Spintronics(自旋电子学)

Spintronics:Fundamentals and applications

Igor Zˇutic′*

Condensed Matter Theory Center,Department of Physics,University of Maryland

at College Park,College Park,Maryland20742-4111,USA

Jaroslav Fabian?

Institute for Theoretical Physics,Karl-Franzens University,Universita¨tsplatz5,8010Graz,

Austria

S.Das Sarma

Condensed Matter Theory Center,Department of Physics,University of Maryland

at College Park,College Park,Maryland20742-4111,USA

(Published23April2004)

Spintronics,or spin electronics,involves the study of active control and manipulation of spin degrees of freedom in solid-state systems.This article reviews the current status of this subject,including both recent advances and well-established results.The primary focus is on the basic physical principles underlying the generation of carrier spin polarization,spin dynamics,and spin-polarized transport in semiconductors and metals.Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyper?ne coupling.The authors discuss in detail spin decoherence mechanisms in metals and semiconductors.Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties.Experimental work is reviewed with the emphasis on projected applications,in which external electric and magnetic?elds and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

CONTENTS

I.Introduction323

A.Overview323

B.History and background325

1.Spin-polarized transport and

magnetoresistive effects325

2.Spin injection and optical orientation328

II.Generation of Spin Polarization329

A.Introduction329

B.Optical spin orientation331

C.Theories of spin injection333

1.F/N junction333

2.F/N/F junction337

3.Spin injection through the space-charge

region338

D.Experiments on spin injection340

1.Johnson-Silsbee spin injection340

2.Spin injection into metals341

3.All-semiconductor spin injection342

4.Metallic ferromagnet/semiconductor

junctions345 III.Spin Relaxation and Spin Dephasing346

A.Introduction346

1.T1and T2347

2.Experimental probes348

B.Mechanisms of spin relaxation349

1.Elliott-Yafet mechanism349

2.D’yakonov-Perel’mechanism351

a.Bulk III-V semiconductors353

b.Two-dimensional III-V semiconductor

systems354

3.Bir-Aronov-Pikus mechanism356

4.Hyper?ne-interaction mechanism357

C.Spin relaxation in metals358

D.Spin relaxation in semiconductors360

1.Bulk semiconductors360

2.Low-dimensional semiconductor structures361

3.Example:Spin relaxation in GaAs363

a.Bulk n-GaAs363

b.GaAs-based quantum wells365 IV.Spintronic Devices and Applications366

A.Spin-polarized transport366

1.F/I/S tunneling366

2.F/I/F tunneling368

3.Andreev re?ection371

4.Spin-polarized drift and diffusion372

B.Materials considerations373

C.Spin?lters376

D.Spin diodes377

E.Spin transistors380

1.Spin?eld-effect transistors380

2.Magnetic bipolar transistor381

3.Hot-electron spin transistors382

F.Spin qubits in semiconductor nanostructures384

V.Outlook385 Acknowledgments386 References386 I.INTRODUCTION

A.Overview

Spintronics is a multidisciplinary?eld whose central theme is the active manipulation of spin degrees of free-

*Present address:Center for Computational Materials Sci-

ence,Naval Research Laboratory,Washington,D.C.20735,

USA.Electronic address:igorz@d72efe37f111f18583d05a9d

?Electronic address:jaroslav.fabian@uni-graz.at

REVIEWS OF MODERN PHYSICS,VOLUME76,APRIL2004

0034-6861/2004/76(2)/323(88)/$40.00?2004The American Physical Society

323

dom in solid-state systems.1In this article the term spin

stands for either the spin of a single electron s,which

can be detected by its magnetic moment?g?B s(?B is the Bohr magneton and g is the electron g factor,in a

solid generally different from the free-electron value of

g0?2.0023),or the average spin of an ensemble of elec-trons,manifested by magnetization.The control of spin

is then a control of either the population and the phase

of the spin of an ensemble of particles,or a coherent spin manipulation of a single or a few-spin system.The goal of spintronics is to understand the interaction be-tween the particle spin and its solid-state environments and to make useful devices using the acquired knowl-edge.Fundamental studies of spintronics include inves-tigations of spin transport in electronic materials,as well as of spin dynamics and spin relaxation.Typical ques-tions that are posed are(a)what is an effective way to polarize a spin system?(b)how long is the system able to remember its spin orientation?and(c)how can spin be detected?

