SERIES ON SEMICONDUCTOR SCIENCE AND TECHNOLOGY SeriesEditors R.J.Nicholas UniversityofOxford H.Kamimura UniversityofTokyo Series on Semiconductor Science and Technology 1. M.Jaros:Physicsandapplicationsofsemiconductor microstructures 2. V.N.DobrovolskyandV.G.Litovchenko:Surfaceelectronic transportphenomenainsemiconductors 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.J.Challis:Electron–phononinteractioninlow-dimensionalstructures 11. V.Ustinov,A.Zhukov,A.Egorov,N.Maleev:Quantumdotlasers 12. H.Spieler:Semiconductordetectorsystems 13. S.Maekawa:Conceptsinspinelectronics Concepts in Spin Electronics Editedby Sadamichi Maekawa InstituteforMaterialsResearch, TohokuUniversity,Japan 1 3 GreatClarendonStreet,OxfordOX26DP OxfordUniversityPressisadepartmentoftheUniversityofOxford. ItfurtherstheUniversity’sobjectiveofexcellenceinresearch,scholarship, andeducationbypublishingworldwidein Oxford NewYork Auckland CapeTown DaresSalaam HongKong Karachi KualaLumpur Madrid Melbourne MexicoCity Nairobi NewDelhi Shanghai Taipei Toronto Withofficesin Argentina Austria Brazil Chile CzechRepublic France Greece Guatemala Hungary Italy Japan Poland Portugal Singapore SouthKorea Switzerland Thailand Turkey Ukraine Vietnam OxfordisaregisteredtrademarkofOxfordUniversityPress intheUKandincertainothercountries PublishedintheUnitedStates byOxfordUniversityPressInc.,NewYork ©OxfordUniversityPress2006 Themoralrightsoftheauthorhavebeenasserted DatabaserightOxfordUniversityPress(maker) Firstpublished2006 Allrightsreserved.Nopartofthispublicationmaybereproduced, storedinaretrievalsystem,ortransmitted,inanyformorbyanymeans, withoutthepriorpermissioninwritingofOxfordUniversityPress, orasexpresslypermittedbylaw,orundertermsagreedwiththeappropriate reprographicsrightsorganization.Enquiriesconcerningreproduction outsidethescopeoftheaboveshouldbesenttotheRightsDepartment, OxfordUniversityPress,attheaddressabove Youmustnotcirculatethisbookinanyotherbindingorcover andyoumustimposethesameconditiononanyacquirer BritishLibraryCataloguinginPublicationData Dataavailable LibraryofCongressCataloginginPublicationData Dataavailable PrintedinGreatBritain onacid-freepaperby BiddlesLtd.,King’sLynn ISBN 0–19–856821–5 978–0–19–856821–6 1 3 5 7 9 10 8 6 4 2 Preface Nowadays information technology is based on semiconductor and ferromagnetic materials.Informationprocessingandcomputationareperformedusingelectron charge by semiconductor transistors and integrated circuits, but on the other hand the information is stored on magnetic high-density hard disks by electron spins. Recently, a new branch of physics and nanotechnology, called magneto- electronics, spintronics, or spin electronics, has emerged, which aims to simul- taneously exploit both the charge and the spin of electrons in the same device and describes thenewphysics raised.One of its tasksisto mergethe processing and storage of data in the same basic building blocks of integrated circuits, but a broader goal is to develop new functionality that does not exist separately in a ferromagnet or a semiconductor. Research in magnetic materials has long been characterized by unusually rapid transitions to technology. A prominent example is the discovery in 1988 of one of the first spin electronics effects, namely the giant magnetoresistance (GMR) effect in magnetic layered structures, which has already found market applicationinreadheadsincomputerharddiskdrivesandalsoinmagneticsen- sors.Recentlynewtechnologybasedonthetunnelingmagnetoresistance(TMR) of magnetic tunnel junctions as magnetic random access memory (MRAM) is emerging into the electronic memory market. It is to be expected that future progressinspinelectronicswillleadtosimilarlyrapidapplications,inparticular once the merging of semiconductor and magnetic technologies is achieved. The aim of this book is to present new directions in the development of spin electronics, both the basic physics and technology in recent years, which will become the foundation of future technology. In the first part we give an introduction to ferromagnetic semiconductors: recent developments, new effects and devices. Further it will demonstrate how a spin current can be created, maintained, measured, and manipulated by light or an electric field, in several types of devices. One very interesting and promising group of such devices, which allow us to control and manipulate a single spin, is ultrasmall systems called quantumdots (QDs), whereCoulombinteraction(Coulombblockade)playsanimportantrole. In quantum dots due tothe control of a single electron charge, the possibilityof manipulatingof asinglespinisopenedup,whichcanbeimportant forquantum computing. On the level of a few spins, the new physics related to exchange interaction, spin blockade, Larmor precession, electron spin resonance (ESR), the Kondo effect, and hyperfine interactions with nuclear spins is raised. Also combining ferromagnetic materials with QDs opens up the new possibility of v vi PREFACE control and manipulation of a QD single spin by direct exchange interactions and construction of ferromagnetic single-electron transistors (F-SET). Recent study of spin-dependent transport in hybrid structures involving a