Spin Current

460 Pages·2012·25.482 MB·English

by  Sadamichi Maekawa, Sergio O. Valenzuela, Eiji Saitoh, Takashi Kimura

SERIES ON SEMICONDUCTOR
SCIENCE AND TECHNOLOGY
SeriesEditors
R.J.Nicholas UniversityofOxford
H.Kamimura UniversityofTokyo
SERIES ON SEMICONDUCTOR SCIENCE AND TECHNOLOGY
1. M.Jaros:Physicsandapplicationsofsemiconductormicrostructures
2. V.N.DobrovolskyandV.G.Litovchenko:Surfaceelectronictransport
phenomenainsemiconductors
3. M.J.Kelly:Low-dimensionalsemiconductors
4. P.K.Basu:Theoryofopticalprocessesinsemiconductors
5. N.Balkan:Hotelectronsinsemiconductors
6. B.Gil:GroupIIInitridesemiconductorcompounds:physicsandapplications
7. M.Sugawara:Plasmaetching
8. M.BalkanskiandR.F.Wallis:Semiconductorphysicsandapplications
9. B.Gil:Low-dimensionalnitridesemiconductors
10. L.Challis:Electron-phononinteractioninlow-dimensionalstructures
11. V.Ustinov,A.Zhukov,A.Egorov,N.Maleev:Quantumdotlasers
12. H.Spieler:Semiconductordetectorsystems
13. S.Maekawa:Conceptsinspinelectronics
14. S.D.Ganichev,W.Prettl:Intenseterahertzexcitationofsemiconductors
15. N.Miura:Physicsofsemiconductorsinhighmagneticfields
16. A.V.Kavokin,J.J.Baumberg,G.Malpuech,F.P.Laussy:Microcavities
17. S.Maekawa,S.O.Valenzuela,E.Saitoh,T.Kimura:Spincurrent
Spin Current
Editedby
Sadamichi Maekawa
Sergio O. Valenzuela
Eiji Saitoh
Takashi Kimura
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Preface
Sincethediscoveryofthegiantmagnetoresistance(GMR)effectinmagneticmul-
tilayersin1988,anewbranchofphysicsandtechnology,calledspin-electronicsor
spintronics, has emerged, where the flow of electrical charge as well as the flow
of electron spin, the so-called “spin current,” are manipulated and controlled
together. Whereas charge current flows without decay (owing to fundamental
charge conservation), spin current decays on a length-scale of less than a few
micrometers. In other words, it exists only at nanometer scales. Recent progress
in the physics of magnetism and the application of spin current has progressed
in tandem with the nanofabrication technology of magnets and the engineering
of interfaces and thin films.
Thisbookisintendedtoprovideanintroductionandguidetothenewphysics
andapplicationofspincurrent.Theemphasisisplaceontheinteractionbetween
spin and charge currents in magnetic nanostructures.
TheInternationalConferenceonMagnetism(ICM),thelargestconferencein
the physics of magnetism, has been held triennially since the first one organized
by Louis N´eel at Grenoble, France in 1958. The Eighteenth Conference in the
ICM series took place in Karlsruhe, Germany in July 2009, where a paradigm
in physics was epitomized by “a flood of spin current,” which introduces a new
front in the evolution of traditional research in magnetism.
I am glad to note that the achievements of the research in spin current by
Sergio O. Valenzuela, Eiji Saitoh, and Takashi Kimura were recognized through
the Young Scientist in Magnetism Awards at the Eighteenth ICM sponsored
by the International Union of Pure and Applied Physics (IUPAP), an agency
under the auspices of UNESCO (United Nations Educational, Scientific, and
Cultural Organization).
In this book, three of them give introductions to spin current, the spin Hall
effect, spin torgues, and the spin Seebeck effect based on their achievements.
Although the chapters make up a coherent whole, each chapter is self-contained
and may be read independently. The physics based on spin current is growing
rapidly. Therefore, we have tried to introduce the most recent results up to
August2011.Ihopethisbookisasoundguidetothenewphysicsandtechnology.
August 2011 Sadamichi Maekawa
(On behalf of the Editors)
