Observation of the spin-Hall Effect

Soichiro Sasaki

Suzuki-Kusakabe Lab. Graduate School of Engineering Science

Osaka University

2005. 4. 18M1 Colloquium in the Frontier Materials Division

Abstract• To realize spin-electronics, techniques to accumulate electron sp

in in semiconductor devices are required. In the usual spin-electronics devices, spin injection is usually realized using ferromagnets.

• The spin-Hall effect is another possibility and we can substitute the effect for ferromagnets. Spin current flows in the direction perpendicular to the direction of charge current when the voltage is applied to an electron system in a semiconductor device. Spin accumulation would occur at two edges as a result. This is called the spin-Hall effect.

• The experiments [1,2] I'll introduce succeeded in observation of the spin-Hall effect that was predicted theoretically. Spin accumulation is observed with a Kerr rotation microscope in the first one. Spin-dependence of polarization in a light-emitting diode structure is an evidence in the second one. There are two origins of spin-Hall effect, extrinsic spin-Hall effect and intrinsic spin-Hall effect. The former experiment shows domination in the extrinsic spin-Hall effect. The latter claims observation of intrinsic one.References[1] Y.K. Kato, et al., Science 306 (2004) 1910.[2] J.Wunderlich,Phy. Rev. Lett. Vol.94 (2005) 047204

Contents

1. Introduction

2. Observation of the spin-Hall Effect2.1 Kato’s spin-Hall device

2.2 Wunderlich’s spin-Hall device

3. Summary

1-1 Spin-electronics devicesGMR device

TMR device

Spin-FET

These devices are realized by using ferromagnets.

Electrode(Ferromagnel)

Electrode(Ferromagnet)

Insulator(Oxides)

Gate bias (Off)

Gate bias (On)

Source (FM)Gate electrode Drain (FM)

No current flows

Spin-FET by Datta & Das

Current flows

Channel

Current flows

Problems of these devices

• We use ferromagnets to fabricate spin-electronic devices. However, it is troublesome to fabricate ferromagnets in semiconductor devices.

• We must use magnetic field to change the direction of a magnetic moment. We can’t make a device structure, which is controllable by applied magnetic field, smaller than a predicted limit.

Solution Spin-Hall effect

1-2 semiconductor hetero junction

Ordinal heterojunction to realizethe two-dimensional electron gas.(The 2DEG)

Heterojunction may used as a n-type device.The electronic system at the interface in AlGaAs/ GaAs junction produces the two-dimensoinal hole gas (The 2DHG), when the bias is introduced as shown by the black lines.

2DEG

2DHG

(a) (b)

1-3 The Hall effect (Carriers are electrons)

xx enhdvI

yzx eFBev )( FeBve

yy hFV

end

BIV zxy Hall voltage ;

Lorentz force ;

Applied current ;

Induced voltage ; (Hall voltage)

en

v

The elementary electric charge ;

Density of carriers ;

Velocity of a carrier ;

1-4 Spin Hall Effect

the vector of spin- current whose spin is polarized along z axis.

Spin accumulation Motion of up-spinsMotion of down-spins

In a nonmagnetic metal, the spin-orbit interaction causesa pure spin current perpendicular to the applied electric field.

Extrinsic Spin-Hall effect

• This effect is caused by spin-orbit interaction of impurities.

0

s

SS

x

q

t

S

S: The spin density

Spin relaxation

H: Magnetic field

=mBgH/h

Magnetic field effectq : Spin flux density

A phenomenological continuous equation of spin density and spin-flux density.

Intrinsic Spin-Hall effect

• The electrons at the valence band top, i.e. the holes follow an equation of motion in the k-space, in which a band-touching point the acts as a Dirac magnetic monopole in the momentum space.

• Valence band top of GaAs is a typical example. Thus p-type doped semiconductor is required to have intrinsic SHE.