combinationofferromagnetic(bothmetallicandsemiconducting)andnormalor superconductingmaterialsisreviewed.Theinterplaybetweenthedifferenttypes of interactions and correlations present in each produces a host of interesting spin-dependent effects, many of which have direct potentials for applications. A very promising new effect and technology of spin current induced mag- netization switching in magnetic nanostructures are discussed, together with potential applications. Another interesting field closely related with the minia- turization of magnetic systems is nanoscopic magnetism, where the cross-over between Stoner magnetism of the bulk magnetism to Hund’s rules in molecular systems using tunneling spectroscopy can be studied. In summary, spin electronics and spin optoelectronics promise to lead to a growing collection of novel devices and circuits that possibly can be integrated intohigh-performancechipstoperformcomplexfunctions,wherethekeyelement willbe integrationof complex magneticmaterials withmainstreamsemiconduc- tor technology. April 2005 Sadamichi Maekawa (On behalf of the authors) Contents ListofContributors xiii 1 Optical phenomena in magnetic semiconductors 1 H. Munekata 1.1 Introduction 1 1.2 Optical properties of III-V-based MAS 2 1.2.1 Brief history 2 1.2.2 Hole-mediated ferromagnetism 3 1.2.3 Optical properties 6 1.3 Photo-induced ferromagnetism 11 1.3.1 Effect of charge injection I: photo-induced ferromagnetism 11 1.3.2 Effect of charge injection II: optical control of coercive force 14 1.4 Photo-induced magnetization rotation effect of spin injection 17 1.5 Spin dynamics 23 1.6 Possible applications 29 1.6.1 Magnetization reversal by electrical spin injection 30 1.6.2 Circularly polarized light emitters and detector 32 References 36 2 Bipolar spintronics 43 Igor Zˇuti´c and Jaroslav Fabian 2.1 Preliminaries 43 2.1.1 Introduction 43 2.1.2 Concept of spin polarization 44 2.1.3 Optical spin orientation 46 2.1.4 Spin injection in metallic F/N junctions 49 2.1.5 Spin relaxation in semiconductors 55 2.2 Bipolar spin-polarized transport and applications 61 2.2.1 Spin-polarized drift-diffusion equations 61 2.2.2 Spin-polarized p-n junctions 65 2.2.3 Magnetic p-n junctions 70 vii viii CONTENTS 2.2.4 Spin transistors 74 2.2.5 Outlook and future directions 86 References 88 3 Probing and manipulating spin effects in quantum dots 93 S. Tarucha, M. Stopa, S. Sasaki, and K. Ono 3.1 Introduction and some history 93 3.2 Charge and spin in single quantum dots 96 3.2.1 Constant interaction model 96 3.2.2 Spin and exchange effect 99 3.3 Controlling spin states in single quantum dots 101 3.3.1 Singlet-triplet and doublet-doublet crossings 101 3.3.2 Non-linear regime for singlet-triplet crossing 104 3.3.3 Zeeman effect 105 3.4 Charge and spin in double quantum dots 109 3.4.1 Hydrogen molecule model 109 3.4.2 Stability diagram of charge states 110 3.4.3 Exchange coupling in the scheme of quantum computing 112 3.5 Spin relaxation in quantum dots 114 3.5.1 Transverse and longitudinal relaxation 114 3.5.2 Effect of spin-orbit interaction 117 3.6 Spin blockade in single-electron tunneling 118 3.6.1 Suppression of single-electron tunneling 118 3.6.2 Pauli effect in coupled dots 119 3.6.3 Lifting of Pauli spin blockade by hyperfine coupling 122 3.7 Cotunneling and the Kondo effect 125 3.7.1 Cotunneling 125 3.7.2 The standard Kondo effect 127 3.7.3 The S-T and D-D Kondo effect 131 3.8 Conclusions 139 References 140 4 Spin-dependenttransportinsingle-electron devices 145 Jan Martinek and J´ozef Barna´s 4.1 Single-electron transport 146 4.2 Model Hamiltonian 148 4.2.1 Metallic or ferromagnetic island 149 4.2.2 Quantum dot – Anderson model 149 4.3 Transport regimes 150 4.4 Weak coupling – sequential tunneling 151 CONTENTS ix 4.4.1 Quantum dot 151 4.4.2 Non-Collinear geometry 155 4.4.3 Ferromagnetic island 159 4.4.4 Metallic island 161 4.4.5 Shot noise 164 4.5 Cotunneling 167 4.5.1 Ferromagnetic island 167 4.5.2 Metallic island 168 4.5.3 Quantum dot 170 4.6 Strong coupling – Kondo effect 171 4.6.1 Perturbative-scaling approach 172 4.6.2 Numerical renormalization group 173 4.6.3 Gate-controlled spin-splitting in quantum dots 177 4.6.4 Non-equilibrium transport properties 182 4.6.5 Relation to experiment 184 4.7 RKKY interaction between quantum dots 184 4.7.1 Flux-dependent RKKY interaction 185 4.7.2 RKKY interaction – experimental results 188 References 190 5 Spin-transfer torques and nanomagnets 195 Daniel C. Ralph and Robert A. Buhrman 5.1 Spin-transfer torques 195 5.1.1 Intuitive picture of spin-transfer torques 196 5.1.2 The case of two magnetic layers 198 5.1.3 Simple picture of spin-transfer-driven magnetic dynamics 200 5.1.4 Experimental results 203 5.1.5 Applications of spin transfer torques 216 5.2 Electrons in micro- and nanomagnets 219 5.2.1 Micron-scale magnets and Coulomb blockade 220 5.2.2 Ferromagnetic nanoparticles 222 5.2.3 Magnetic molecules and the Kondo effect 227 References 234 6 Tunnel spin injectors 239 Xin Jiang and Stuart Parkin 6.1 Introduction 239 6.2 Magnetic tunnel junctions 241 6.2.1 Tunnelingspin polarization 245 6.2.2 Giant tunneling using MgO tunnel barriers 247 6.3 Magnetic tunnel transistor 256 6.3.1 Hot electron devices 256