Topical review Research frontiers in magnetic materials at ...In analogy with semiconductor materials, mod-ern magnetic materials have complex structures with important functional

Journal of Magnetism and Magnetic Materials 207 (1999) 7}44

Topical review

Research frontiers in magnetic materials at soft X-raysynchrotron radiation facilities

J.B. Kortright!,*, D.D. Awschalom", J. StoK hr#, S.D. Bader$, Y.U. Idzerda%,S.S.P. Parkin#, Ivan K. Schuller&, H.-C. Siegmann'

!Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA"Physics Department, University of California } Santa Barbara, Santa Barbara, CA 93106, USA

#IBM Almaden Research Center, San Jose, CA 95120-6099, USA$Materials Sciences Division, Argonne National Laboratory, Argonne, IL 60439, USA

%Naval Research Laboratory, Washington, DC 20375, USA&Physics Department-0319, University of California } San Diego, La Jolla, CA 92093, USA

'ETH Zurich, CH-8093 Zurich, Switzerland

Received 19 March 1999; received in revised form 2 June 1999

Abstract

Current and anticipated future research frontiers in magnetism and magnetic materials are discussed from a perspect-ive of soft X-ray synchrotron utilization. Topics covered include dimensionality (including e!ects of spatial dimensionsand di!ering time scales), magneto-electronics, structure/property relationships, and exploratory materials, with anemphasis on challenges that limit the understanding and advancement of these areas. Many soft X-ray spectroscopies canbe used to study magnetism associated with transition and rare earth metals with element- and chemical-state speci"cityand large cross-sections associated with dipole transitions from pPd and dPf states. Established electron spectro-scopies, including spin-resolved techniques, yield near-surface sensitivity in conjunction with linear and circular magneticdichroism. Emerging photon-based scattering and Faraday and Kerr magneto-optical measurements can be usedbeyond the near-surface region and in applied magnetic "elds. Microscopies based on either electron or photonspectroscopies to image the magnetization at 50 nm resolution are also emerging, as are time-resolved measurements thatutilize the natural time structure of synchrotron sources. Examples of research using these techniques to impact ourfundamental understanding of magnetism and magnetic materials are given, as are future opportunities. ( 1999Elsevier Science B.V. All rights reserved.

Keywords: Magnetic "lms; Synchrotron radiation; Electron spectroscopy; X-ray spectroscopy; X-ray scattering; Microscopy

*Corresponding author. Tel.: #1-510-486-5960; fax: #1-510-486-5530.

E-mail address: [email protected] (J.B. Kortright)

1. Introduction

This report discusses current and anticipatedresearch frontiers in magnetism and magneticmaterials with an emphasis on the utilizationof vacuum-ultraviolet (VUV) and soft X-ray

0304-8853/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 4 8 5 - 0

techniques. The report grew out of a workshop inBerkeley, CA, in March of 1998 to explore newscienti"c directions with the unique capabilitiesprovided by high-brightness, low-energy synchro-tron-radiation facilities, such as the AdvancedLight Source (ALS) at the Ernest O. LawrenceBerkeley National Laboratory. Select examples ofpast accomplishments within the "eld of magnet-ism are enumerated and emerging capabilities areidenti"ed. The report then addresses how the ALScan impact the frontiers of magnetism research andwhere the unique strengths and greatest opportuni-ties of such facilities lie. Finally, it provides a road-map for future investment in research in magnetismand magnetic materials at ALS.

Interactions among electrons in solids give riseto many interesting physical properties that lead tosuch practical applications as superconductivityand magnetism. The consequences of magnetismand magnetic materials for society are enormous,ranging from recording heads and media in in-formation-storage technology to components inthe most basic transformers and motors involvedin the generation and application of electric power.While magnetic materials have been used with in-creasing sophistication since the lodestone com-passes of the Phoenicians, many basic questionsabout the microscopic physical interactions andproperties in these materials remain unanswered.Increasingly sophisticated applications, driven inpart by advances in the synthesis of complex struc-tures, often with nanometer-scale dimensions, re-quire increasingly sophisticated experimentaltechniques that can directly probe electronic andspin states as well as the magnitudes of atomicmagnetic moments and the magnetic microstruc-tures. Such basic properties are ultimately respon-sible for the remarkable usefulness of thesematerials.

In many ways, research in magnetism and mag-netic materials is experiencing a renaissance [1]that has been enabled by developments in the lastseveral decades in the "elds of semiconductor phys-ics and materials. The ability to control thin-"lmgrowth at the atomic level to form epitaxial andheteroepitaxial semiconductor structures has morerecently been extended to magnetic nanostructures,including metallic, oxide, and semiconducting

phases. Fig. 1 shows a wide variety of atomicallyengineered magnetic nanostructures that can cur-rently be grown and studied. This new era is drivenboth by the interesting physics of these materials,and by the $50 billion per year magnetic-storageindustry ($150 billion per year if magnetic-record-ing tape, video, etc., are also included). This is inclose analogy to the scienti"c impetus provided bythe $150 billion per year semiconductor industry.The pervasive role of hard and soft magnets inelectric-power production and utilization also con-tinues to motivate research at all levels into newmaterials that might save considerable amountsannually by reducing energy losses and savingnatural resources consumed in the generation anduse of electricity. The considerable relevance ofmagnetism and magnetic materials to societythrough both technology and economic factorscannot be disputed.

Many examples can be found to illustrate theclose link between the fundamental physics of mag-netic phenomena and their technological applica-tion; consider interlayer magnetic coupling inmultilayer structures. Initial studies of coupling inmultilayers were motivated primarily by scienti"ccuriosity. (For an early claim of RKKY coupling inCu/Ni see Ref. [2], for magnetic dipolar coupling inMo/Ni superlattices see Ref. [3], for oscillatorycoupling in Gd/Y superlattices see Ref. [4], forspiral coupling in Dy/Y superlattices see Ref. [5],for antiferromagnetic coupling in Co/Cu see Ref.[6] and for antiferromagnetic coupling in Fe/Cr seeRefs. [7,8].) As giant magnetoresistance [9}11](GMR) and the concomitant phenomenon of oscil-latory interlayer magnetic coupling [12}14] werediscovered, the technological relevance of these ini-tially purely scienti"c discoveries [7,8] werebrought to the marketplace as vital products withinan extraordinarily short ten-year period. Fig. 2shows a diagram of a magnetic recording head thatuses a spin-valve read transducer based on theGMR e!ect. With the ability to synthesize novelmagnetic "lms and nanostructures [15] exhibitinga variety of magnetic properties come prospects fornew generations of devices. An example of an ap-plication based on magnetoelectronics [16], is themagnetic random-memory cells illustrated inFig. 3; here, spin-dependent transport introduces

8 J.B. Kortright et al. / Journal of Magnetism and Magnetic Materials 207 (1999) 7}44

Fig. 1. Some geometrical arrangements of magnetic nanostructures of current interest are illustrated here. In general, dark featuresrepresent magnetic material. The top row indicates ordered monolayers and nanoscale thin "lms, sandwich structures, and multilayers.Wedged thin "lms present a range of thicknesses for study in a single sample. Decorated steps and quantum corrals have been grownwith atomic-level control. Laterally patterned structures in the bottom include thin "lms, magnetic dots and wires, and arrays ofmagnetic-multilayer columns. The broad range of materials that can be grown in these nanostructures present many interestingfundamental questions and potential technological applications. They also present many experimental challenges, such as distinguishingmagnetism in buried ultrathin layers from that at the interfaces between them ("gure courtesy of S.D. Bader, Argonne NationalLaboratory).

J.B. Kortright et al. / Journal of Magnetism and Magnetic Materials 207 (1999) 7}44 9

Fig. 2. Magnetic recording heads typically consist of a write head and a read head within the same lithographically de"ned structure.The write head consists of a coil and a yoke that guides the magnetic #ux created by the coil to a pole tip. The large magnetic "eldemerging from the pole tip is used to write the magnetic bits into a thin magnetic "lm on a rotating magnetic-recording disk. The readhead is used to retrieve the information written on the disk. It senses the magnetic #ux emerging from the transition regions between thebits on the disk (see Fig. 12). In the spin-valve giant-magnetoresistance (GMR) head shown here, the #ux from the disk is large enough tochange the magnetization direction in one of the ferromagnetic layers which comprise the head. The magnetization direction in thesecond ferromagnetic layer is pinned by exchange coupling to an antiferromagnet and does not be rotate. Owing to the so-called GMRe!ect, a sense current #owing through the spin valve experiences a resistance that is higher by about 10% when the two ferromagneticlayers are magnetically aligned antiparallel rather than parallel ("gure courtesy of J. StoK hr, IBM Almaden Research Center).

magnetism into the realm of electronic devices, withthe promise of large-scale nonvolatile memory.However, there is still no accepted detailed theoret-ical model for the mechanism of this e!ect.

In analogy with semiconductor materials, mod-ern magnetic materials have complex structureswith important functional properties over multiplelength scales, ranging from the atomic to the mac-roscopic level. (See, for example, articles in Ref.[17] and see, for example articles in special issue inRef. [18].) Moreover, experimental and theoreticale!orts have shown that these properties evolvewithin time scales spanning femtoseconds to micro-seconds to hours and beyond. For example, not

only are magnetic phase transitions and moments(both spin and orbital) important parameters, buttheir dynamical behavior at nanometer length andultrashort time scales can determine macroscopicproperties. Yet the underlying physics in manycases remains largely unexplained. Structural com-plexity can take many forms. Nominally homo-geneous binary alloys of magnetic 3d transitionelements can exhibit Invar-like phase transitionslinking structure and magnetism as electron}elec-tron interactions change with composition see Ref.[19] and related articles in the same issue. Manyimportant magnetic materials contain three or fourdi!erent elements in structures with quite large unit


Fig. 3. Schematic illustration of a magnetic memory cell. The cell consists of a tunnel junction in which two ferromagnetic layers (e.g.,cobalt) are separated by an insulator (e.g., Al

2O

3). The tunnel current #owing through the read line senses a resistance that depends on

the relative orientation of the two magnetic layers, i.e., whether they are parallel (1) or antiparallel (0). As in the spin valve shown in Fig.2, the magnetization direction in one of the magnetic layers is pinned by exchange coupling to an antiferromagnet. The magnetizationdirection in the other magnetic layer can be rotated by the magnetic "eld of a current #owing in a nearby write line ("gure courtesy of J.StoK hr IBM Almaden Research Center).

cells, such as NdFeB hard magnets [20] and themanganites that exhibit colossal magnetoresistance(CMR). (For a review, see Ref. [21] and for a reviewof metal}insulator transitions in manganites,see Ref. [22].) As in the related cuprate high-¹

Cmaterials, interesting properties in these complexstructures appear related to anisotropies and in-homogeneities inherent in the atomic arrangementswithin their unit cells. Increasingly common areheterogeneous structures consisting of "nelylayered or granular "lms of similar or di!erentphases that themselves may be simple orcomplex. In these heterogeneous materials, anisot-ropy associated with size or structural aspects ofthe interfaces are likely to control the magneticproperties of interest. For an interesting recentexample of enhanced magnetoresistance in insulat-ing granular systems, see Ref. [23].

In general, the complexities faced in magnetic-materials research are frequently due to coupling ofstructural and electronic degrees of freedom overmany scales. This results in intimate relationshipsbetween the structural, magnetic, and optical prop-erties, which are increasingly utilized for new tech-nologies* generating applications in sectors fromcommunication to sensors to electro-optics to mag-netic recording. The consequences of couplingsbetween spin, charge, and lattice degrees of free-dom are apparent in both inorganic and organicmaterials. Indeed, even opportunities for hybridorganic}inorganic materials and multifunctionaldesigns are now emerging in the literature. Many ofthese strategies are evident in biological materialsand are inspiring exciting new directions in mater-ials design and synthesis of soft and biomimeticmaterials, starting from the molecular level. The


emergence of such materials provides new oppor-tunities for studies that shed light on the funda-mental mechanisms governing spin-polarizedtransport in low-dimensional systems, carrier-me-diated magnetism, and magnetic-"eld-induced car-rier localization. Such spin-engineered materialsserve as model systems to test fundamental ques-tions in quantum transport theory and local mag-netism. For example, recent attempts to bridge the"elds of semiconductor physics and magnetismhave developed the basis for a new class of &spinelectronics' with fascinating technological oppor-tunities [24]. Moreover, a number of theoreticiansare posing scenarios in solids where spin andcharge dynamics may be decoupled, with resultingpredictions of new mechanisms for intrinsic spintransport.