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Contents
Contributors xv
Part I Spin current
1 Introduction 3
E. Saitoh
1.1 Spin of electrons 3
1.1.1 Spin angular momentum 3
1.1.2 Dirac equation and spin 5
1.1.3 Nonrelativistic approximation 7
1.2 Spin current 9
1.2.1 Concept of spin current 9
1.2.2 An exact definition of spin current 10
References 14
2 Incoherent spin current 15
K. Ando and E. Saitoh
2.1 Fermi–Dirac distribution 15
2.2 Diffusion equation 18
2.3 Spin diffusion equation 19
References 24
3 Exchange spin current 25
E. Saitoh and K. Ando
3.1 Magnetic order and exchange interaction 25
3.2 Exchange spin current 25
3.2.1 Landau–Lifshitz–Gilbert equation 25
3.2.2 Rewriting the Landau–Lifshitz–Gilbert
equation 27
3.3 Spin-wave spin current 28
3.3.1 Spin-wave formulation 28
3.3.2 Spin current carried by a spin wave 31
References 32
viii CONTENTS
4 Topological spin current 33
E. Saitoh
4.1 Bulk topological spin current 33
4.2 Surface topological spin current 35
References 35
5 Spin polarization in magnets 36
K. Takanashi and Y. Sakuraba
5.1 Spin polarization in ferromagnets 36
5.2 Half-metallic ferromagnets 37
5.3 Experimental techniques for spin-polarization
measurement 39
5.3.1 Point-contact Andreev reflection (PCAR) 39
5.3.2 Superconducting tunneling spectroscopy (STS) 41
5.3.3 Spin-resolved photoemission spectroscopy (SP-PES) 43
5.4 Magnetoresistive devices with half-metals 43
5.4.1 Magnetic tunnel junctions with half-metals 43
5.4.2 Current-perpendicular-to-plane magnetoresistive
device with half-metals 45
References 46
6 Optically induced and detected spin current 49
A. Hirohata
6.1 Introduction 49
6.1.1 Optical generation of spins 49
6.1.2 Spin polarization in GaAs 49
6.1.3 Photoexcitation model 50
6.2 Optical spin injection 55
6.2.1 Photoexcitation 55
6.2.2 Schottky diodes 55
6.2.3 Spin-polarized scanning tunneling microscopy
(spin STM) 57
6.3 Optical spin detection 58
6.3.1 Spin-polarized light-emitting diodes (spin LED) 58
6.3.2 Schottky diodes 60
6.3.3 Spin injection into Si 62
References 62
7 Theory of spinmotive forces in ferromagnetic
structures 65
S. E. Barnes
7.1 Introduction 65
7.2 Spin Faraday’s law 65
CONTENTS ix
7.2.1 Magnetic momentum, spin electric, and magnetic
fields 66
7.2.2 Calculation of magnetic momentum for adiabatic
electrons 66
7.2.3 The spin fields E(cid:2) and B(cid:2) and Faraday’s law 68
s s
7.2.4 Landau–Lifshitz equations 69
7.2.5 Spin-torque-transfer (STT), spin valves, and magnetic
tunnel junctions (MTJs) 69
7.2.6 Spinmotive force (smf), spin valves and MTJs 71
7.2.7 The spin electric field E(cid:2) 73
s
7.3 Examples of spinmotive forces 74
7.3.1 A plain N´eel domain wall 74
7.3.2 Reciprocal relations 75
7.3.3 The spinmotive force and magnons 76
7.3.4 Spin forces and Doppler shifts for phonons 77
7.3.5 Voltage steps and magnetoresistance 77
7.3.6 Static magnetic vortices 78
7.3.7 Field-driven magnetic vortices 78
7.4 Ferromagnetic resonance (FMR) and spinmotive forces 80
7.4.1 Field-driven FMR 80
7.4.2 STT-FMR and a-FMR resonance 80
7.4.3 The spin Seebeck effect and the smf 80
7.4.4 FMR with thermal gradients 81
7.5 Spinmotive forces, magnons, and phonons 82
7.5.1 The s=1 nature of magnons and phonons 82
7.5.2 Realization of smf effects with magnons and
phonons 83
References 84
8 Spin pumping and spin transfer 87
A. Brataas, Y. Tserkovnyak, G. E. W. Bauer,
and P. J. Kelly
8.1 Introduction 87
8.1.1 Technology pull and physics push 87
8.1.2 Discrete versus homogeneous 87
8.1.3 This chapter 88
8.2 Phenomenology 89
8.2.1 Mechanics 89
8.2.2 Spin-transfer torque and spin pumping 89
8.2.3 Onsager reciprocity relations 99
8.3 Microscopic derivations 103
8.3.1 Spin-transfer torque 103
8.3.2 Spin pumping 109