~ σxyα with α>1
Skew dominated regime
22Nanoelectronics, spintronics, and materials control by spin-orbit coupling
n, q
n’≠n, q
Intrinsic AHE approach in comparing to experiment: phenomenological “proof”
•DMS systems (Jungwirth et al PRL 2002, Jungwirth, Sinova, et al APL 03)
•layered 2D ferromagnets e.g. SrRuO3 ferromagnets (Taguchi et al, Science 01, Fang et al, Science 03)
•CuCrSeBr compounds ( Lee et al, Science 04)
•Fe (Yao et al PRL 04) Experiment: σAH 1000 (Ω cm)∼ -1
Theory: σAH 750 (Ω cm)∼ -1
AHE in Fe
AHE in GaMnAs
23Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Valenzuela et al Nature 06
Inverse SHE
Anomalous Hall effect: more than meets the eye
Wunderlich, Kaestner, Sinova, Jungwirth PRL 04
Kato et al Science 03
IntrinsicExtrinsic
V
Mesoscopic Spin Hall Effect
Intrinsic
Brune,Roth, Hankiewicz, Sinova, Molenkamp, et al 09
Wunderlich, Irvine, Sinova, Jungwirth, et al, Nature Physics 09
Spin-injection Hall Effect
Anomalous Hall Effect
I
_ FS
OFS
O
_ _majority
minority
V
Spin Hall Effect
I
_ FS
OFS
O
_ _
V
24Nanoelectronics, spintronics, and materials control by spin-orbit coupling
I. Role of basic research in technology development
• Control of material and transport properties through spin-orbit coupling:•Ferromagnetic semiconductors•Tunneling anisotropic magnetoresistance•Anomalous Hall effect and spin-dependent Hall effects
• Spin-injection Hall effect device concept•Spin based FET: old and new paradigm
in charge-spin transport•Theory expectations and modeling•Experimental results
• Future challenges and perspectives
Nanoelectronics, spintronics, and materials control in multiband complex
systems through spin-orbit coupling
Nanoelectronics, spintronics, and materials control in multiband complex
systems through spin-orbit coupling
Wunderlich, Irvine, Sinova, Jungwirth, et al, Nature Physics 09
Spin-injection Hall Effect
25Nanoelectronics, spintronics, and materials control by spin-orbit coupling
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
Unfortunately it has not worked :•no reliable detection of spin-polarization in a diagonal transport configuration •No long spin-coherence in a Rashba spin-orbit coupled system (Dyakonov-Perel mechanism)
Towards a realistic spin-based non-magnetic FET device
26Nanoelectronics, spintronics, and materials control by spin-orbit coupling
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).
New paradigm using SO coupling: SO not so bad for dephasing
1) Can we use SO coupling to manipulate spin AND increase spin-coherence?
• Can we detect the spin in a non-destructive way electrically?
Use the persistent spin-Helix state and control of SO coupling strength(Bernevig et al 06, Weber et al 07, Wünderlich et al 09)
Use AHE to measure injected current polarization at the nano-scale electrically (Wünderlich, et al 09, 04)
27Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Spin-dynamics in 2D electron gas with Rashba and Dresselhauss SO coupling
a 2DEG is well described by the effective Hamiltonian:
Something interesting occurs when
• spin along the [110] direction is conserved• long lived precessing spin wave for spin perpendicular to [110]The nesting property of the Fermi surface:
Bernevig et al PRL 06, Weber et al. PRL 07
Schliemann et al PRL 04
28Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Effects of Rashba and Dresselhaus SO coupling
α= -β
[110]
[110]_
ky [010]
kx [100]
α > 0, β = 0[110]
[110]_
ky [010]
kx [100]
α = 0, β < 0[110]
[110]_
ky [010]
kx [100]
29Nanoelectronics, spintronics, and materials control by spin-orbit coupling
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]_
30Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Spatial variation scale consistent with the one observed in SIHE
Spin-helix state when α ≠ β
Wunderlich, Irvine, Sinova, Jungwirth, et al, Nature Physics 09
31Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Type (i) contribution much smaller in the weak SO coupled regime where the SO-coupled bands are not resolved, dominant contribution from type (ii)
Crepieux et al PRB 01Nozier et al J. Phys. 79
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
Lower bound estimate of skew scatt. contribution
AHE contribution to Spin-injection Hall effect
Wunderlich, Irvine, Sinova, Jungwirth, et al, Nature Physics 09
32Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Local spin-polarization → calculation of AHE signal
Weak SO coupling regime → extrinsic skew-scattering term is dominant
Lower bound estimate
Spin-injection Hall effect: theoretical expectations