The diversity of interactions underlying magneticphenomena require probes that are sensitive to thelocal atomic and extended electronic structure (in-cluding electron spin), as well as to the geometricstructure, in varied materials and over a wide rangeof spatial and temporal dimensions. Both lateraland vertical (depth) dimensions must be probed.Many experimental probes and techniques, such asmagneto-optics with lasers [25], magnetometry,MoK ssbauer, electron spin resonance, neutron scat-tering, electron microscopy, and computationalmodeling, can currently provide information bear-ing on the questions of interest [17]. In roughly thelast decade, techniques based on synchrotron radi-ation, such as spin-resolved photoemission, X-raymagnetic dichroism, and magnetic X-ray scattering,have been shown to provide unique capabilities forthe study of magnetic phenomena and magneticmaterials. Spin-resolved valence-band photoemis-sion has provided unique insight into the mecha-nism of oscillatory exchange coupling in magneticmultilayers [26,27] and proven the existence ofhalf-metallic ferromagnets [28] where only onespin subband contributes to electron transport.X-ray magnetic circular dichroism (XMCD) spec-troscopy in the hard [29] and soft [30,31] X-rayrange, through quantitative sum-rule separation ofspin and orbital moments [32}35], has given usa clearer picture of the origin of the magnetocrys-talline anisotropy [34,36] and revealed the exist-ence and size of interfacial magnetic moments in

&non-ferromagnetic' metals, such as platinum [37]or copper [38,39], when adjacent to a ferromagnet.Spin-resolved core-level photoemission [40}44]has provided an element-speci"c probe of short-range magnetic order and magnetic phasetransitions near surfaces. Hard X-ray magneticscattering techniques are being extended into thesoft X-ray to probe magnetic structure at nano-meter lengths and above [45}48]. Faraday andKerr magnetooptical rotation techniques commonin near-visible regimes are likewise being extendedinto the soft X-ray, gaining element speci"city andresonant signals much larger than at longerwavelengths [49,50]. Emerging techniques for softX-ray magnetic microscopy [51] utilize several ofthese e!ects for elemental, magnetic contrast. To-gether these capabilities have clear potential toimpact the study of magnetic materials and nanos-tructures in many ways.

2. Research frontiers in magnetic materials andphenomena

While it can be risky to try to identify long-termfuture trends in any scienti"c discipline, the processof doing so sensitizes the community and chal-lenges us to transcend the known and the obvious,so as to go beyond the mainstay incremental ad-vances and suggest potential new paradigms of thefuture.

The broad scienti"c issues that will form thefabric of the future can be categorized in manyways. Several de"ning attributes of magnetic ma-terials cut across any classi"cation scheme and arecentral in current and likely future research. Exam-ples include the spin-polarized electronic structure,magnetic anisotropy, and the phase transitionsthat result in ferromagnets, antiferromagnets, andferrimagnets. Underlying these attributes is theparadigm by which magnetic interactions and phe-nomena can be understood in terms of the geomet-rical and electronic structure that are inseparablylinked in host materials. Table 1 categorizes re-search trends in magnetism emphasizing: (1) theunderlying role of dimensionality [1,52], includingboth spatial and temporal degrees of freedom,(2) the emergence of magnetoelectronics [16],


Table 1Current and future trends in magnetism and magnetic-materialsresearch

Dimensionality*space and timeMagnetic anisotropyDomain walls, domain correlationsMagnetic dynamics/#uctuations/meltingWeakly interacting systemsPhase transitionsQuantum tunneling

MagnetoelectronicsSpin injection and transportQuantum con"nementMagnetic semiconductorsDisorder

Structure and magnetic orderMagnetic anisotropyFrustrationProximity e!ectsDisorderInterfacial e!ects

Exploratory materialsHybrid structures/competing interactions/frustrationActive interfacesBiomagnetsMolecular magnets

including spin injection and transport in hetero-structures that can serve in nonvolatile memory andlogic-element arrays of the future, and magneticsemiconductors that may integrate spin-dependenttransport and magnetooptics into semiconductordevices [53], (3) the broad fundamental materials-science area involving correlation of structure withmagnetic properties [52,54}57], and (4) the excitingarea of exploratory materials. Within each broadcategory are examples of areas perceived to provideopportunities for new fundamental understandingthat may impact the science and technology of mag-netism. In many cases, the same or related topicsappear under di!erent categories, indicating in partthe pervasive role of the essential features of mag-netic materials mentioned above.

2.1. Dimensionality * space and time

In magnetic materials, the spatial dimensions ofinterest range from the intra- and inter-atomic

scale, de"ning how electrons within and betweenatoms interact, to the macroscopic size of thesample as controlled by magnetostatic interactions.Research challenges range over this entire scale,from understanding how spin and orbital momentscontribute to properties in speci"c materials tomicromagnetic modeling of the magnetostatic in-teractions so as to yield both equilibrium proper-ties and switching rates. Time scales of interestrange from femtoseconds (and shorter) relevant toelectronic transitions and electron}electron scatter-ing to the very long times over which materialsretain properties of interest. Switching of domainsoccurs by processes at intermediate time scales inthe nano- and picosecond range. The ability tocontrol spatial dimensions of magnetic features atthe nanometer level, as illustrated in Fig. 1, opensthe study of the dependence of all fundamentalmagnetic interactions on this important new ex-perimental variable. Spatial and temporal dimen-sions are necessarily coupled, and as dimensionsare reduced, the relative importance of di!erentmechanisms for controlling dynamics changes. Forexample, of key importance in future magnetic-storage technologies is the &superparamagneticlimit' at which the inherent magnetic anisotropy ofa small magnetic particle is no longer strongenough compared to thermal energies to yieldstable magnetization over the extended timesneeded in nonvolatile magnetic memory [58]. Theprevalence of buried, ultrathin layers and interfacesin magnetic nanostructures or nanoparticles under-scores the fact that the controlled growth and char-acterization of buried interfaces remains one of thegreat experimental challenges in materials science,as discussed in Section 2.3 below. In many cases,these magnetic nanostructures represent modelsystems for the study of interesting physical phe-nomena, while at the same time having direct re-levance to applications.

2.1.1. AnisotropyMagnetic anisotropy ultimately derives from the

symmetries de"ning magnetic interactions, and re-mains at the heart of current research both fromfundamental and applied materials perspectives.Fig. 4 shows magnetic anisotropy constants fora variety of materials and reveals this link between


Fig. 4. Anisotropy constants for various magnetic materials show trends that reveal correlations between atomic structure, crystalstructure and magnetism. Anisotropy is a fundamental property de"ning the suitability of magnetic materials for di!erent applicationswhose microscopic origin is still being unraveled. Experimental probes that are sensitive to the spin-resolved electronic states underlyinganisotropy in complex materials are needed to explain these trends at a microscopic level ("gure courtesy of D. Weller, IBM, based ondata taken from B.D. Cullity, Introduction to Magnetic Materials, Addison-Westley, Reading, MA, 1972, p. 381, and T. Klemmer et al.,Scripta Metallurgica et Materialia 33 (1995) 1793).

anisotropy energy and symmetry through crystalstructure. Anisotropy is known to have two major,often competing contributions. One is the dipolarcoupling between individual atomic spins, which inpractice leads to a dependence of the anisotropy onthe macroscopic shape of the material (shape an-isotropy). The other is the coupling of the electronic

spin to the lattice via the spin}orbit coupling (mag-netocrystalline anisotropy). Because of the di$cultyin separating various anisotropy contributions andthe small size of typical anisotropy energies(10~4}10~6 eV/atom), the microscopic origins of an-isotropy in speci"c materials generally remain poor-ly understood. The in-plane anisotropy typically


exhibited by thin magnetic "lms has found greatutility in magnetic recording, yet "lms exhibitingperpendicular magnetic anisotropy are highly desir-able for some applications, such as magneto-opticalrecording (see, for example Ref. [59]). Some thin"lms (e.g., a-TbFe, CoPt, and FePt) are found or canbe grown to exhibit perpendicular magnetic anisot-ropy [57], and some nanoscale epitaxial "lms (e.g.,iron "lms) exhibit reorientation transitions from in-plane to out-of-plane magnetization with changinggrowth conditions [60}64].

The ability to control the dimensionality of mag-netic structures opens up the "eld of tuning mag-netic properties by engineering anisotropy. Beloware three examples based on e!ects resulting fromlayered or multilayered "lms in which the thicknessof individual layers is of order 1}10 nm [65].

f Nonmagnetic materials (e.g., gold, platinum, andpalladium) interleaved between soft ferromag-nets (e.g., cobalt) yield new composites withunique magneto-optic properties, namely per-pendicular magnetic anisotropy [66,67]. Under-standing the origin of the magnetic anisotropyand magneto-optic response requires a ratherdetailed understanding of the interfacial regionin terms of possible intermixing and topologicalroughness, in addition to the inherent asym-metry imposed by the interface.

f When a ferromagnet is grown in a magnetic "eldon top of an antiferromagnet, the magnetizationdirection in the ferromagnet becomes unidirec-tionally pinned [68] (for a recent review see Ref.[69]). This phenomenon of anisotropic exchangebiasing is not understood, despite the fact that itis already utilized in the manufacturing of mag-netic recording heads. Understanding the mech-anisms of exchange biasing again requires thedetailed magnetic and structural characteriza-tion of buried layers and interfaces between thetwo materials.

f New permanent-magnet nanostructures can po-tentially be tailored with maximum energy prod-ucts exceeding that of NdFeB by factors of twoor more [70,71]. These exchange spring magnetscomprise hard, permanent-magnet layers contri-buting high anisotropy and coercivity, owing tothe presence of a rare-earth component, and soft

layers, which could be as simple as elementaliron, contributing a high magnetization per unitvolume. The resultant coupling ideally yieldshigh magnetization and coercivity. After mag-netization, a reversed applied "eld in#uences thesoft component "rst, but turning o! such a "eldallows the system to spring back to the originalstate of full remanence, hence the term &spring'magnets to denote these remarkable, newly in-troduced structures. The physics of the couplingacross coherent interfaces, the spin dynamics,and switching processes o!er new challenges toand rewards for understanding the energetics ofmagnetic-domain walls in a fundamental way.The layered nanostructures provide model sys-tems [72] to test concepts that may open majorvistas for the miniaturization of electrical motorsand the bene"t of energy e$ciency both in elec-tricity and fossil-fuel utilization. The latter is dueto the preponderance of electrical motors in au-tomotive and other transportation vehicles.

Similar strategies of combining high-anisotropy,hard magnets with soft magnetic layers are relevantin the quest to overcome the superparamagneticlimit in conventional magnetic-recording media,as discussed in more detail in Section 2.1.3 below.Thus, the concept of engineering anisotropythrough judicious choice of magnetic nanostruc-tures is creating a paradigm shift in our thinkingabout diverse magnetic applications that a!ect so-ciety in fundamental ways.

2.1.2. Domain walls/domain correlationsDomain walls themselves are systems of reduced

dimensionality that are rich in physics yet exceed-ingly di$cult to characterize on a microscopicscale. As layer thickness and lateral dimensions ofnanostructures decrease below the length scalesassociated with domain walls, dimensional e!ectsalter domain and domain-wall con"gurations. Un-derstanding the changing energetics, structure,pinning, and dynamics of domain walls in systemswith reduced dimensionality is important, and isespecially challenging in buried layers of nano-structures. In particular, there are a number ofoutstanding issues concerning the spatio-temporalpro"le of a moving domain wall in real materials,


where nonlinear e!ects become important near theWalker limit, the magnetostatic mode associatedwith the shape of the magnetic domain and thespeed of sound. While classical mechanisms, suchas wall}wall interactions, need to be understoodand controlled for technology, a number of funda-mental physics questions on the macroscopic quan-tum tunneling of domain walls and correlationmeasurements need to be addressed, with their an-swers possibly providing solutions to existing limitsof high-density information storage.

