曾经的曾经One of the keys to the success of magnetoresistance-based applications is their ability to control14the relative orientation of M1and M2.An interesting realization of such control was proposed independently by Berger (1996)and Slonczewski(1996).While in GMR or TMR structures the relative orientation of magnetizations will affect the?ow of spin-polarized current,they predicted a reverse effect.The?ow of spin-polarized current can transfer angular momentum from carriers to ferromag-net and alter the orientation of the corresponding mag-netization,even in the absence of an applied magnetic ?eld.This phenomenon,known as spin-transfer torque, has since been extensively studied both theoretically and experimentally(Bazaliy et al.,1998;Tsoi et al.,1998;My-ers et al.,1999;Sun,2000;Waintal et al.,2000;Stiles and Zangwill,2002),and current-induced magnetization re-versal has been demonstrated at room temperature(Ka-tine et al.,2000).It was also shown that the magnetic ?eld generated by passing the current through a CPP giant magnetoresonance device could produce room-temperature magnetization reversal(Bussmann et al., 1999).In the context of ferromagnetic semiconductors additional control of magnetization was demonstrated optically,by shining light(Koshihara et al.,1997; Boukari et al.,2002;Oiwa et al.,2002)and electrically, by applying gate voltage(Ohno,Chiba,et al.,2000; Boukari et al.,2002;Park et al.,2002)to perform switching between the ferromagnetic and paramagnetic states.
Jullie`re’s model also justi?es the continued quest for highly spin-polarized materials—they would provide large magnetoresistive effects,desirable for device appli-cations.In an extreme case,spins would be completely polarized even in the absence of magnetic?eld.Numeri-cal support for the existence of such materials—the so-called half-metallic ferromagnets15—was provided by de Groot,Janner,and Mueller(1983),and these materials were reviewed by Pickett and Moodera(2001).In addi-tion to ferromagnets,such as CrO2(Soulen et al.,1998; Parker et al.,2002)and manganite perovskites(Park et al.,1998a),there is evidence for high spin polarization in III-V ferromagnetic semiconductors like(Ga,Mn)As (Braden et al.,2003;Panguluri,Nadgorny,et al.,2003). The challenge remains to preserve such spin polarization above room temperature and in junctions with other materials,since the surface(interface)and bulk mag-netic properties can be signi?cantly different(Fisher, 1967;Mills,1971;Falicov et al.,1990).
While many existing spintronic applications(Hart-man,2000;Hirota et al.,2002)are based on the GMR effects,the discovery of large room-temperature TMR (Miyazaki and Tezuka,1995;Moodera et al.,1995)has renewed interest in the study of magnetic tunnel junc-tions,which are now the basis for the several magnetic random-access memory prototypes16(Parkin,Roche, et al.,1999;Tehrani et al.,2000).Future generations of magnetic read heads are expected to use MTJ’s instead of CIP giant magnetoresonance.To improve the switch-ing performance of related devices it is important to re-duce the junction resistance,which determines the RC time constant of the MTJ cell.Consequently,semicon-ductors,which would provide a lower tunneling barrier than the usually employed oxides,are being investigated both as the nonferromagnetic region in MTJ’s and as the basis for an all-semiconductor junction that would dem-
onstrate large TMR at low temperatures(Tanaka and Higo,2001;Tanaka,2002).Another desirable property of semiconductors has been demonstrated by the ex-traordinary large room-temperature magnetoresistance in hybrid structures with metals,reaching750000%at a magnetic?eld of4T(Solin et al.,2000),which could lead to improved magnetic read heads(Solin et al.;2002; Moussa et al.,2003).Magnetoresistance effects of similar magnitude have also been found in hybrid metal/ semiconductor granular?lms(Akinaga,2002).Another approach to obtaining large room-temperature magne-toresistance(?100%at B?100G)is to fabricate ferro-magnetic regions separated by a nanosize contact.For simplicity,such a structure could be thought of as the
14For example,with small magnetic?eld(Parkin,2002)or at high switching speeds(Schumacher et al.,2003a,2003b).
15Near the Fermi level they behave as metals only for one spin,the density of states vanishes completely for the other spin.
16Realization of the early magnetic random-access memory proposals used the effect of anisotropic magnetoresistance (Pohn et al.,1987,1988).
FIG.3.(Color in online edition)Schematic illustration of(a) the current in plane(CIP),(b)the current perpendicular to the plane(CPP)giant magnetoresistance geometry.327
Zˇutic′,Fabian,and Das Sarma:Spintronics:Fundamentals and applications Rev.Mod.Phys.,Vol.76,No.2,April2004
limiting case of the CPP giant magnetoresonance scheme in Fig.3(b).This behavior,also known as ballis-tic magnetoresistance,has already been studied in a large number of materials and geometries(Bruno,1999;Gar-cia et al.,1999;Tatara et al.,1999;Imamura et al.,2000; Versluijs et al.,2001;Chung et al.,2002).