33Nanoelectronics, spintronics, and materials control by spin-orbit coupling
2DHG
2DEG
e
h
ee
ee
e
hhh
h h
Vs
Vd
VH
Spin-injection Hall effect device schematics
For our 2DEG system:
Hence α ≈ -β
34Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Spin-injection Hall device measurements
trans. signal
σσooσσ++σσ-- σσoo
VL
35Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Spin-injection Hall device measurements
trans. signal
σσooσσ++σσ-- σσoo
VL
SIHE ↔ Anomalous Hall
Local Hall voltage changes sign and magnitude along a channel of 6 μm
36Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Further experimental tests of the observed SIHE
37Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Non public slides deleted. Please contact
Sinova if interested
38Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Summary of spin-injection Hall effect
Basic studies of spin-charge dynamics and Hall effect in non-magnetic systems with SO coupling Spin-photovoltaic cell: solid state polarimeter on a semiconductor chip requiring no magnetic elements, external magnetic field, or bias
SIHE can be tuned electrically by external gate and combined with electrical spin-injection from a ferromagnet (e.g. Fe/Ga(Mn)As structures)
39Nanoelectronics, spintronics, and materials control by spin-orbit coupling
I. Role of basic research in technology development
• Control of material and transport properties through spin-orbit coupling:•Ferromagnetic semiconductors•Tunneling anisotropic magnetoresistance•Anomalous Hall effect and spin-dependent Hall effects
• Spin-injection Hall effect device concept•Spin based FET: old and new paradigm in charge-spin transport•Theory expectations and modeling•Experimental results
• Future challenges and perspectives
Nanoelectronics, spintronics, and materials control in multiband complex
systems through spin-orbit coupling
Nanoelectronics, spintronics, and materials control in multiband complex
systems through spin-orbit coupling
40Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Effects of spin-orbit coupling in
multiband systems
AsAsGaGaMnMn
New magnetic materials
Nano-transport
Spintronic Hall effects
Magneto-transport
Caloritronics
Topological transport effects
Theory techniques and development in our studies
•First principles DFT (LDA,LDA+U)
•Tight-binding•Phenomenological k.p models
•Exact diagonalization study of disorder
•Non-equilibrium Green’s function formalism in real space
•Landuaer-Buttiker formalism
•phenomenological k.p DOS•NEGF tunneling formalism
•Semiclassical transport•NEGF/Keldysh formalism
•Kubo microscopic method
•Numerical diagonalization studies
41Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Non public slides deleted. Please contact
Sinova if interested
42Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Allan MacDonald U of Texas
Tomas JungwirthTexas A&M U.
Inst. of Phys. ASCRU. of Nottingham
Joerg WunderlichCambridge-Hitachi
Laurens MolenkampWürzburg
Xiong-Jun LiuTexas A&M U.
Mario BorundaTexas A&M Univ.
Harvard Univ.
Nikolai SinitsynTexas A&M U.
U. of TexasLANL
Alexey KovalevTexas A&M U.
UCLA
Liviu ZarboTexas A&M Univ.
Xin LiuTexas A&M U.
Ewelina Hankiewicz(Texas A&M Univ.)
Würzburg University
Sinova’s group
Principal Collaborators
Gerrit BauerTU Delft
Bryan GallagherU. of Nottingham
and many others
43Nanoelectronics, spintronics, and materials control by spin-orbit coupling
44Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Mesoscopic SHE
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Advantages:•No worries about spin-current definition. Defined in leads where SO=0•Well established formalism valid in linear and nonlinear regime•Easy to see what is going on locally•Fermi surface transport
Charge based measurements of SHE: SHE-1
Non-equilibrium Green’s function formalism (Keldysh-LB)
59
Hankiewiecz,JS, Molenkamp et al PRB 05
Nanoelectronics, spintronics, and materials control by spin-orbit coupling60
H-bar structures for detection of Spin-Hall-Effect
(electrical detection through inverse SHE)
E.M. Hankiewicz et al ., PRB 70, R241301 (2004)
Nanoelectronics, spintronics, and materials control by spin-orbit coupling
sample layout
Molenkamp et al (unpublished)
insu
lati
ng
p-c
onduct
ing
n-conducting
Strong SHE-1 in HgTe
theory
48Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Strong SHE-1 in HgTe
Molenkamp et al (unpublished)
49Nanoelectronics, spintronics, and materials control by spin-orbit coupling
Strong SHE-1 in HgTe
Molenkamp et al (unpublished)
Narrower sample (non-inverted)