The spatial correlation of magnetic domainswithin and between the magnetic layers of layeredstructures is of fundamental importance for manyapplications of spin-conductance devices, whetherfor magnetoresistive, spin-tunneling, or spin-tran-sistor operation. For systems where magnetic-domain sizes are large compared to the relevantspin-#ip path length, it is the orientation of thelocal relative magnetic moment that de"nes thelocal and global conductance behavior [73]. If do-main sizes are comparable to or smaller than thispath length, more complex behavior is expected.For a meaningful comparison between the mea-sured and calculated spin conductance, a quantitat-ive description of the applied-magnetic-"eld de-pendence of these magnetic-domain correlations isrequired. Experimental measurements of domaincorrelations within these layered structures area great challenge and generally require develop-ment of new measurement techniques with spatialand chemical sensitivity at the appropriate lengthscales. Advances in micromagnetic modeling ofthese complex structures also require reliable ex-perimental data.

2.1.3. Magnetic dynamics/yuctuations/meltingMagnetic dynamics includes both magneti-

zation-reversal processes (whether dynamic oradiabatic) and important issues associated withnoise inherent in magnetic devices. Fluctuationscan be associated with the reversal process and alsowith magnetic phase transitions. Since all magneticorder results from temperature-dependent phasetransitions, the term &melting' applies very broadlyto the loss of order with temperature. Anotherimportant distinction is that between long-rangemagnetic order (e.g., as manifested in macroscopic

susceptibility) and short-range magnetic order (asnow thought to be important in such phenomenaas high-temperature superconductivity and CMR).All of these fundamental dynamic properties inturn depend on the dimensionality of systems ofinterest, and so they present continued opportuni-ties for fruitful research in the context of nanostruc-tures. For example, in a system with nanometerdimensions, there is a merging of long- and short-range order. Some speci"c questions associatedwith the picosecond time scales for such processesare considered below.

Traditionally, magnetization processes havebeen studied only at the nanosecond time scale orlonger. Studies at shorter time scales have beenprecluded by induction, which limits both the speedwith which one can increase the magnetic "eld andthe speed with which one can measure the changein magnetization by an induced current in a pick-up coil. With the advent of powerful pulsed lightsources, such as lasers and synchrotrons, magneti-zation can now be measured either by magnetoop-tic e!ects or by observing the spin polarization ofelectrons ejected from the magnetic material ona time scale that is limited only by the length of thelight pulse.

The femtosecond time scale is important in thestudy of electron}electron scattering [74] and ispresently reserved for fast pulsed lasers [75,76].The picosecond time scale, however, should be ac-cessible with the pulsed light emitted from synchro-trons, and there are important reasons to studymagnetism on this time scale. One is to observedirectly the postulated spin blocks formed at themagnetic transition in two-dimensional objects,such as thin "lms. Such spin blocks are spontan-eously magnetized regions with a diameter of mi-crons or less that form and dissolve on a time scaleof picoseconds. It has not yet been possible toobserve such spin blocks directly, but indications oftheir presence appear mostly in neutron-scatteringexperiments, where they are seen as enhancedscattering (critical scattering). Picosecond mag-netization measurements (if possible with ele-ment sensitivity) would make it possible to directlydetermine the magnitude of the spontaneousmagnetization in the spin blocks. More gener-ally, picosecond studies of the motion of the


magnetization vector when external conditions,such as applied magnetic "eld, temperature, orpressure are changed, are of interest.

Of particular interest is the dynamics of magne-tization reversal [77], because it is the process bywhich magnetic bits are written into recording me-dia. The industry needs to reduce the time forwriting a bit from its present value of 10 to 1 ns.Experimental studies of magnetization reversal aregenerally not in agreement with the familiar modelof coherent spin rotation. Instead, complex pro-cesses, such as curling and buckling, are observed[78]. However, if the magnetic-"eld excitation oc-curs at an angle to the magnetization direction andon much shorter time scales in the picosecondrange, such complexity is suppressed and reversalshould simply follow the model of coherent spinrotation. The time for the elementary process ofmagnetization reversal is given by the Larmor pre-cession in the anisotropy "eld and lies in thepicosecond regime. Therefore, "eld pulses of a fewpicoseconds duration with peak amplitudes up to2 T are required to study magnetization reversal.

So far, such "eld pulses can only be produced byelectron bunches of high current density passingthrough the sample. The remanent magnetizationpattern generated in a premagnetized sample by the"eld pulse can be imaged by magnetic-microscopytechniques. The results show that the application of"eld pulses as short as a few picoseconds producecoherent rotation of the magnetization. The pro-cess of magnetic recording can thus be made fasterby several orders of magnitude [79]. The compari-son of the observed magnetization pattern witha micromagnetic model yields information on thedamping behavior after application of the "eldpulse. The study of the damping and its under-standing on a quantum-mechanical level is a pres-ently unexplored "eld of crucial importance forincreasing the data rates in magnetic recording.

2.1.4. Weakly interacting systemsAs with all electronic interactions, the correla-

tions among spin-polarized states depend on di-mensionality. Low-dimensional systems containingone- and two-dimensional magnetic states exhibitexciting properties of both theoretical and poten-tially practical interest. Especially interesting are

states consisting of half-integer spins. Long ago,Bethe demonstrated that the spin-1

2Heisenberg

chain would not order because of the presence ofquantum #uctuations [80]. More recently, Hal-dane has conjectured that half-integer spin chainspossess an excitation spectrum with an energygap [81], whereas integer spin chains are gapless.Similar fascinating properties are shown by two-dimensional systems, such as the S"1

2square cop-

per}oxygen layers that are present in high-¹#

superconductors. An intermediate case is represent-ed by spin ladders, which are arrays of coupledchains [82]. More complex systems involve quan-tum chains and quantum ladders. The static anddynamical properties of such systems are of funda-mental interest because they may relate to theoccurrence of superconductivity and colossalmagnetoresistance.

Fundamental physics and potentially signi"cantpractical applications are joined in the naturallylayered magnetic structures, represented by thecolossal magnetoresistive (CMR) manganites ofthe Ruddlesden}Popper phases SrO(LaMnO

3)/,

where SrO represents a blocking layer that separ-ates n layers of perovskite-based octahedra [83,84].The trivalent lanthanum sites are partiallysubstitutionally "lled by a divalent species, such asstrontium, to realize a mixed-valent state in manga-nese that is characteristic of double-exchange fer-romagnets. Even though double exchange is analmost 50-year-old concept [85], the resurgence ofinterest in these materials is due to their beingcandidates for magnetoresistance materials, beinghalf-metallic ferromagnets with possible 100%spin-polarized carriers, and to their forming com-patible junctions with high-¹

#cuprate supercon-

ductors [86]. These phases are analogous to thoseencountered in the high-¹

#cuprates, as well as in

the exotic ruthenate materials [87]. Research op-portunities include the manipulation of dimen-sionality and structural anisotropy to (1) tailor themagnetotransport properties and (2) to explore theunderlying physics of double-exchange, polarons,short-range magnetic order above the Curie tem-perature, the competition with antiferromagneticsuperexchange inter-actions, and the charge-and orbital-ordering states that occur at specialcompositions. Characterization of these complex


materials demands techniques that can identify lo-cal structure (and occupancy) [88], valence, mag-netic moment, and magnetic anisotropy for eachconstituent with element and site speci"city. Inshort, what is needed is a complete atomic, elec-tronic, and magnetic structure description that in-cludes each constituent in a structure which maycontain 10}100 atoms per unit cell. While this maybe a daunting goal, many new spectroscopic andstructural techniques will be employed to elucidatethe physics of these materials. It is a challenge to thecollective imagination of the research community toreach the goal of fully understanding these wonder-fully complex and potentially very useful materials.

2.1.5. Phase transitionsAs with all phase transitions, magnetic phase

transitions have a pronounced dependence on di-mensionality. Magnetic nanostructures containingmore than one distinct magnetic component bringboth dimensionality and competing interactions to-gether in the study of magnetic phase transitions.Distinguishing between the responses of di!erentconstituents in these phase transitions presents ex-perimental challenges. It is likely that pulsed syn-chrotron radiation may be used to drive phasetransitions in magnetic materials as a means ofprobing magnetic behavior and local interactionswith unprecedented resolution. This class ofnonequilibrium dynamics is largely unexplored,and the ability to induce transitions in magneticmoment with chemical speci"city o!ers a uniqueopportunity to examine this behavior in low-di-mensional materials. The possible role of quantum#uctuations and tunneling in phase transitionsis an emerging area that appears rich in futureopportunity.

2.1.6. Quantum tunnelingThe process of miniaturizing magnetic materials

has unexpectedly revealed fascinating new intrinsicclassical and quantum-mechanical phenomena.Even the simplest magnetic system, the isolatedsingle-domain particle, exhibits a wealth of exoticbehavior that pushes us to the limits of our presentunderstanding of the fundamentals of magnetism.The simple picture of classical magnetism suggeststhat the magnetic orientation of a small particle will

remain stable in one of two orientations inde"nitelybelow the blocking temperature, de"ned as thetemperature at which thermal #uctuations are ener-getic enough to overcome the volume anisotropyenergy. However, quantum mechanics tells us thatthe states in the two potential wells can be coupledby tunneling, thereby leading to strikingly di!erentdynamics. Several recent experiments have demon-strated the presence of these quantum dynamics insmall systems and raised tantalizing questionsranging from the fundamental limits of informationstorage to the observation of macroscopic quantumphenomena [89]. One-dimensional barrier pen-etration is a misleadingly oversimpli"ed depictionof quantum tunneling of magnetization* there isno simple SchroK dinger equation that describes thisprocess, since it is not an elementary particle that istunneling, but a collective coordinate, the magnet-ization direction of a collection of spins. The area ofmagnetic quantum tunneling o!ers exciting newtheoretical and experimental opportunities for re-search including the role of dissipation in quantummagnetic phenomena and quantum-measurementtheory.

It might appear that this process points to limita-tions and restrictions on the use of small magneticparticles for the storage of information, since theswitching of magnetic domains depends on a myr-iad of detailed features of the particles, and quan-tum e!ects ultimately limit the length of time thata magnetic bit can remain stable (e.g., through thesuperparamagnetic limit). Nevertheless, it seemsequally plausible that these investigations will pro-vide fundamentally new ways of using magneticstructures in technology. For example, theoreticalinvestigations of magnetic quantum tunneling ledto the surprising discovery that a selection rulequite generally forbids quantum tunneling for par-ticles with an odd number of electrons! [90]. Thus,limitations imposed by quantum mechanics may beovercome. Another area for new research concernsthe use of magnetic systems not for memory, but forlogic. Some computational problems can be greatlyaccelerated in a &quantum computer' in which bitsand gates are implemented at the level of individualspins, thereby permitting calculations that exploitquantum interference e!ects within the computeritself [91].


2.2. Magnetoelectronics

During the past few years, quantum electronicsand micromagnetics have begun converging to-wards a new "eld known as magnetoelectronics or&spintronics', which focuses on low-dimensionalelectronic systems that display magnetically driven,spin-dependent phenomena [16]. On the one hand,quantum electronics has successfully exploitednanofabrication techniques to establish quantizedenergy levels, and charge manipulation betweenstates produces a variety of electronic and opticaldevices. In parallel, research in micromagnetismhas used miniaturization to produce submicronferromagnetic structures whose switching, stability,and transport properties are controlled to createmagnetic-storage and reading media. The mergingof these two areas of research is giving rise tomagneto-electronic phenomena where the spin ofquantum-con"ned charge carriers is controlled us-ing local magnetic "elds. While there are severalrecent examples of systems where magnetic nano-structures are used in conjunction with semicon-ductor and metallic heterostructures to directelectron #ow, the integration of magnetic and elec-tronic quantum structures also o!ers opportunitiesto probe qualitatively new physics by obtainingadditional dynamical information about the fullquantum-mechanical nature of electronic states inreduced geometries. A fundamental understandingof incoherent and coherent electronic processes innanometer-scale geometries will play an importantrole in the development of future technologies thatrely on the quantum-mechanical control of elec-tronic states. The eventual goal of such studies is toestablish, store, and manipulate the coherence ofelectronic and magnetic spins in solid-state systems[92]. In order to attain this level of control, it iscritical to develop an understanding of the micro-scopic mechanisms underlying spin injection,transport, and collection in practical materialsystems.