2.Spin injection and optical orientation
Many materials in their ferromagnetic state can have a substantial degree of equilibrium carrier spin polariza-tion.However,as illustrated in Fig.1,this alone is usu-ally not suf?cient for spintronic applications,which typi-cally require current?ow and/or manipulation of the nonequilibrium spin(polarization).17The importance of generating nonequilibrium spin is not limited to device applications;it can also be used as a sensitive spectro-scopic tool to study a wide variety of fundamental prop-erties ranging from spin-orbit and hyper?ne interactions (Meier and Zakharchenya,1984)to the pairing symme-try of high-temperature superconductors(Vas’ko et al., 1997;Wei et al.,1999;Tsuei and Kirtley,2000;Ngai et al.,
2004)and the creation of spin-polarized beams to mea-sure parity violation in high-energy physics(Pierce and Celotta,1984).
Nonequilibrium spin is the result of some source of pumping arising from transport,optical,or resonance methods.Once the pumping is turned off the spin will return to its equilibrium value.While for most applica-tions it is desirable to have long spin relaxation times,it has been demonstrated that short spin relaxation times are useful in the implementation of fast switching(Nish-ikawa et al.,1995).
Electrical spin injection,an example of a transport method for generating nonequilibrium spin,has already been realized experimentally by Clark and Feher(1963), who drove a direct current through a sample of InSb in the presence of a constant applied magnetic?eld.The principle was based on the Feher effect,18in which the hyper?ne coupling between the electron and nuclear spins,together with different temperatures representing electron velocity and electron spin populations,is re-sponsible for the dynamical nuclear polarization(Slich-ter,1989).19Motivated by the work of Clark and Feher (1963)and Tedrow and Meservey(1971b,1973)and the principle of optical orientation(Meier and Zakharch-enya,1984),Aronov(1976a,1976b),and Aronov and Pikus(1976)established several key concepts in electri-cal spin injection from ferromagnets into metals, semiconductors,20and superconductors.Aronov(1976b) predicted that,when a charge current?owed across the F/N junction(Fig.4),spin-polarized carriers in a ferro-magnet would contribute to the net current of magneti-zation entering the nonmagnetic region and would lead
to nonequilibrium magnetization?M,depicted in Fig. 4(b),with the spatial extent given by the spin diffusion length(Aronov,1976b;Aronov and Pikus,1976).21Such
a?M,which is also equivalent to a nonequilibrium spin accumulation,was?rst measured in metals by Johnson
and Silsbee(1985,1988d).In the steady state?M is re-
17Important exceptions are tunneling devices operating at low bias and near equilibrium spin.Equilibrium polarization and the current?ow can be potentially realized,for example,in spin-triplet superconductors and thin-?lm ferromagnets (Ko¨nig et al.,2001),accompanied by dissipationless spin d72efe37f111f18583d05a9ding an analogy with the quantum Hall effect,it has been suggested that the spin-orbit interaction could lead to dissipationless spin currents in hole-doped semiconductors (Murakami et al.,2003).Rashba(2003b)has pointed out that similar dissipationless spin currents in thermodynamic equilib-rium,due to spin-orbit interaction,are not transport currents which could be employed for transporting spins and spin injec-tion.It is also instructive to compare several earlier proposals that use spin-orbit coupling to generate spin currents,dis-cussed in Sec.II.A.
18The importance and possible applications of the Feher ef-fect(Feher,1959)to polarize electrons was discussed by Das Sarma et al.(2000c)and Suhl(2002).
19Such an effect can be thought of as a generalization of the Overhauser effect(Overhauser,1953b),in which the use of a resonant microwave excitation causes the spin relaxation of the nonequilibrium electron population through hyper?ne cou-pling to lead to the spin polarization of nuclei.Feher(1959) suggested several other methods,instead of microwave excita-tion,that could produce a nonequilibrium electron population and yield a dynamical polarization of nuclei(see also Weger, 1963).
20In an earlier work,spin injection of minority carriers was proposed in a ferromagnet/insulator/p-type semiconductor structure.Measuring polarization of electroluminescence was suggested as a technique for detecting injection of polarized carriers in a semiconductor(Scifres et al.,1973).
21Supporting the?ndings of Clark and Feher(1963),Aronov calculated that the electrical spin injection would polarize nu-clei and lead to a measurable effect in the electron spin reso-nance.Several decades later related experiments on spin injec-tion are also examining other implications of dynamical nuclear polarization(Johnson,2000;Strand et al.,
2003). FIG.4.(Color in online edition)Pedagogical illustration of the concept of electrical spin injection from a ferromagnet(F)into a normal metal(N).Electrons?ow from F to N:(a)schematic device geometry;(b)magnetization M as a function of position—nonequilibrium magnetization?M(spin accumula-tion)is injected into a normal metal;(c)contribution of differ-ent spin-resolved densities of states to both charge and spin transport across the F/N interface.Unequal?lled levels in the density of states depict spin-resolved electrochemical poten-tials different from the equilibrium value?0.
