Spintronics(自旋电子学)(2)

There have been many other reviews written on spin-tronics,most focusing on a particular aspect of the?eld. We pide them here,for an easier orientation,into two groups,those that cover the emerging applications3and those covering already well-established schemes and materials.4The latter group,often described as magne-toelectronics typically covers paramagnetic and ferro-magnetic metals and insulators,which utilize magnetore-sistive effects,realized,for example,as magnetic read heads in computer hard drives,nonvolatile magnetic random access memory(MRAM),and circuit isolators (Wang et al.,2002).These more established aspects of spintronics have also been addressed in several books5 and will be discussed in another review6complementary to ours.

Spintronics also bene?ts from a large class of emerg-ing materials,such as ferromagnetic semiconductors (Ohno,1998;Pearton et al.,2003),organic semiconduc-tors(Dediu et al.,2002),organic ferromagnets(Pejak-ovic′et al.,2002;Epstein,2003),high-temperature super-conductors(Goldman et al.,1999),and carbon nanotubes(Tsukagoshi et al.,1999;Zhao et al.,2002), which can bring novel functionalities to the traditional devices.There is a continuing need for fundamental studies before the potential of spintronic applications can be fully realized.

After an overview,Sec.I covers some basic historical and background material,part of which has already been extensively covered in the context of magnetoelec-tronics and will not be discussed further in this review. Techniques for generating spin polarization,focusing on optical spin orientation and electrical spin injection,are described in Sec.II.The underlying mechanisms respon-sible for the loss of spin orientation and coherence, which impose fundamental limits on the length and time scales in spintronic devices,are addressed in Sec.III. Spintronic applications and devices,with the emphasis on those based on semiconductors,are discussed in Sec. IV.The review concludes with a look at future prospects in Sec.V.

B.History and background

1.Spin-polarized transport and magnetoresistive effects

In a pioneering work,Mott(1936a,1936b)provided a basis for our understanding of spin-polarized transport. Mott sought an explanation for an unusual behavior of resistance in ferromagnetic metals.He realized that at suf?ciently low temperatures,where magnon scattering becomes vanishingly small,electrons of majority and mi-nority spin,with magnetic moment parallel and antipar-allel to the magnetization of a ferromagnet,respectively, do not mix in the scattering processes.The conductivity can then be expressed as the sum of two independent and unequal parts for two different spin projections—the current in ferromagnets is spin polarized.This is also known as the two-current model and has been extended by Campbell et al.(1967)and Fert and Campbell(1968). It continues,in its modi?cations,to provide an explana-tion for various magnetoresistive phenomena(Valet and Fert,1993).

Tunneling measurements played a key role in early experimental work on spin-polarized transport.Studying N/F/N junctions,where N was a nonmagnetic7metal and F was an Eu-based ferromagnetic semiconductor(Ka-suya and Yanase,1968;Nagaev,1983),revealed that I-V curves could be modi?ed by an applied magnetic?eld (Esaki et al.,1967)and now show potential for develop-ing a solid-state spin?lter.When unpolarized current is passed across a ferromagnetic semiconductor,the cur-rent becomes spin-polarized(Moodera et al.,1988;Hao et al.,1990).

A series of experiments(Tedrow and Meservey, 1971b,1973,1994)in ferromagnet/insulator/ superconductor(F/I/S)junctions has unambiguously proved that the tunneling current remains spin polarized even outside of the ferromagnetic region.8The Zeeman-

3Reviews on emerging applications include those of Das Sarma et al.(2000a,2000b,2000c,2001);Wolf and Treger (2000);Das Sarma(2001);Wolf et al.(2001);Oestreich et al. (2002);Rashba(2002c);Zˇutic′(2002a,2002b).

4Established schemes and materials are reviewed by Tedrow

and Meservey(1994);Prinz(1995,1998);Gijs and Bauer (1997);Gregg et al.(1997);Ansermet(1998);Bass and Pratt, Jr.(1999);Daughton et al.(1999);Stiles(2004).

5See,for example,the books of Hartman(2000);Ziese and Thornton(2001);Hirota et al.(2002);Levy and Mertig(2002); Maekawa et al.(2002);Parkin(2002);Shinjo(2002);and Chtchelkanova et al.(2003).

6In preparation by S.S.P.Parkin for Reviews of Modern Physics.

7Unless explicitly speci?ed,we shall use the terms‘‘nonmag-netic’’and‘‘paramagnetic’’interchangeably,i.e.,assume that they both refer to a material with no long-range ferromagnetic order and with Zeeman-split carrier spin subbands in an ap-plied magnetic?eld.

8It has been shown that electrons photoemitted from ferro-magnetic gadolinium remain spin polarized(Busch et al., 1969).