2.2.1. Spin injection and transportSince the early part of this decade magnetoresis-

tive "lms have served as read transducers to pro-vide higher performance than their inductive coilpredecessors in magnetic-recording technology.

However, the rapid pace of the disk drive industry,with magnetic bit areal densities increasing ata compound growth rate of about 60% per year,has already outdated these traditional MR sensorswhich cannot provide enough read signal for arealdensities greater than &5 Gbit/in2. Currently, thedisk drive industry is undergoing a revolution withthe rapid introduction of novel GMR sensors,which can provide orders of magnitude more signalthan conventional MR sensors. While the promiseof GMR is high there are signi"cant risks involvedwith the introduction of such a major new techno-logy. Of particular concern is the long-term mag-netic and structural stability of these novelstructures within the relatively harsh environmentof a disk drive enclosure.

The #ux from the transitions between magneticbits in conventional magnetic recording disk drivesgives rise to a "eld at the read sensor of the order ofa few tens of Oersteds. Thus, even though GMRmultilayers, such as those formed from alternatinglayers of Co and Cu, each a few atomic layersthick, can exhibit MR values at room temperatureexceeding 70% [93], these structures cannot beused for disk drive MR read sensors. Such multi-layers require large magnetic "elds of order&10}30 kOe to saturate their resistance becausethe interlayer antiferromagnetic exchange couplingprovided by the ultrathin Cu layers is very large.Whilst it is possible to reduce the saturation "eld byusing thicker Cu layers, it is preferable to usea spin-valve GMR structure which exhibits stillsigni"cant changes in resistance (&10%) but atmagnetic "elds many orders of magnitude lowerthan GMR multilayered structures. The simplestspin valve structure comprises a soft ferromagneticlayer separated from a second harder ferromagneticlayer by a metallic spacer layer, typically copper.The resistance of the structure is changed when themagnetization of the soft layer is a!ected by ex-ternal "elds such as the #ux from transitions ina magnetic recording medium (see Fig. 12). Thedetailed origin of the GMR e!ect in such structureshas been the subject of much debate. An importantquestion concerning the origin of the spin-depen-dent scattering processes which leads to GMR isnow resolved in favor of a dominant contribu-tion arising from spin-dependent scattering at the


interfaces between the ferromagnetic and metallicspacer layers [94]. The most sensitive spin valveMR sensors take advantage of this perhaps surpris-ing result by doping the interfaces with thin mag-netic layers, just a few atomic layers thick, whichgive rise to strong spin-dependent scattering. It isnot surprising that the GMR exhibited by a spin-valve is very sensitive to details of its structure,which can be strongly in#uenced by the depositionmethod used to form the structure, and by theprocessing techniques used to de"ne the MR sensoritself.

A second structure similar in construction to thespin-valve but whose MR arises from a quite di!er-ent mechanism is the magnetic tunnel junction(MTJ). In an MTJ a thin insulating layer throughwhich the electrons tunnel separates two ferromag-netic layers: the tunneling conductance of the junc-tion depends on the relative orientation of the twoferromagnetic layers. MTJ, a curiosity for morethan 20 years since its "rst discovery in 1977[95,96] has become the focus of intense interest inrecent years since large room temperatue MR ef-fects were found in such structures ranging from10}18% [97,98] to more than 40% [99]. Themechanism of MR in an MTJ clearly is related tothe extent to which the tunneling current is spinpolarized. However, there are many remainingpuzzles including, for example, the sign of the spinpolarization of the tunneling electrons, and therelationship of the magnitude of the spin polariza-tion to the magnetization and the detailed elec-tronic structure of the ferromagnetic electrodes.Just as for GMR, the details of the structure of theinterfaces between the ferromagnetic electrodesand, in this case, the tunnel barrier, are critical todetermining not only the magnitude of the MR butalso the bias voltage and temperature dependenceof the MR.

Another important potential application of bothGMR and MTJ structures is the use of such struc-tures to form non-volatile magnetic memory cells(see Fig. 3b). For this application the MTJ or spin-valve devices are engineered to have two stablemagnetic states in small magnetic "elds. Typically,these states will correspond to parallel or anti-parallel alignment of the ferromagnetic layersin the sandwich. Note that a spin-valve MR read

head sensor is, by contrast, designed to have anonhysteretic magnetic structure which respondsapproximately linearly to small applied "elds. Un-derstanding and controlling the magnetic responseof these devices in both the spatial and temporaldomains is a formidable challenge, particularly asthe size of such devices decreases to deep sub-micron dimensions. The science of spin-dependenttransport in GMR and MTJ structures is fascinat-ing but progress in the detailed understanding ofthese phenomena, and in their technological ap-plications, depends on a detailed characterizationof their physical, electronic and magnetic structureboth in unpatterned and in patterned structures.

The recent developments in GMR and MTJ ma-terials has led to intensi"ed interest in the possibil-ity of building useful nonvolatile magnetic randomaccess memory (MRAM). While there is stronginterest in such nonvolatile magnetic memoriesfor defense and aerospace applications, there isalso great commercial potential for a nonvolatileMRAM technology that can compete, in perfor-mance, with existing memory technologies. In 1995,the defense advanced research projects agency(DARPA) funded three industry-led consortia todevelop competing magnetic-memory technologies.Of these consortia, those led by Honeywell andMotorola have proposed using GMR elements andthat led by IBM has proposed using memory ele-ments comprised of magnetic tunnel junctions.While the latter give much higher magnetoresis-tance values ('40% at room temperature) thanGMR structures of similar complexity, a concern isthe control and reliability of the very thin tunnelbarrier layers required. The success of any of theseconsortia depends on developing a magnetic mem-ory technology that has excellent properties (shortread and write times, low power, integration withstandard complementary metal-oxide-semiconduc-tor (CMOS) electronics) and scalability to ultra-high densities. For both GMR and MTJ elements,controlling the structure, especially the size andshape, and the micromagnetics of sub-micron-sizedelements are critical issues that advanced micro-scopy techniques, now under development at theALS, should help to address. Similarly, the thermalstability of these elements and their compatibilitywith standard CMOS processes is important. As


discussed above the interfaces between the fer-romagnetic and the nonferromagnetic spacer layers(metallic for GMR and insulating for magnetic tun-nel junctions) are critical to the properties exhibitedby these devices. Advanced element-speci"c charac-terization techniques at the ALS should be helpfulin understanding materials, processing, and relia-bility issues.

2.2.2. Quantum conxnementQuantum con"nement of electrons takes on the

added dimension of spin in magnetic nanostruc-tures. Di!erent spins experience di!erent potentialsand hence di!erent degrees of con"nement. Dimen-sionality is again important, and many types ofstructures and con"nement can be envisaged. Themagnetoresistive and interlayer coupling propertiesof layered GMR structures are one clear example ofthis type.

One of the last open basic questions in the phys-ics of the GMR exhibited by layered structures isunderstanding the origin of the magnetic couplingin terms of momentum-resolved electronic struc-ture. In particular, the &long-period' oscillation ofiron/chromium layers remains a controversial issuewith multiple contradictory theoretical predictions[100,101] and partial experimental "ndings [102].Angle-resolved photoemission studies to monitorthe emergence of spin-polarized quantum-wellstates across the Fermi energy as a function of thethickness of the spacer medium (e.g., chromium) areneeded to solve this problem. Many transition-metal spacers (chromium, niobium, molybdenum)yield provocative predictions, and the band struc-ture of transition metals yields Fermi surfaces thatare complex and rich in possibilities. It is thusa major challenge to move beyond understandingthe electronic structural origins of the couplings innoble metals (copper, silver, gold) to these morecomplicated structures that will require experi-mental probes capable of momentum and spin-resolved detection of electrons involved in thequantum-well states at surfaces. Probing spin-po-larized quantum-well states in buried layers poseseven more experimental challenges. And "nally,being able to more directly probe the degree ofinterface mixing and the atomic and magneticstructures of atoms near the interfaces that are

crucial to the GMR e!ect (as well as other magneticnanostructures) is a key area of future experimentalneed.

2.2.3. Magnetic semiconductorsA very appealing class of materials with unex-

plored spin dynamics is provided by semiconductornanostuctures doped with magnetic moments[103]. These systems provide a unique nexus be-tween low-dimensional magnetism and semicon-ductor quantum con"nement. By tailoring thequantum-con"ning potential, one can systemati-cally control the overlap between quantized elec-tronic states and the local moments, while themagnetic environment generated by the local mo-ments may be varied through factors such as strain,dilution, and dimensionality. The exchange interac-tion between the electronic band states and thelocal moments leads to a greatly enhanced spinsplitting of the con"ned carriers in the presence ofan applied magnetic "eld, resulting in spin-depen-dent and magnetically tunable con"ning potentials.Since the various interactions involved (e.g., thed}d and sp}d exchange) are well characterizedbecause of the extensive work in both bulk mag-netic-semiconductor crystals and magnetic-semiconductor heterostructures, these magneticnanostructures provide clean model systems forbasic studies of electron-spin dynamics in low di-mensions. Furthermore, these materials have excel-lent optical properties that are characterized bysmall inhomogeneous line widths and well-de"nedexcitonic resonances. Hence, the development ofpowerful probes of spin dynamics and magneticinteractions has been possible recently using state-of-the-art femtosecond-laser magnetooptical tech-niques [53].

Within this "eld during the past few years, therehave been remarkable advances in materialsscience that are fueling a resurgence of activity.Recently, it has been shown that one may fabricate&digital magnetic heterostructures' in which interac-tions between localized magnetic spins and theirwavefunction-overlap with quantized electronicstates is tuned through a controlled distributionof two-dimensional magnetic layers [104]. Theseengineered planar structures reduce clustering ofthe magnetic moments, thereby resulting in an


enhanced paramagnetism, allow a large magneto-electronic overlap, and display qualitatively andquantitatively di!erent dynamical interactions ascompared to bulk alloys. In addition, it has nowbecome possible to electronically dope these struc-tures with either n-type or p-type dopants, repres-enting a marked advance in potential applicationsof these materials. As in traditional semiconductorphysics, the formation of a two-dimensional elec-tron gas in modulation-doped nanostructures hasproven to be a mainstay of contemporary interestsin both condensed matter physics and quantumdevice physics. The recent fabrication of magnetictwo-dimensional electron gases has opened a newmodel system, in which a two-dimensional popula-tion of electrons interacts ferromagnetically withlocal moments, in materials readily accessible toquantum transport and magnetooptical studies[105]. Measurements have shown that such a mag-netic two-dimensional electron gas is easily spin-polarized in modest applied "elds, even at large"lling fractions, since the spin splitting at cryogenictemperatures is much greater than the Landau-level splitting. Furthermore, the last few years haveseen the creation of semiconductor nanostructureswith dimensions less than two. Zero-dimensionalquantum dots have attracted substantial attentionbecause of recent successes with in situ fabricationof defect-free, self-assembled ensembles of strainedsemiconductor quantum dots with surprisinglyhigh quantum e$ciencies and relatively narrowsize distributions [106].