Generation of spin polarization usually means creat-ing a nonequilibrium spin population.This can be achieved in several ways.While traditionally spin has been oriented using optical techniques in which circu-larly polarized photons transfer their angular momenta to electrons,for device applications electrical spin injec-tion is more desirable.In electrical spin injection a mag-

netic electrode is connected to the sample.When the current drives spin-polarized electrons from the elec-trode to the sample,nonequilibrium spin accumulates there.The rate of spin accumulation depends on spin relaxation,the process of bringing the accumulated spin population back to equilibrium.There are several mechanisms of spin relaxation,most involving spin-orbit coupling to provide the spin-dependent potential,in combination with momentum scattering to provide a randomizing force.Typical time scales for spin relax-ation in electronic systems are measured in nanosec-onds,while the range is from picoseconds to microsec-onds.Spin detection,also part of a generic spintronic scheme,typically relies on sensing the changes in the signals caused by the presence of nonequilibrium spin in the system.The common goal in many spintronic de-vices is to maximize the spin detection sensitivity to the point that it detects not the spin itself,but changes in the spin states.

Let us illustrate the generic spintronic scheme on a prototypical device,the Datta-Das spin?eld-effect tran-sistor(SFET;Datta and Das,1990),depicted in Fig.1. The scheme shows the structure of the usual FET,with a drain,a source,a narrow channel,and a gate for control-ling the current.The gate either allows the current to ?ow(ON)or does not(OFF).The spin transistor is simi-lar in that the result is also a control of the charge cur-rent through the narrow channel.The difference,how-ever,is in the physical realization of the current control. In the Datta-Das SFET the source and the drain are ferromagnets acting as the injector and detector of the electron spin.The drain injects electrons with spins par-allel to the transport direction.The electrons are trans-ported ballistically through the channel.When they ar-rive at the drain,their spin is detected.In a simpli?ed picture,the electron can enter the drain(ON)if its spin points in the same direction as the spin of the drain. Otherwise it is scattered away(OFF).The role of the gate is to generate an effective magnetic?eld(in the direction of?in Fig.1),arising from the spin-orbit cou-pling in the substrate material,from the con?nement ge-ometry of the transport channel,and the electrostatic potential of the gate.This effective magnetic?eld causes the electron spins to precess.By modifying the voltage, one can cause the precession to lead to either parallel or antiparallel(or anything between)electron spin at the drain,effectively controlling the current.

Even though the name spintronics is rather novel,2 contemporary research in spintronics relies closely on a long tradition of results obtained in perse areas of physics(for example,magnetism,semiconductor phys-ics,superconductivity,optics,and mesoscopic physics) and establishes new connections between its different sub?elds(Rashba,2002c;Zˇutic′,2002a).We review here both well-established results and the physical principles

1While there are proposals for spintronic devices based on

deoxyribonucleic acid(DNA)molecules(Zwolak and Di Ven-tra,2002),the whole device,which includes electrodes, voltage/current source,etc.,is still a solid-state system.

2The term was coined by S.A.Wolf in1996,as a name for a DARPA initiative for novel magnetic materials and

devices. FIG.1.(Color in online edition)Scheme of the Datta-Das spin ?eld-effect transistor(SFET).The source(spin injector)and the drain(spin detector)are ferromagnetic metals or semicon-ductors,with parallel magnetic moments.The injected spin-polarized electrons with wave vector k move ballistically along a quasi-one-dimensional channel formed by,for example,an InGaAs/InAlAs heterojunction in a plane normal to n.Elec-tron spins precess about the precession vector?,which arises from spin-orbit coupling and which is de?ned by the structure and the materials properties of the channel.The magnitude of ?is tunable by the gate voltage V G at the top of the channel. The current is large if the electron spin at the drain points in the initial direction(top row)—for example,if the precession period is much larger than the time of?ight—and small if the direction is reversed(bottom).

324Zˇutic′,Fabian,and Das Sarma:Spintronics:Fundamentals and applications Rev.Mod.Phys.,Vol.76,No.2,April2004

relevant to present and future applications.Our strategy is to give a comprehensive view of what has been accom-plished,focusing in detail on a few selected topics that we believe are representative for the broader subject within which they appear.For example,while discussing the generation of spin polarization,we survey many ex-perimental and theoretical studies of both optical orien-tation and electrical spin injection and present a detailed and self-contained formalism of electrical spin injection. Similarly,when we discuss spin relaxation,we give a catalog of important work,while studying spin relax-ation in the cases of Al and GaAs as representative of the whole?eld.Finally,in the section on spin devices we give detailed physical principles of several selected de-vices,such as,for example,the above-mentioned Datta-Das SFET.

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