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split quasiparticle density of states in a superconductor (Tedrow et al.,1970;Fulde,1973)was used as a detector of spin polarization of conduction electrons in various

magnetic materials.Jullie

`re (1975)measured tunneling conductance of F/I/F junctions,where I was an amor-phous Ge.By adopting the Tedrow and Meservey (1971b,1973)analysis of the tunneling conductance

from F/I/S to the F/I/F junctions,Jullie

`re (1975)formu-lated a model for a change of conductance between the parallel (↑↑)and antiparallel (↑↓)magnetization in the two ferromagnetic regions F1and F2,as depicted in Fig.2.The corresponding tunneling magnetoresistance 9(TMR)in an F/I/F magnetic tunnel junction (MTJ)is de?ned as

TMR ?

?R R ↑↑?R ↑↓?R ↑↑R ↑↑?G ↑↑?G ↑↓

G ↑↓

,(1)

where conductance G and resistance R ?1/G are la-beled by the relative orientations of the magnetizations in F1and F2(it is possible to change the relative orien-tations,between ↑↑and ↑↓,even at small applied mag-netic ?elds ?10G).TMR is a particular manifestation of a magnetoresistance that yields a change of electrical resistance in the presence of an external magnetic ?eld.10Historically,the anisotropic magnetoresistance in bulk ferromagnets such as Fe and Ni was discov-ered ?rst,dating back to the experiments of Lord Kelvin (Thomson,1857).Due to spin-orbit interaction,electri-cal resistivity changes with the relative direction of the

charge current (for example,parallel or perpendicular)

with respect to the direction of magnetization.

Within Jullie

`re’s model,which assumes constant tun-neling matrix elements and that electrons tunnel without spin ?ip,Eq.(1)yields

TMR ?2P 1P 2

1?P 1P 2

,

(2)

where the polarization P i ?(N Mi ?N mi )/(N Mi ?N mi )is expressed in terms of the spin-resolved density of states N Mi and N mi ,for majority and minority spin in F i ,re-spectively.Conductance in Eq.(1)can then be expressed

as (Maekawa and Ga

¨fvert,1982)G ↑↑?N M 1N M 2?N m 1N m 2and G ↑↓?N M 1N m 2?N m 1N M 2to give Eq.

(2).11While the early results of Jullie

`re (1975)were not con?rmed,TMR at 4.2K was observed using NiO as a

tunnel barrier by Maekawa and Ga

¨fvert (1982).The prediction of Jullie

`re’s model illustrates the spin-valve effect :the resistance of a device can be changed by manipulating the relative orientation of the magnetiza-tions M 1and M 2,in F1and F2,respectively.Such ori-entation can be preserved even in the absence of a power supply,and the spin-valve effect,12later discov-ered in multilayer structures displaying the giant magnetoresistance 13(GMR)effect (Baibich et al.,1988;Binasch et al.,1989)can be used for nonvolatile memory applications (Hartman,2000;Hirota et al.,2002;Parkin,

9

Starting with Jullie

`re (1975)an equivalent expression (G ↑↑?G ↑↓)/G ↑↑has also been used by different authors and is often referred to as junction magnetoresistance (Moodera and Mathon,1999).10

The concept of TMR was proposed independently by R.C.Barker in 1975[see Meservey et al.(1983)]and by Slonczewski (1976),who envisioned its use for magnetic bubble memory (Parkin,2002).

11

In Sec.IV we address some limitations of the Jullie

`re model and its potential ambiguities to identify precisely which spin polarization is actually measured.12

The term was coined by Dieny et al.(1991)in the context of GMR,by invoking an analogy with the physics of the TMR.13

The term ‘‘giant’’re?ected the magnitude of the effect (more than ?10%),as compared to the better known aniso-tropic magnetoresistance (?

1%).

FIG. 2.(Color in online edi-tion)Schematic illustration of

electron tunneling in ferro-magnet /insulator /ferromagnet (F/I/F)tunnel junctions:(a)Parallel and (b)antiparallel orientation of magnetizations with the corresponding spin-resolved density of the d states in ferromagnetic metals that have exchange spin splitting ?ex .Arrows in the two ferro-magnetic regions are deter-mined by the majority-spin sub-band.Dashed lines depict spin-conserved tunneling.

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ˇutic ′,Fabian,and Das Sarma:Spintronics:Fundamentals and applications Rev.Mod.Phys.,Vol.76,No.2,April 2004

2002).GMR structures are often classi?ed according to whether the current?ows parallel(CIP,current in plane) or perpendicular(CPP,current perpendicular to the plane)to the interfaces between the different layers,as depicted in Fig.3.Most of the GMR applications use the CIP geometry,while the CPP version,?rst realized by Pratt et al.(1991),is easier to analyze theoretically(Gijs and Bauer,1997;Levy and Mertig,2002)and relates to the physics of the tunneling magnetoresistance effect (Mathon and Umerski,1997).The size of magnetoresis-tance in the GMR structures can be expressed analo-gously to Eq.(1),where parallel and antiparallel orien-tations of the magnetizations in the two ferromagnetic regions are often denoted by‘‘P’’and‘‘AP,’’respectively (instead of↑↑and↑↓).Realization of a large room-temperature GMR(Parkin,Bhadra,and Roche,1991; Parkin,Li,and Smith,1991)enabled a quick transition from basic physics to commercial applications in mag-netic recording(Parkin,Jiang,et al.,2003).

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