These scienti"c opportunities in magneticsemiconductors have helped stimulate new fabrica-tion and nonequilibrium growth techniques aimedat producing truly integrated magnetoelectronics,with nothing short of spectacular results. A numberof remarkable discoveries suggest that detailedstructural studies may have a profound impact inthis area. For example, it has been historicallydi$cult to produce magnetic semiconductors usingtechnologically common semiconductors; more-over, the introduction of magnetic moments, inthese materials has typically resulted in Heisenbergantiferromagnetic interactions, thus limiting thesesystems to behaving as heavily diluted para-magnetic media. However, recently it has beendemonstrated that nonequilibrium molecular-

beam-epitaxy (MBE) growth techniques may beused to incorporate Mn2` ions substitutionallyinto GaAs semiconductors, where they also act asacceptors or p-type dopants. In contrast to all the-oretical predictions, this combination of events hasproduced the "rst ferromagnetic semiconductorgrown by MBE that may be used to constructnanometer-scale ferromagnetic devices [107]. Thisrevolutionary discovery has rapidly led to demon-strations of spin-polarized resonant-tunnelingdiodes and the genuine possibility of a new familyof spin-injection devices based on semiconductortechnology. The advent of highly magnetoresistiveferromagnetic materials that are compatible withthe semiconductor industry has profound eco-nomic implications. There is a great deal of fascin-ating scienti"c work to do in understanding themechanism driving ferromagnetism withina semiconductor host and understanding in generalhow the electronic structure of wide-gap semicon-ductors may be tuned to modify exchange interac-tions in solids. This knowledge may subsequentlybe used to generate new families of arti"cially en-gineered band gap systems that incorporate fer-romagnetic media for science and technology.

2.2.4. DisorderThe e!ects of disorder and defects on transport

properties in conventional metals and semiconduc-tors are quite important in practice and hence fairlywell established. The e!ects of magnetic disorder onspin-dependent transport are less well understood,although likely to be just as important in mag-netoelectronic applications and more complex innature. Disorder can take many forms in magneticmaterials, including the usual chemical and struc-tural disorder but now with local magnetic proper-ties varying site by site [108,109]. Simple examplesof this magnetic disorder are nanoscale chem-ical/magnetic clustering in metallic alloys thatintroduce spin-dependent scattering centers andincrease magnetoresistance over that of a nominal-ly homogeneous alloy. Interfaces and domain wallsare themselves forms of disorder that can signi"-cantly in#uence magnetotransport, as mentionedabove [110,111].

Recently, it has become clear that the presence ofdisorder may be bene"cial in certain classes of


materials. For example, the presence of manganeseions introduced through nonequilibrium-growthtechniques into the III}V semiconductor GaAs hasproduced ferromagnetic GaMnAs. In this example,the addition of magnetic moments into the latticeresults in a ferromagnetic moment, p-type doping,and potential disorder. The combination of theselatter two attributes result in a new band structurethat generates a long-range spin interaction andsubsequent ferromagnetism in a MBE-grownsemiconductor host. While the applications of thesenew materials are numerous, ranging from mag-netically tunable resonant tunneling diodes to mag-netic "eld-e!ect transistors, the fundamentalmechanism for ferromagnetic exchange remainspoorly understood. Resonant tunneling diodesbased on GaAs/ErAs, where magnetization-con-trolled quantum transport has been demonstratedin a materials system with highly mismatched inter-faces and dislocations, are another outstandingexample [112]. In addition, strong optically activematerials, such as GaN, have "ve to six ordersof magnitude higher defect density than today'scommercial semiconductors, yet are amongstthe most promising materials for blue lasers anddisplay technologies [113]. It is expected that therole of disorder and its e!ect on micromagneticsand new hybrid magnetic-semiconductor materialsmay be uniquely explored with synchrotron radi-ation.

2.3. Structure and magnetic order

Textbooks illustrate the interdependence of in-teratomic structure, electronic structure, and mag-netic order in the bulk of prototypical ionic anditinerant ferromagnets, ferrimagnets, and antifer-romagnets. The foundations provided by modelsof these prototypical systems provide startingpoints for understanding the increasingly complexmaterials of current and likely future interest.Experimental techniques capable of providingelement-resolved information about local structure,electronic structure, magnetic moments, and mag-netic structure at relevant length scales (e.g., short-range order versus long-range order over a con-tinuum of lengths) are essential for future studies ofmagnetic materials.

2.3.1. Magnetic anisotropyAs discussed in Section 2.1.1, magnetic anisot-

ropy is linked to structural anisotropy, and themechanisms linking structural to magnetic aniso-tropies can di!er from material to material. Smallstrains, surface or interface e!ects, texturing, andgrowth in applied "elds are examples of e!ectsknown to in#uence anisotropies in speci"cmaterials.

2.3.2. FrustrationThe role of structure in quenching moments and

frustrating spin alignments has been well studied inmany bulk materials, and now it must be extendedinto magnetic nanostructures where the e!ects ofsurfaces and interfaces must be understood. Aninteresting example is the interface between fer-romagnets and antiferromagnets involved in ex-change biasing [68,69], as discussed in Section 2.1.1and used in some GMR structures [114] such asthe prototypical iron/chromium multilayer system.In each case, the issues revolve around spin frustra-tion in the interfacial region. In the iron/chromiumcase, it is also of interest to understand how thespin-density waves in the chromium are a!ected bythe con"ned geometries and how placement ofnodes in the spin-density waves in these regionsmay minimize the frustration energy by limiting themagnitude of the chromium moments in the inter-facial region [115]. Neutron scattering has servedas a prime tool to explore the interfacial physics inthese structures [116], but VUV/soft X-ray scatter-ing and spectroscopic measurements with third-generation synchrotron radiation should provideadditional information. The physics of noncollinearspin structures in magnetic heterostructures repres-ents a general area of ongoing research that is richin opportunities to be explored by element-speci"c,spin-polarized spectroscopies and spectromicro-scopies.

2.3.3. Proximity ewects/induced magnetismMany basic- and applied-research problems re-

quire the use of magnetic materials in contact withother magnetic, nonmagnetic, superconducting,semiconducting, or insulating materials. By anal-ogy with the superconducting proximity e!ect, itis natural to question if there exists a magnetic


proximity e!ect whereby the magnetic properties ofone material in#uence the properties of the other[117}119]. Many di!erent issues must be distin-guished in considering this question. For example,interdi!usion, reaction, and roughness at interfacescan have important physical consequences, some ofwhich might include induced moments in nominal-ly nonmagnetic atoms through hybridization ofelectronic states [120]. Such induced momentshave been observed with element-speci"c synchro-tron techniques and can be thought of as one typeof proximity e!ect mediated by overlap of elec-tronic states [37}39]. Such induced moments raiseinteresting questions about itinerant magnetismbut are distinct from a more general notion ofproximity e!ects. Thus, it is important to under-stand whether the magnetism in one material cancreate and a!ect magnetism in another materialthat is in close physical proximity. Simple mean-"eld theory predicts the presence of this type ofe!ect in any system undergoing a second-orderphase transition and was the subject of some theor-etical research 15}20 years ago. Layered magneticsystems are again candidates for the investigationof such e!ects, which in addition require sensitiveexperimental probes capable of isolating the mag-netic responses of individual layers and constituentelements. Indeed, element-speci"c XMCD has re-cently been used to study the e!ect on the Curietemperature of two di!erent magnetic layers separ-ated by an ultra-thin nonmagnetic layer [121]. Itwould be interesting to investigate the dependenceof proximity e!ects on their coherence lengths, tem-perature, and other experimental parameters. Suche!ects would clearly impact the study of all mag-netic nanostructures, and they could be particularlyuseful in device applications if they could be ma-nipulated or controlled.

2.3.4. Interfacial ewectsWith the rapid trend towards ever-smaller mag-

netic nanostructures, the characterization of theinterfaces inherent in these structures becomes in-creasingly important [122]. Real interfaces can al-ways exhibit imperfections and roughness, even ifthere are examples of atomically smooth interfacesbetween certain pairs of materials as grown in care-fully controlled, often UHV, environments. Rough-

ness can be categorized in many ways, but oneuseful classi"cation is based on spatial frequencies.Roughness at the highest (atomic-scale) frequenciescorresponds to intermixing between atoms at theinterface; it could result from growth-induced(knock-on) mixing, interdi!usion during or aftergrowth, or chemical driving forces leading to com-pound formation at the interface. At progressivelylower spatial frequencies, roughness can be de-scribed as topological and characterized by heightvariations of a distinct interface and their correla-tion with in-plane distance. A variety of magneticphenomena are a!ected by interfacial roughness,and indeed magnetic roughness can be de"ned asthe variation in magnetic properties over this samerange of spatial frequencies. Early neurtron scatter-ing [108,109], and recent soft X-ray magnetic-scat-tering measurements [46,48], suggest that chemicaland magnetic roughness at interfaces are not neces-sarily the same. The characterization of these vari-ous types of roughness at real (buried) interfaces ischallenging, and it will become increasingly im-portant both for fundamental understanding andbecause these imperfections may directly in#uencemagnetotransport and other properties of interestin device applications. Techniques capable of quan-tifying magnetic roughness and magnetic correla-tions in heteromagnetic structures will thereforelikewise become increasingly important.

In the areas of surface, monolayer, and thin-"lmmagnetism, the last decade was driven by the questto verify theoretical predictions of enhanced mag-netic moments [123,124], reduced or increasedmagnetic transition temperatures, and enhancedand/or perpendicular anisotropies. We anticipatethat the next decade will involve further explora-tions of the role of unique magnetic surface andinterface states in underlying the phenomenon ofsurface-altered magnetic order. Exchange biasingof a ferromagnetic layer by an antiferromagneticlayer is one area where interfacial magnetic statesappear to control important properties [125]. Fun-damental issues will also include the elucidation inmodel systems of the collapse of the exchange split-ting above the Curie transition in itinerant fer-romagnets as a function of k-vector. Experimentaltechniques capable of providing spin-polarizedelectronic structural information, as well as


geometrical structural information, are valuable inthis area. The surface magnetooptic Kerr e!ect,valence and core photoemission, X-ray absorption,X-ray emission, and neutron scattering have pro-vided valuable input to date in these areas, and weexpect future developments to enhance most ofthese techniques.

2.4. Exploratory materials

In this section, we consider some novel magneticmaterials that are currently beginning to be ex-plored. Although perhaps not immediately amen-able to technological application, these materialsprovide new and exciting ways to look at magnet-ism in the future, as well as additional fundamentalquestions to be explored.

2.4.1. Hybrid structures/competing interactions/frustrations

All of the magnetic nanostructures discussedabove can be thought of as hybrid structures. Struc-tures containing normal-metal, antiferromagnetic-metal, and insulating-spacer layers have alreadybeen mentioned. Superconducting interlayers alsosuggest new concepts for spin-injection, all-metallicspin-transistors, and novel magnetoresistive mem-ory elements. For example, a new type of super-conductive coupling across ferromagnetic layersinvolves a change in the phase of the order para-meter by 1803 (p) across the ferromagnet [126,127].The phase shift oscillates, according to theoreticalprediction, as a function of the thickness of theferromagnetic layer, from its ordinary value of 0 toits &exotic' value of p, and physical properties, suchas the superconductive transition temperature,critical "eld, and critical current are all predicted tobe nonmonotonic functions of the thickness of theferromagnetic spacer [128]. This &p-phase' super-conductivity is a challenge to create in the laborat-ory and to characterize with credibility [129]. All ofthe tools of precision growth, as well as interfacialcharacterization, come into play here.

The converse problem of interleaving ferromag-nets and superconductors in order to perturbmagnetism (rather than superconductivity) o!ersunusual challenges as well [130}134]. It is wellknown that magnetism and superconductivity

represent competing interactions that tend to pre-clude each other. This is because spin breaks thetime reversal symmetry of the Cooper pairs(#kC, !kB). Thus, by extending the recent dra-matic advances in the use of photoemission toobserve the superconducting energy gap and itsanisotropy in exotic high-¹

#materials, the possi-

bility exists to observe the superconducting/fer-romagnetic interface and characterize the breakingof Cooper pairs and the introduction of states andeven impurity bands into the superconducting en-ergy gap [135,136]. While such observations arepossible in tunnel junctions [135,136], the k-spaceorigin of the states and the anisotropies can beexplored uniquely via photoelectron-spectroscopytechniques. This capability can add to our funda-mental knowledge base and might help formulatenew paradigms that challenge the quasiparticleconcept.

