expecting the unexpected in the spin hall effect: from...

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8th International Workshop on Nanomagnetism & Superconductivity

Coma-ruga July 2nd, 2012

Expecting the unexpected in the spin Hall effect: from fundamental to practical

JAIRO SINOVA

Texas A&M UniversityInstitute of Physics ASCR

Hitachi CambridgeJoerg Wünderlich, A. Irvine, et al

Institute of Physics ASCRTomas Jungwirth, Vít Novák, et al

1

U. of WurzbergLaurens Molenkamp, E. Hankiewicz, et al

2

Talk dedicated to “THE” dream team

CAMPEONES! OLE !

Nanoelectronics, spintronics, and materials control by spin-orbit coupling 3

I. Introduction:•Basics of AHE: SOC origins and mechanism•SHE phenomenology

II. Spin Hall effect: the early days •First proposals: from theory to experiment•First observations of the extrinsic and intrinsic (optical)

III. Inverse spin Hall effect: SHE as a spin current detector•Direct iSHE in metals•Spin pumping and iSHE•Intrinsic mesoscopic SHE

IV.SHE-FET: first steps towards practicality (but perhaps not)•Spin Hall injection and spin precession manipulation•iSHE device with spin-accumulation modulation

V. FMR measurement of SHE angle: giant SHE as a spin current generator•FMR and SHE angle •Giant intrinsic SHE and STT: Future MRAM technology?

VI.Conclusion

Expecting the unexpected in the spin Hall effect:

from fundamental to practical

⇢xy

= � �xy

�2xx

+ �2xy

⇡ ��xy

�2xx

⇡ ��xy

⇢2xx

⇡ �A⇢xx

� B⇢2xx

Simple electrical measurement of out of plane magnetization (or spin polarization ~ n↑-n↓)

InMnAs

4

Spin dependent “force” deflects like-spin particles

ρH=R0B ┴ +4π RsM┴

Anomalous Hall Effect: the basics

I

_ FSO

FSO

_ __majority

minority

V

�AHxy

⇡ B + A�xx

M⊥

AHE is does NOT originate from any internal magnetic field created by M⊥; the field would have to be of the order of 100T!!!

Nagaosa, Sinova, Onoda, MacDonald, Ong, Rev. Mod. Phys. 2010

e-

Nanoelectronics, spintronics, and materials control by spin-orbit coupling 5

(one of the few echoes of relativistic physics in the solid state)

HSO = �~µB · ~Beff

This gives an effective interaction with the electron’s magnetic moment

Classical explanation (in reality it arises from a second order expansion of Dirac equation around the non-relativistic limit)

~E = �1erV (~r)• “Impurity” potential V(r) Produces

an electric field

∇V

Beff

ps

In the rest frame of an electronthe electric field generates an effective magnetic field

• Motion of an electron ~Beff= � ~~k

cm⇥ ~E

Consequence #1: Spin or the band-structure Bloch states are linked to the momentum. Coupled multi-band system.

Internal communication between spin and charge:spin-orbit coupling interaction

Consequence #2: Mott scattering

6

Cartoon of the mechanisms contributing to AHE �AHxy

⇡ B + A�xx

independent of impurity density

xc =@"(~k)~@

~

k

+e

~~

E ⇥ ~⌦

⌦z

(~k, n) = 2Im⌧

@

@y

n~k

��@

@x

n~k

�

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) or ∝ 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) or ∝ λ*∇Vimp(r) (Δso<ħ/τ) A

Spin Hall effectTake now a PARAMAGNET instead of a FERROMAGNET:

Spin-orbit coupling “force” deflects like-spin particles

I

_ FSO

FSO

_ __

V=0

non-magnetic

Transverse spin-current generation in paramagnets without external magnetic fields by spin-depedent deflection of electrons

Carriers with same charge but opposite spin are deflected by the spin-orbit coupling to opposite sides.

