Research fueled by:

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




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


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

http://www.amd.com/us-en/

http://www.ibm.com/us/


http://www.intel.com/

http://www.amd.com/us-en/



http://www.intel.com/



• Control of material and transport properties through spin-orbit coupling:•Ferromagnetic semiconductors•Tunneling anisotropic magnetoresistance•Anomalous Hall effect and spin-dependent Hall effects








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


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


Nano-transport


Magneto-transport

Caloritronics



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



AsAsGaGaMnMn


Nano-transport


Magneto-transport

Caloritronics



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


Jungwirth, Sinova, et al RMP 06


AsAsGaGaMnMn


Nano-transport


Magneto-transport

Caloritronics



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 ?


11

Effects of spin-orbit coupling in multiband

systems

Nanoelectronics, spintronics, and materials control by spin-orbit coupling

AsAsGaGaMnMn


Nano-transport


Magneto-transport

Caloritronics



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→


12

Effects of spin-orbit coupling in multiband

systems


AsAsGaGaMnMn


Nano-transport


Magneto-transport

Caloritronics


Magneto-transport in GaMnAs

G(T)MR~ 100% MR effect


Fert, Grunberg et al. 1988


AsAsGaGaMnMn


Nano-transport


Magneto-transport

Caloritronics



multiband systems


G(T)MR~ 100% MR effect


Exchange split bands:σ~ TDOS(↑↓) < TDOS(↑↑)

Fert, Grunberg et al. 1988


AsAsGaGaMnMn


Nano-transport


Magneto-transport

Caloritronics



multiband systems


TMR ~ 100% MR effect


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


AsAsGaGaMnMn


Nano-transport


Magneto-transport

Caloritronics



multiband systems



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 !!



multiband systems

AsAsGaGaMnMn


Nano-transport


Magneto-transport

Caloritronics





multiband systems

AsAsGaGaMnMn


Nano-transport


Magneto-transport

Caloritronics



Anomalous Hall effects

I

FSO

FSO

majority

minority

V

Nagaosa, Sinova, Onoda, MacDonald, Ong, RMP 10


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


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


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


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


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


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




• Spin-injection Hall effect device concept•Spin based FET: old and new paradigm

in charge-spin transport•Theory expectations and modeling•Experimental results







Spin-injection Hall Effect


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


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)


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


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]


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]_


Spatial variation scale consistent with the one observed in SIHE

Spin-helix state when α ≠ β



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



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


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


Spin-injection Hall device measurements

trans. signal

σσooσσ++σσ-- σσoo

VL


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


Further experimental tests of the observed SIHE


Non public slides deleted. Please contact

Sinova if interested


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)



multiband systems

AsAsGaGaMnMn


Nano-transport


Magneto-transport

Caloritronics


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


Non public slides deleted. Please contact

Sinova if interested


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


Mesoscopic SHE


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)


sample layout

Molenkamp et al (unpublished)

insu

lati

ng

p-c

onduct

ing

n-conducting

Strong SHE-1 in HgTe

theory




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