2.4.2. Active interfacesMixing and matching &active' materials

like superconductors and ferromagnets withsemiconductors will produce material systems withnew properties important for future electronics,photonics, and magnetics. The important materials,physics and device issues center upon: (1) thegrowth and character of the interface between wild-ly dissimilar materials, (2) understanding phase-coherent or spin transport across the interface, and(3) the interaction between nanostructured &active'materials each in intimate contact with high-mobility semiconductor structures. The electronicnature of the interface typically determines thebehavior of the composite system. For example,although there has been great progress inunderstanding the coherent transport across theNb-InAs superconducting-semiconducting inter-face, control of the quality of that interface remainsthe most critical issue in reproducible preparationof systems that demonstrate phase-coherent trans-port [137]. Less well developed but scienti"callyand technically important are ferromagnet-semiconductor heterostructures, where the orderparameter in the contact is the magnetization, andthe critical issue is the transport of spin polariza-tion across the interface and through a two-dimen-sional electron gas.


A prototypical active interface may be viewed asa semiconductor quantum heterostructure in directcontact with a ferromagnetic "lm. For example,one may consider the structure consisting of a res-onant-tunneling device, where the local magnetic"elds will have a dramatic e!ect on the electronicbehavior of the structure through proximity e!ects.This class of systems o!ers the potential of exploit-ing ultrafast quantum electronics to sense magneticswitching, thereby giving rise to another family ofcomposite magnetoelectronic materials. The natureof the ferromagnetic}semiconductor interface, localband structure, and chemical potentials must beunderstood and engineered in order to producethese systems. Synchrotron radiation will play animportant role in determining the character of suchinterfaces in concert with in situ growth character-ization and modeling.

2.4.3. BiomagnetsAn ordered magnetic microstructure may be fab-

ricated by either a top-down approach by deposi-ting bulk material and using standard lithographictechniques or a bottom-up approach by assemblingnanometer-scale ferromagnets created by chemicalsynthesis. The "rst method allows direct spatialcontrol and can produce individual magnets assmall as 10}100 nm using electron-beam lithogra-phy or local deposition with a scanning tunnelingmicroscope. The second approach includes mag-netic clusters and implanted ion species. Althoughthere is little spatial control in the chemical syn-thesis beyond direct masking, they can producestructures of smaller dimensions than lithography.Another intriguing possibility is to use biologicalsystems to direct the self-assembly of ordered mag-netic structures. For example, certain organisms,such as magnetotactic bacteria, assimilate magneticmaterials and create microscopic ferromagnets ofremarkably high purity and uniformity [138].Others create complex inorganic structuresthrough biomineralization, an approach that maybe used as a template for arti"cial magnetic struc-tures [139]. Thus, bacteria-based systems are nowbeing used as frameworks for ferromagnetic micro-structures, such as bacterial threads, typically oforder 100 lm in diameter and several tens of cen-timeters long, that are fabricated by pulling many

strings of cells together out of a culture solution[140]. The resulting &bionites' have high densities ofparticles about 100 nm in size and extremely hightensile strengths. Biomaterials-engineering tech-niques, such as bacterial templating, are likely toplay an increasing role in the development of fu-ture technologies, with the potential of providingordered arrangements of microscopic particlesover macroscopic length scales. Chemical speci-"city for nucleating magnetic entities within thesecomplexes require atomic-scale characterizationtechniques that may be addressed by synchrotronradiation.

2.4.4. Molecular magnetsRemarkable new techniques in molecular chem-

istry are now making it possible to create true&molecular magnets', in which the magnetic ions areadded one at a time and the resulting magnet hasa precisely de"ned atomic weight [141]. Many ofthese new magnetic molecules are constructed suchthat the magnetic cluster is secluded from contactwith its external environment by a shell of organicligands. For example, one such magnet, &Fe10', isdubbed the &ferric wheel' by its inventors because ofthe ring structure formed by the ten iron atomsand the bridging organic anions. Magnetizationmeasurements reveal that this particular cluster isliterally a perfect molecular antiferromagnet withan S"0 quantum ground state [142]. The success-ive transitions from one discrete quantum spinstate to the next, evident for magnetic "elds up to45 T, resemble a sequence of phase transitions inthe magnetic order of bulk layered antiferromag-nets. The ability to fabricate macroscopic singlecrystals of these materials containing perfectlyspaced arrays of identical molecules o!ers thepromise of fascinating new science with potentialapplications to technology. This is thus mag-netoelectronics at the most basic level. These mo-lecular magnets may be used to experimentallybridge the gap in our understanding of micromag-netics from the atomic to the mesoscopic lengthscale, and they have already been shown to benearly ideal systems in which to explore the macro-scopic quantum tunneling of magnetization [143](as discussed in Section 2.1.6 for more complexsystems).


Table 2Capabilities of VUV/soft X-ray techniques in the study of mag-netism and magnetic materials

General capabilities of VUV/soft X-ray synchrotron techniquesSpin-dependent electronic structureElemental speci"cityChemical speci"citySensitivity to local magnetic structureAngular momentum speci"citySensitivity to spin and orbital momentSensitivity to ferro-, ferri-, and antiferromagnetic alignmentsBulk and surface sensitivity

Unique capabilities of high-brightness synchrotron facilitiesMagnetic spectromicroscopyEnhanced capabilities for all spectroscopies and scatteringexperiments, including spin and time resolution

3. VUV/soft X-ray capabilities

Valence and core-level spectroscopies in theVUV/soft X-ray region couple directly to the elec-tronic states responsible for magnetism in metals,insulators, and semiconductors, and so they areimmensely relevant in the study of magnetism andmagnetic materials. VUV radiation in the 10}50 eVspectral range provides optimum access to valencelevels and hence the electronic spin structure of allmaterials. The spectral range from 50}2000 eV is ofprime importance for core-level photoemission andhigh-resolution X-ray absorption, emission, andscattering spectroscopies. In this range, an abund-ance of core levels exist from which electric dipoletransitions couple spin}orbit split initial states tospin-polarized "nal states. Long core hole lifetimesresult in strong, sharp lines in the photoelectriccross section and spectacular resonances in refrac-tive properties that together give the optical re-sponse of materials. Of particular importance formagnetism are the 2p core levels (L edges) of themagnetic 3d transition metals (chromium, manga-nese, iron, cobalt and nickel), which lie in the550}900 eV range, and the 3d core levels (M edges)of the rare earths, which lie in the 800}1600 eVrange. In X-ray absorption and resonant scattering,electric dipole transitions from these core levelsallow direct access to the magnetic properties of theimportant 3d valence shell of the transition metalsand the 4f valence shell of the rare earths. Core-level photoemission from the 3s, 3p, and 2p levels inthe 3d elements [40,42,43,144,145], and 4p and 4dlevels in the rare earths [44] also permits studyingmagnetic properties through multiplet e!ects andmagnetic dichroism. Other core levels of interestinclude those of the chemically important elementsboron, carbon, nitrogen, and oxygen, which have 1slevels (K edges in the 150}550 eV range and arefound frequently in interesting materials. In gen-eral, absorption and resonant-scattering (includingre#ectivity) spectroscopies directly probe the emptystates at and above the Fermi level or band gaps,while valence photoemission and X-ray emissionspectroscopies directly probe the "lled electronicstates. Resonant inelastic or Raman scattering mayprovide sensitivity to magnetic excitations. Somegeneral capabilities of using VUV/soft X-ray

probes for the study of magnetic materials are enu-merated in Table 2.

Spin-polarized photoemission [146] is well-es-tablished as the premier tool for the determinationof the spin-dependent electronic structure. Becauseof factors such as cross-section e!ects, energy res-olution, and momentum resolution, the spin-de-pendent electronic valence-band structure is beststudied by use of VUV radiation in the 10}50 eVrange. At higher photon energies spin-resolvedcore-level photoemission [40}44], opens up thestudy of element- and chemical-state-speci"c mag-netic properties. Chemical-state and site informa-tion is present in core-binding-energy shifts; forexample, it has been possible at the ALS to resolvethe atoms at a nonmagnetic/ferromagnetic inter-face and to study their local atomic structure viasite-speci"c photoelectron di!raction [147]. In ad-dition, photoelectrons excited from the core levelscontain magnetic information through multipletcore-valence coupling in the emitting atom orthrough spin polarization imparted by excitation ofa spin}orbit-split level with polarized radiation.This can lead to exchange scattering of the photo-electrons by magnetic neighbor atoms. The latere!ect has been termed spin-polarized photo-electron di!raction, and it provides information onthe short-range spin structure around a given typeof emitting atom in the sample. It thus directlydetermines the local magnetic structure and is


therefore applicable to systems without long-rangegeometric order. To date, this technique has beenapplied to both antiferromagnets [148}150] andferromagnets [151}153]. It has also been proposedto carry out spin-polarized photoelectron holo-graphy [147] by measuring the di!erences inphotoelectron di!raction patterns for spin-up andspin-down electrons (e.g., from multiplets). Thistechnique would permit directly imaging magneticspin moments in space at atomic resolution, and itrepresents an interesting future experiment whoseexperimental di$culties would make it impossibleto perform without a high-brightness third-genera-tion source.

The natural polarization of synchrotron radi-ation enables all magnetooptical e!ects (magneticcircular and linear dichroism, Faraday and Kerrmagneto-optical rotation, etc.) common in thenear-visible spectral regions to be extended into thesoft X-ray range, where they gain element speci"-city when applied in spectroscopic fashion nearcore-level absorption edges. X-ray magnetic circu-lar dichroism is very commonly used today tomeasure the di!erence in absorption of circularpolarization with opposite helicity. Faraday andKerr magneto-optical-rotation signals result fromthe di!erence in refraction of circular polarizationcomponents with opposite helicity, and are mea-sured using tunable linear polarizers [49,50]. TheFaraday magneto-optical-rotation spectrum is re-lated to the XMCD via a Kramers}Kronig disper-sion transformation. Scattering techniques relyingon intensity measurements generally measure a sig-nal in#uenced by both absorptive and refractivemagnetooptical responses.

Powerful sum rules directly link XMCD spectrato the value of the atomic spin and orbital moments[32,33] and their anisotropies [34]. The directseparation and determination of atomic spin andorbital moments is a unique capability of thecore-level magneto-optical spectroscopies. Likecore-level photoemission, such measurements areelement speci"c and, through additional near-edge"ne structure, also provide sensitivity to the localbonding environment of a given atom [154,155].Because the allowed core-to-valence transition isdictated by the dipole selection rule (i.e., sPp,pPd, dPf), excitation at di!erent absorption

edges provides selectivity to the angular mo-mentum of the valence-electron states. Forexample, L-edge (2p) excitation in cobalt probes thecobalt 3d valence subshell while K-edge (1s) excita-tion probes the cobalt 3p valence electrons [38,39].The large size of the circular dichroism e!ects (upto 30% in XMCD and tens of degrees in magneto-optical rotation) allow the study of very diluteand/or weak moments. Taken together, these gen-eral capabilities of X-ray magneto-optical spectro-scopies provide powerful tools to dissect the netmagnetic behavior of complex samples (both ho-mogeneous and heterogeneous) into the contribu-tions from the atomic and chemical constituents.

Core-level dichroism in X-ray absorption andphotoemission also allows the determination of themagnetic axis and the study of magnetic correla-tions in antiferromagnets. Such systems are of greatimportance, but their study is often impeded bytheir compensated overall moment. For example,linearly-polarized X-ray absorption spectroscopycan sense the orientation of the magnetic axis rela-tive to the orientation of the electric "eld vector[156}160]. In the presence of a magnetic axis, thespin}orbit coupling in the valence shell leads toa slight anisotropy of the valence charge, which canbe detected by polarized X-ray absorption. Tem-perature-dependent measurements further allowthe separation of the magnetic e!ect from a val-ence-charge anisotropy caused by the electrostaticcrystal potential. The linear magnetic dichroism isparticularly large when the X-ray absorption spec-trum exhibits sizeable multiplet structure. In addi-tion, multiplet e!ects in core photoemission can beused to probe short-range magnetic order in anti-ferromagnets (e.g., using spin-polarized photo-electron di!raction), as each emitter in this case actsas its own spin reference.