1-5 Hall effect v.s. Spin-Hall effect

Applied field

Accumulated

quantity

Hall effect

Magnetic field & Electric

field

Elctric carge

Spin-Hall effect

Spin-orbit interaction &

Electic field

Electron spin

Spin-Hall effect

1-6 Kerr rotational microscopy

• When incident linearly polarized light (i) interacts with a magnetic system the reflected light (r) turns out to be elliptically polarized (the orientation of the magnetization M is perpendicular to the surface).

Kerr effect

Magnetic substance

Kerr rotational microscopeKerr rotaional microscopy(in the Santa Barbara)

Phys. Today Vol. 58 Nov.2 17 (Feb. 2005)

Schematic of the experimental geometry

Observations of spin-Hall effect

1. Y. K. Kato, R. C. Myers, A. C. Gossard, D. D. Aschalom, Science Vol.306,1910 (2004);published online 11 November 2004 (10.1126/science.1105514).

2. J. Wunderlich, B. Kaestner, J. Sinova, T.Jungwirth, Phys. Rev. Lett. Vol. 94,047204 (2005).

2.1 Y. K. Kato’ s experiment

Method of detection ; Kerr rotational microscopy

Strong point ; It can observe spatial map of spin.

Weak point ;The limit of detected spot

(more than 1.1μm)

Long spin life time is needed to observe the spin-Hall effect.

Kerr rotation in an unstrained sample

Schematics of the unstrained GaAs sample Kerr rotation, peak KR,and a spin lifetime as afunction of x or Bext

KR, peak KR, and a spin lifetime as afunction of x, Bext & E

Spin accumulation

• Figure A: Two-dimensional images of spin density ns.

• Figure B: Two-dimensional images of reflectivity.

• These pictures are direct observation of the spin-Hall effect.

Hanle effect of unstrained sample

120

SL

A

extB

L

Bg

Larmor precession frequency;

Electron spin lifetime; sg

B

Electron g factor;

Bohr magneton;

0AThe peak Kerr rotation;

Kerr rotation in strained sampleSchematics of the strained InxGa1-xAs sample Kerr rotation, peak KR,

Bint and R as afunction of x or Bext

Hanle effect of strained sample

]1)[(]1)[( 21

21

ss

AA

1A

)( intBBg extB

)( intBBg extB

Kerr rotation ;

Precession frequency;

Kerr rotation peak ;

The spin polarization is maximumAt when cancels out .

extBintB

intBBext

Result of Y. K. Kato’ s experiment

• The band structure that is the origin of intrinsic one is broken by straining of the sample. This experiment is caused, they think, by extrinsic one.

• If the effect originated in the lattice, then it should depend on the strain direction.

2.2. J. Wunderlich’s experiment

Method of detection ;Electroluminescence of LED

Strong point ;It can detect magnetic field at very small spot.

The crystal used at this experiment is very clean crystal.

It has possibility of intrinsic spin-Hall effect.Weak pint ;

It can’t detect spatial map of spin .

p-n junction LED deviceSchematic cross section of the device Conduction and valence band profiles

Schematic of sample and data

b: Polarization along z axis measured with active LED 1.c: Polarization with fixed Ip for LED 1 or LED 2.

SEM image of the SHE LED device

Result of J. Wunderlich’s experiment

• Hitach researchers believe their experiment to be too low in signal to account for the polarization they observed. The spin-Hall effect in their sample is, they believe,intrinsic in origin.

Summary

• Using the Kerr rotational microscope, the extrinsic Spin-Hall effect was observed in a n-GaAs thin layer. (Kato’s experiment)

• In a strained InGaAs device, no clear evidence of the strain effect on the Spin-Hall effect was observed. Thus, the intrinsic Spin-Hall effect is not observed in this sample. (Kato’s experiment)

• Using electrolumimescemce from LED made of 2DHG/2DEG junction, spin polarization is found, which depends on direction of the electric field across 2DHG. This result suggest the intrinsic Spin-Hall effect. (Wunderlich’s experiment)