7

Spin Hall Effect(Dyaknov and Perel 1971)

InterbandCoherent Response

∼ (EFτ) 0

Occupation # Response

`Skew Scattering‘

[Hirsch, S.F. Zhang] 2000

Intrinsic`Berry Phase’

[Murakami et al, Sinova et al]

2003

Influence of Disorder[Inoue et al, Misckenko et

al, Chalaev et al…]

8

First experimental observations at the end of 2004

Wunderlich, Kästner, Sinova, Jungwirth, cond-mat/0410295PRL January 05

Experimental observation of the spin-Hall effect in a two dimensional spin-orbit coupled semiconductor system

Co-planar spin LED in GaAs 2D hole gas: ~1% polarization

Kato, Myars, Gossard, Awschalom, Science Nov 04 Observation of the spin Hall effect bulk in semiconductors

Local Kerr effect in n-type GaAs and InGaAs: ~0.03% polarization (weaker SO-coupling, stronger disorder)

9

I

FSO

FSO

_majority

minority

V

I

_ FSO

FSO

_ __

V=0non-magnetic

Ispin

FSO

FSO _

Vnon-magnetic

Mz

Mz=0

I=0

Mz=0

magnetic

optical detection

AHE

SHESHE-1

10

✓

✓

Completing the spin dependent Hall family: SHE-1

Valenzuela, S. O. & Tinkham, M, Nature‘06

xy

zBz

Electrical non-local spin valve detection by FM and by iSHE

extrinsic SHE-1 in metals

iSHENL spin detection

11

SHE-1 magnetoresistance measurement

12

Kimura et al PRL 98, 156601 (2007)

(SHE angle HIGHLY underestimated)

Magnetoresistance signals from SHE and inverse SHE θSH ~ 0.0037

Originally proposed by Shufeng Zhang 2000

4 nm thick Pt

80 nm thick Cu

100 nm junction width

Reason for the underestimate of θSH by Kimura et al.

The Cu shunts most of the charge current, so the charge current density in the Pt was much smaller than assumed.

Courtesy of D.C. Ralph13

Spin pumping and SHE-1

14

Saitoh et al APL 06

Theory based on ref: Silsbee, Janossy, and Monod, PRB 19, 4382 (1979)

sample layout

Brune et al, Nature Physics

insu

lating

p-co

nduc

ting

n-conducting

Mesoscopic intrinsic SHE-1 in HgTe

theory

15

16





V. FMR measurement of SHE angle: giant and SHE as a spin current generator•FMR and SHE angle •Giant intrinsic SHE: Future MRAM technology?

VI.Conclusion



17

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).

From DD-FET to new paradigm using SO coupling

1) Can we use SO coupling to manipulate spin AND increase spin-coherence?

2) Can we detect the spin in a non-destructive way electrically?

3) Can this effect be exploited to create a spin-FET logic device?

Use the persistent spin-Helix state or quasi-1D-spin channels and control of SO coupling strength (Bernevig et al 06, Weber et al 07, Wünderlich et al 09, Zarbo et al 10)

Use AHE to measure injected current polarization electrically (Wünderlich, et al Nature Physics. 09, PRL 04)

Spin-Hall AND-gate device (Wünderlich, Jungwirth, et al Science 2010)

DD-FET

VH2I

VbVH1

x

VH2

VbVH1

x

Spin Hall effect transistor:Wunderlich, Jungwirth, et al, Science 2010

SiHE

inverse SHE

18

iSHE transistor

Spin-FET with two gates → logic AND function

Wunderlich et al., Science.‘10

SHE transistor AND gate

19

Electrical spin modulator

Bx=0

Olejník, K. et al. arxiv.org/abs/1202.0881, to appear in PRL (2012).20

21





V. FMR measurement of SHE angle: giant and SHE as a spin current generator•FMR and SHE angle •Giant intrinsic SHE: Future MRAM technology?

VI.Conclusion



T. Kimura et al.PRL 98, 156601 (2007)

Magnetoresistance signals from SHE and inverse SHE

θSH ~ 0.0037

K. Ando et al.PRL 101, 036601 (2008)Effect of inverse SHE on

magnetic dampingθSH ~ 0.08

SHE angle measurements in Pt Vary by a Factor of 20

O. Mosendz et al.PRL 104, 046601 (2010), PRB

82, 214403 (2010)Magnetically-excited Py

precession produces voltage by inverse SHEθSH ~ 0.013

(assumes λSF= 10 nm in Pt)

Courtesy of D.C. Ralph

22

DC-Detected Spin-Transfer-Driven Ferromagnetic Resonance (ST-FMR)

Resonant resistance oscillations generate a DC voltage component by mixing

S

VmixDCcircuitry

Main source of signal at low bias:

Related work: Tulapurkar et al., Nature 438, 339 (2005)€

V (t) = I(t)R(t) ~ IRF cos(ωt)ΔRcos(ωt + δ)

=12IRFΔR cos(δ) + cos(2ωt + δ)[ ]

Vmix ~ 12IRFΔRcos(δ) ∝ d(Torque)

dV


Slonczewski torque: symmetric Lorentzian

A. A. Tulapurkar, et al., Nature 438, 339 (2005).J. N. Kupferschmidt et al., PRB 74, 134416 (2006).A. A. Kovalev et al., PRB 75, 104403 (2007).