The buried layers and interfaces often critical inreal magnetic nanostructures can be uniquelystudied with soft X-ray techniques. Photoemissionor X-ray absorption techniques relying on electrondetection are typically surface or near-surface sensi-tive, with the information depth given by the escapedepth of the particular photoelectrons or secondaryelectrons detected. In typical materials for magneticapplications, these depths range from a minimumof approximately 5 As for photoelectrons at about


50 eV to 15 to 20 As for photoelectrons at 1000 eV.In soft X-ray absorption measurements, the elec-tron-yield sampling depth is determined primarilyby secondary electrons, and it varies from 20 to40 As for metals to 50 to 100 As for insulators.Emerging techniques relying on photon detectioncan have much greater information depths rangingfrom 0.1 to 1 lm at energies away from any absorp-tion edges to a few tens of Angstroms at edges andin a total-re#ection geometry. Working with elec-tron detection at either much lower or much higherenergies is another avenue for increasing bulk sen-sitivity in even these measurements. Thus, surfaceand bulk sensitivity can be obtained by choosingthe detected particle and the relevant energies, andwith careful attention to the relevant penetrationand escape depths, it is possible to vary the sensitiv-ity from the "rst few atomic layers to depthsthroughout the bulk (a true bulk measurement),thus isolating signals associated with buried layersor interfaces. Photon-based techniques can also beapplied in strongly varying "elds, thereby allowingelement-speci"c hysteresis measurements of inter-est in understanding reversal behavior in complexsystems [50,161]. With soft X-ray wavelengthsranging from 0.5 to 50 nm, scattering techniqueso!er interesting possibilities to study magnetic cor-relation lengths on the nanometer scale and above,and they do not require long-range order.

Many of the above capabilities have been pion-eered at existing synchrotron-radiation facilities,and hence they do not require the brightness ofthird-generation facilities for continued applica-tion. However, even for these capabilities, the in-creased brightness of third-generation sources willtranslate into new experimental opportunities,some of which can be extrapolated from existingwork. For example, in photoemission-spectroscopymeasurements, the increased brightness can trans-late directly into increased energy and k-space res-olution and into much decreased data-collectiontimes, particularly for spin-resolved measurementsusing inherently ine$cient (10~4) Mott or otherdetectors. Additional advantages in photoemissionwill derive from the ability to resolve core-levelshifts, multiplet splittings, and spin-dependentscattering e!ects that would be prohibitively slowto measure with a second-generation source. In

photon-based measurements, increased brightnesscan enhance di!use magnetic scattering measure-ments, and the transverse coherence of undulatorsources o!ers the possibility of dynamic magnetic-speckle scattering. In general, the increased inten-sities associated with higher brightness will allowall types of experiments to move farther into thetime-resolved domain, whether it be in situ studiesof the growth of magnetic "lms or time-resolvedstudies of the dynamics of magnetization-reversalprocesses.

A key advantage of the ALS in magnetism re-search lies in its unique capabilities for magneticspectromicroscopy that results from the higherbrightness. Magnetic microscopy with soft X-rayso!ers several unique capabilities that are similar tothose of X-ray linear- and circular-dichroism spec-troscopies; i.e., high magnetic spatial resolution iscomplemented by elemental, chemical, and vari-able-depth sensitivity [162]. The latter ability, forexample, allows one to image magnetization distri-butions separately in di!erent magnetic layers ofmagnetic heterostructures. High spatial resolutionis achieved by several means [162], three of whichare illustrated in Fig. 5. The "rst is based on produ-cing a focused X-ray beam by means of suitableX-ray optics, such as a zone plate, and creatinga microscopic image of the sample by scanning itlaterally relative to the focal spot. Either the trans-mitted X-ray intensity or a signal from the sample,such as the #uorescence or electron yield, can bedetected, with di!erent depth resolutions for eachcase. In the scanning technique, the resolution isdetermined by the size of the X-ray spot and todaylies in the 30}40 nm range, but a resolution of 20nm is expected in the near future. In the secondmethod, a condenser optic such as a zone plateilluminates the object while an imaging zone plateproduces an image on a detector. In the thirdmethod the sample is illuminated by a X-ray beamthat is only moderately focused, e.g., to tens ofmicrometers, so that the spot size matches the max-imum "eld of view of a photoelectron electronmicroscope (PEEM). The lateral resolution is de-termined by the electron optics in the PEEM anda resolution of 22 nm has already been obtained[163]. Designs for future spectromicroscopes indi-cate that a 2 nm resolution may be possible [164].


Fig. 5. Principles of scanning X-ray microscopy and two imaging X-ray microscopy techniques are shown. In the scanning mode (a)a small X-ray spot is formed by a suitable X-ray optic, e.g., a zone plate lens as shown, and the sample is scanned relative to the X-rayfocal spot. The spatial resolution is determined by the spot size. The intensity of the transmitted X-rays or the #uorescence or electronyield from the sample is detected as a function of the sample position and thus determines the contrast in the image. In imagingtransmission X-ray microscopy shown in (b), a condenser zone plate in conjunction with a pinhole before the sample producesa monochromatic photon spot on the sample. A micro-zone plate generates a magni"ed image of the sample that can be viewed in realtime by a X-ray sensitive CCD camera. The spatial resolution is determined by the width of the outermost zones in the micro zone plate.In imaging X-ray photoelectron microscopy shown in (c), the X-rays are only moderately focused in order to match the "eld of view of anelectron microscope. Electrons emitted from the sample are projected with magni"cation onto a phosphor screen and the image can beviewed in real time at video rates. The spatial resolution is determined by the electron optics within the microscope, the size of theaperture and the operation voltage (from Ref. [162]).

To date, magnetic images using soft X-rays have beenobtained by imaging emitted secondary electrons[57], Auger electrons [165], or photoelectrons [163],and also in transmission using zone-plate optics andphoton detection in imaging [166] and scanning [50]modes. Other forms of magnetic microscopy with softX-rays, such as Kerr microscopy using photons inre#ection, can also be developed.

4. Current and future science with VUV/soft X-rays

In this section, we discuss a few particularlystimulating examples of recent work in magnetismthat have made use of VUV/soft X-ray radiation,

and other areas for fruitful future work at a third-generation source like the ALS.

4.1. Magnetic anisotropy

Core-level X-ray magnetic dichroism spectro-scopy o!ers the unique capability of separating thespin from the orbital moment through sum-ruleanalysis. It is, therefore, ideally suited to study oneof the fundamental issues in magnetism, the micro-scopic origin of the magnetocrystalline anisotropy,as discussed in Section 2.1.1. Recent angle-depen-dent XMCD studies in high magnetic "elds havebeen used to directly link the thickness-dependentmagnetic anisotropy in Au/Co/Au multilayers with


the preferred direction of the orbital moment[34,36], as shown in Fig. 6. These studies haveprovided a way to visualize the microscopic originof the magnetocrystalline anisotropy in a simplepicture based on moment anisotropy rather thanthe less intuitive concept of energy anisotropy. Inthis picture the anisotropy of the lattice or localenvironment causes a preferred direction of theorbital moment, which then redirects the isotropicspin moment into a parallel or antiparallel align-ment (Hund's rule) by means of the spin}orbitcoupling. Early work, which has addressed multi-layer and thin-"lm systems where the magnetocrys-talline anisotropy is large [36,167,168], points tomany future opportunities. Improved measurementmethods, e.g., transverse geometries [169] andstanding-wave techniques, will extend these studiesto samples with smaller anisotropies and allowdetailed investigations of dimensionality and in-homogeneity e!ects with both lateral and depthresolution. The quantitative link between macro-scopic measurements of anisotropy energy and theaverage moments obtained from these spectro-scopies is one direction for future research [170].

One example of a future application of X-raycapabilities is the study of spring magnets, as dis-cussed in Section 2.1.1. Here, the elemental speci"-city of core-level magnetooptic techniques willallow the separation of the magnetic response ofthe soft and hard materials in the hybrid structure.Not only could the average spin and orbital mo-ments on individual species in di!erent layers beprobed, but magneto-optical rotation and othermethods could be utilized to measure the hysteresisresponse of the di!erent layers individually to morefully understand magnetization reversal in thesestructures. Such studies should provide a betterunderstanding of the unusual twist structure pre-dicted in the soft layer, and they also should pro-vide information concerning the interfacial regionwhere little is know about how the transition fromhard, high-anisotropy material to soft, low-anisot-ropy material is made.

4.2. Structure and magnetic properties

A key issue in nanoscale magnetics is the inter-relation of structure and magnetic properties. Stud-

ies of surface and ultrathin-"lm magnetism are ex-tending beyond elemental overlayers, such as epi-taxial iron layers, to surface and thin-"lm alloysystems, where structural aspects of pseudomor-phic or more complex growth modes can in#uenceboth structure and magnetism in overlayers. Anexample of the strong interplay of geometric struc-ture and magnetic properties can be seen in Fig. 7,where Invar quenching is observed in NiFe ultra-thin "lms despite the absence of the phase changefrom FCC to BCC that is observed in the bulk[171]. Here in the ultrathin "lm, the phase changeis frustrated, and instead a decrease in the unit-cellvolume is correlated with the loss of magnetization.In turn, understanding these NiFe ultrathin "lmshas a technological signi"cance because they arefrequently used in GMR and spin-valve devices.Increased intensities resulting from higher bright-ness should enable in situ growth studies of layeredstructures, perhaps with dynamic sensitivity. Beingable to probe the local atomic structure aroundeach elemental constituent via photoelectron dif-fraction will also assist in characterizing such struc-tures fully.

Probing magnetic structure both laterally andinto the depth of thin "lm, layered and bulk struc-tures is feasible using scattered or re#ected pho-tons. Resonant and nonresonant elastic andresonant inelastic scattering have been applied inthe hard X-ray range to study a variety of funda-mental aspects of magnetic structure [172}176]. Athard X-ray wavelengths these techniques often in-volve di!raction to study magnetic structure atinteratomic distances. Similar techniques areemerging in the soft X-ray range, where longerwavelengths do not couple directly to interatomicdistances but rather to magnetic and chemicalstructure at nanometer and above dimensions thatis of fundamental importance in many areas asdiscussed in Section 2. Early examples in the softX-ray include the measurement of di!use scatteringaway from the specular re#ection from a magnetic"lm [46,48]. These studies suggest that the mag-netic and chemical roughness that produce thisdi!use scattering are not necessarily identical.In another example, the magnetooptical sensitivityin the specular re#ectivity intensity from amagnetic multilayer has been used to observe an



$&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&

Fig. 6. Origin of the magnetocrystalline anisotropy illustrated by XMCD results for a Au/Co/Au wedge sample. The wedge shown hasan in-plane easy axis at the thick end and an out-of-plane easy axis at its thin end. The measured angle-dependent orbital moment isfound to become increasingly anisotropic towards the thin end where it strongly favors a perpendicular direction and redirects the spinmoment from an in-plane orientation, the one favored by the dipolar coupling of the isotropic atomic spins (shape anisotropy), to theunusual perpendicular direction. The measured size of the independently determined isotropic spin moment is also shown (see Ref. [36]).