Vmix Vmix

If both Slonczewski and out-of-plane spin-torque components are present then the FMR response is a simple sum of two contributions.

Out-of-Plane torque: antisymmetric Lorentzian

Out-of-Plane Torque

Slonczewski Torque

Mfree

Mfixed

Accurate measurement of SHE angle

€

Vmix ∝A

1+[( f − f0) /Δ0]2

€

Vmix ∝B[( f − f0) /Δ0]1+[( f − f0) /Δ0]

2

Courtesy of D.C. Ralph 24

FMR Peak Shape Analysis

Spin torque FMR measurement of the SHE

Spin current in plane torque τST symmetric peak

Oersted field perpendicular torque τH antisymmetric peak

The two driving forces induce oscillations with 90° phase differenceDC readout of the FMR signal using the anisotropic magnetoresistance of Py


25

Pt(1.5-15 nm)/Py(2-15 nm), room temperature

+=

Luqiao Liu et al., PRL 106, 036601 (2011)

Spin torque FMR measurement of the SHE

Results: θSH = 0.068 ± 0.005 for Pt

This is big!control tests


Why is JS/JC ~ 0.06 is Big?

The spin Hall angle is a relationship between current densities.

To calculate the efficiency of total spin current generation, must take into account a difference in areas

A = Lw

t

a = tw

can be >> 1 even with θSH ~ 0.06

With the spin Hall effect, the traversal of one electron through the sample can transfer more than angular momentum to a magnet!

€

θSH =JS /( /2)J /e


27

spin Hall effect in Ta

•ab initio calculation: θSH(Ta) has opposite sign compared to θSH(Pt)

•for highly resistive case, θSH(Ta) can be very large

Tanaka, T. et al, Phys. Rev. B 77, 165117 (2008)

INTRINSIC spin Hall conductivity calculated for 4d, 5d elements

28

ST-FMR induced by the SHE in Ta

CoFeB/Ta CoFeB/Pt

• antisymmetric peak, same sign (Oersted field)

• symmetric peaks, opposite sign (spin torque)

JS/JC = 0.15 ± 0.04! Narrower linewidth – less added damping from Ta compared to Pt


Liu et al. Science 336, 555 (2012)

SHE as a source for spin current

Jc Js

FM

FM

FM

NM

Spin Hall Device Conventional Magnetic Tunnel Junction

JS and JC travel perpendicular paths JS and JC travel the same path

What is in various metals?


Text

Using the Spin Hall Torque to Switch In-Plane-Polarized Magnetic Layers -- A 3-Terminal Device

Ta strip 1 µm wide

MTJ 100×300 nm2

DC current in Ta strip to write

Resistance measurement across the MTJ to read

Liu et al. Science 336, 555 (2012)

31

• Switch the magnetic moment using the SHE via an anti-damping mechanism• Use a magnetic tunnel junction to read out the magnetic orientation

DC current induced switching

•Ramp-rate measurement of critical currents:

Ic0 = 2.0 mA E0 ~ 46 kBT

JS/JC ≈ 0.12 ± 0.03 agrees with ST-FMR and perpendicular switchingmeasurements

No barrier degradation, better read-out signal compared to conventional devices.Switching currents well below 100 µA should be possible.

Liu et al. Science 336, 555 (2012)May 4th 2012


Spin Hall Effect: from fundamentals to applications

33

SOC Effects

SHE

AHEAMR

CITCurrent induced

torque

SHE-1

IntrinsicExtrinsic

Optical

Electrical

Spin current detectorSpin current generator

FMRSpin Pumping

Spin Caoloritronics STT

SHE-MRAM??Jungwirth, Wunderlich, Olejnik, Nat. Mat. 11,382 (2012)

expecting the unexpected in the spin hall effect: from...

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