Fig. 7. Changing structure and magnetism with composition insix-monolayer thick Fe

xNi

1~x/Cu(1 0 0) "lms are revealed using

photoemission in conjunction with magnetic linear dichroism.The composition dependence of the dichroism at the iron andnickel 3p levels (top) and of the atomic volume of the "lms(bottom) reveal a clear correlation between structure and mag-netism. (from Ref. [171]). Fig. 8. Longitudinal X-ray magnetooptical Kerr rotation hys-

teresis loops reveal the individual magnetization response foriron and chromium layers in a polycrystalline Fe/Cr multilayer.In this sample the Fe layers couple ferromagnetically and the Crlayers possess a signi"cant moment oriented opposite to that ofFe. The net Cr moment results from the interfaces where a signif-icant number of Cr moments are uncompensated, possibly dueto intermixing with Fe to yield ferrimagnetic local order. Ob-tained using a low 23 grazing-incidence angle, these curves areprimarily sensitive to the in-plane magnetization in just the topFe/Cr bilayer, while measurements at higher angles in re#ectionor in transmission would weight the curves with signi"cantlyincreased contributions from deeper layers (from Ref. [49]).

antiferromagnetic multilayer interference peak un-observable with charge scattering alone [46]. Otherexamples emphasize the compatibility of scatteringtechniques with variable applied magnetic "elds tosense switching of individual layers in layeredstructures, providing information on the degree ofmagnetic correlation between layers [177,178]. Theabove examples have relied on scattered-intensitymeasurements. However, measurements of polar-ization changes in re#ected (scattered) and trans-mitted beams have also been demonstrated,thereby extending Kerr and Faraday rotation spec-troscopies and hysteresis measurements into the

VUV/soft X-ray range [49,50]. Fig. 8 shows hyster-esis loops of iron and chromium in a polycrystallineiron/chromium multilayer, revealing that chro-mium possesses a net moment opposite to the iron


moment in these structures. As in both visible mag-netooptics and hard X-ray magnetic scattering, theability to measure polarization as well as intensityof scattered beams is often essential in rigorousinterpretation of data.

Careful consideration of the magneto-opticalproperties and experimental details will allowthese emerging scattering techniques to be appliedwith varying depth sensitivity. Simply increasingthe grazing-incidence angle results in increasedpenetration depth, and so changes in hysteresisloops with angle would indicate changing magne-tization of a given element with depth. A higherdegree of depth sensitivity will be obtained by usingoptical standing waves to probe the magnetizationacross buried interfaces. Standing waves producedby the interference from multilayer structures read-ily modulate di!use scattering [179], suggestingmany extensions into magnetic multilayers nearspectral resonances. Opportunities exist to applythese capabilities to study magnetism of the buriedlayers and interfaces that are often critical in de"n-ing magnetic properties of interest for applications.Examples include studying buried layers (includingspacer layers) in spin valve and MTJ structuresand the antiferromagnetic/ferromagnetic interfacewhere the mechanisms of exchange biasing arethought to be associated with uncompensated in-terfacial spins of the antiferromagnetic layer [125].It should be possible to study magnetic #uctuationsand inhomogenieties associated with reversal pro-cesses and phase transitions with scattering, pos-sibly in a time-resolved fashion.

4.3. Spin-resolved electronic structure

Following inverse photoemission studies[26,27], spin-polarized photoemission studies ofnoble-metal thin "lms deposited on ferromagneticsubstrates [180,181] provided direct evidence thatspin-polarized quantum-well states are available tomediate the coupling in the associated magneticmultilayers. These studies focused the discussion ofthe proposed mechanism of oscillatory coupling onthe relative binding energies of the spin-dependentband gaps in the ferromagnetic layers, thereby pro-viding a mechanism for oscillatory coupling interms of spin-polarized quantum con"nement of

electrons in the noble-metal "lms resulting from thespin-polarized band structure in the ferromagneticparent materials. When the spacer-layer thicknessis varied, the quantum-well states move up to theFermi level, at which point the total energy of thesystem reaches a local maximum. The energy ofthe system is reduced by switching to antiferro-magnetic alignment which moves the band gapaway from the Fermi level and reduces the con"ne-ment of the quantum-well state.

These seminal early measurements have recentlybeen extended at the ALS to include moderatespatial resolution and to investigate coupling be-tween quantum-well states in di!erent spacer layers[182]. These extensions are possible only withsmall focused beam spots (50}100 lm at 1 : 1 focus-ing), which result directly from the brightness ofthe ALS, in conjunction with in situ grown, wedgedsamples to provide an entire range of samplethickness for study from one deposition session.High #ux and resolution ('1012 photons per sec-ond at a resolving power of 10 000) then enablecombining high-resolution photoemission mea-surements with lateral scanning of the sample toobtain spatially resolved maps of photoemissionspectra. In this case, the diameter of the focusedbeam spot and "lm-thickness gradient translatedinto a resolution of 0.5 monolayers in layer thick-ness. Single-wedged layers of Co/Cu(0 0 1) havebeen studied, as have more complicated structuresconsisting of double-wedged structures of Cu/Co/Ni/Co/Cu(1 0 0), where the copper and top co-balt layer were wedged to detect the inter-ference of the quantum well states in the copperand nickel layers. The results shown in Fig. 9clearly indicate how the intensity of the copperquantum-well state depends on the cobalt thick-ness (copper-nickel separation), thus identifyingthis interference e!ect. The results stronglysupport the quantum-well model of the oscillatorymagnetic coupling in the newly discovered GMRmagnetic multilayers. These techniques can be ex-tended by including circularly polarized incidentradiation and/or spin-polarized electron detectionto more "nely resolve the spin-polarized nature ofthe quantum-well states. They can also be extendedto many other classes of magnetic systems andproblems.


Fig. 9. Interference between quantum-well states in di!erent layers of magnetic multilayers is shown in the image of photoemission-intensity modulations with varying thickness of two di!erent layers (see sample schematic). In addition to the double-wedged sample(grown in situ), an intense X-ray spot 50}100 lm in size made possible by the undulator brightness was needed to observe these features.The clear dependence of the interference on multiple "lm thicknesses provides strong support for a model in which spin-polarizedquantum-well states mediate oscillatory coupling in magnetic multilayers that in turn exhibit giant magnetoresistance (from Ref. [182]).

The colossal-magnetoresistance manganitesand related oxide materials exhibit a range of inter-esting phenomena as a function of doping thatdemand a microscopic, and ultimately atomic,characterization of structure, electronic structure,and dynamics. At low temperature below themetal-insulator transition, they are predicted toexist as half-metallic ferromagnets. A recent spin-polarized photoemission study has provided thebest experimental evidence that this is the case,as shown in Fig. 10. More detailed studies ofthese materials will require spin-polarized studieswith energy resolution comparable to k¹

C, where

¹C

is the Curie temperature. In the case ofLa

0.7Sr

0.3MnO

3for example, this resolution

would be of the order of 25 meV, and it can beobtained at a third-generation source such as theALS by combining high-resolution photoelectronspectrometers with micro-Mott or other spin de-tectors of improved-e$ciency. Additional impor-tant information concerning these materials willcome from companion core-level photoemissionstudies with variable photon polarization and spinresolution; for example, it should be possible toresolve the two types of manganese ions and to

study their short-range magnetic order separately,perhaps using spin-polarized photoelectron di!rac-tion. Clearly, photoemission techniques will con-tinue to evolve to higher spatial, energy, angular,spin, and time resolution so as to provide spin- andmomentum-resolved electronic structure and mag-netic structure for many important problems inmagnetic and other materials.

In most cases photoemission and inverse photo-emission are the techniques of choice for studies ofelectronic and spin structure, owing to their highenergy and momentum resolution and, in the caseof core levels, also their element speci"city. In casesof buried layers and interfaces, X-ray emissionspectroscopy (for a review see Ref. [183]), a tech-nique that has already undergone a renaissance atthe ALS, o!ers unique capabilities. Similar to val-ence photoemission, the X-ray emission techni-que probes the "lled electronic states, but becauseof the initial core hole, it does so with elementalspeci"city. Thus, X-ray emission spectroscopyallows an investigation of chemical bonding andhybridization e!ects. Application of this techniqueto cobalt/copper magnetic multilayers [120], forexample, has shown the hybridization of the cobalt


Fig. 10. Spin-polarized photoemission spectra of a 1900-As thick"lm of La

0.7Sr

0.3MnO

3taken at ¹"40 K (¹

C"350 K). The

photon energy and experimental resolution were 40 and 0.2 eV,respectively. A magnetic pulse coil with a magnetic "eld of about200 Oe was used for magnetization of the sample. The insetshows the magnetization (M) versus applied magnetic "eld (H)hysteresis loop, which was obtained by monitoring manganeseL2-edge absorption of circularly polarized incident light (from

Ref. [28]).

and copper densities of states at the interfaces,giving direct evidence for the origin of the magneticmoment on copper interface atoms found byXMCD absorption spectroscopy.

In addition to the extensive capabilities of photo-emission for characterizing the electronic structureof magnetic materials, XMCD and other core-levelspectroscopies are having an impact on magnetismthrough probing electronic structure. Examples re-lating to anisotropy are given above. Other exam-ples include alloys, oxides, and compounds inwhich XMCD spectra of di!erent elements areprobed. Induced moments on nominally non-

magnetic elements in multilayers and alloys havebeen observed by XMCD absorption. Momentshave been observed on oxygen in CrO

2[184]

and the colossal-magnetoresistance compoundsLa

1~xSr

xMnO

3[185]. These intriguing observa-

tions have direct relevance to the microscopic ex-change mechanisms that are thought to mediatemagnetic interactions. This atomic-scale under-standing of magnetism in multicomponent struc-tures may be used to test proposed exchangemodels, in the CMR materials for example, andto motivate further theoretical research in a widerange of materials. Another example is understand-ing the origin and nature of carrier-induced mag-netism in compound magnetic semiconductors.Capabilities of probing both surface and bulk orburied-layer/interface properties are again impor-tant considerations in these types of studies.

While demonstrated potential and new op-portunities clearly exist to study spin-resolvedelectronic structure with core-level XMCD spectro-scopies, important questions remain about thecore-level spectroscopies themselves. The detailedmechanisms of the spectroscopies are quite com-plicated and may in#uence the very properties ofinterest to investigate. In particular, the large MOsignals near core levels result from processes inwhich a core hole is created, which may perturb the"nal states whose unperturbed spin and orbitalpolarization are of primary interest. The X-rayphysics at ultrashort time scales associated withtransition processes and the many-body responsesto the creation of the core hole are thus criticallyimportant in assessing the degree to which themeasurement distorts the system. This implies anessential close coupling of the theory of such experi-ments with the next generation of such experi-mental work.

It is also worthwhile to note that the develop-ment of new techniques for probing the atomic andelectronic structure of magnetic materials shouldbe possible at a facility like the ALS. One earlyexample of this is the interatomic multiatom reson-ant-photoemission e!ect that has been observed forthe "rst time in several metal oxides (MnO, Fe

2O

3,

and La0.7

Sr0.3

MnO3) [186]. This e!ect, which

couples the photoelectron excitation of one corelevel (e.g., oxygen 1s in MnO) with a resonant


absorption on a neighboring atom (e.g., manganese2p in MnO), has the unique potential for identify-ing which atoms are neighbors to a given atom, fordoing spectroscopy only on interface atoms thathave some degree of hetero-atomic bonding be-tween homogeneous layers, for studying magnetismvia XMCD e!ects in the resonance, and for study-ing more deeply buried interfaces by using second-ary X-ray emission processes for detection.

4.4. Time domain/dynamics

Some early experiments using synchrotron radi-ation [187] and accelerator-produced electron be-ams [79] have studied time-domain issues, but thisis a largely unexplored area in the context of syn-chrotron radiation where interesting opportunitiesmay exist. The issues associated with dynamics areof central importance to research in magnetism andmagnetic materials as a whole, where pushing fromnano- to picosecond time scales are especially im-portant. One general limitation at these time scalesis the ability to produce intense, picosecond mag-netic-"eld pulses.

Nanosecond XMCD spectroscopy has beendemonstrated at the European Synchrotron Radi-ation Facility, where 25 and 50 ns magnetic-"eldpulses with an amplitude of 0.1 T were appliedrepeatedly to GdCo

2amorphous thin "lms. Data

were collected over three minutes, averaging over80 million minor hysteresis cycles. Time resolutionin the XMCD measurement was achieved throughsynchronization and delay of electron bunches withrespect to the "eld pulses. The magnetization oscil-lated in time after the "eld pulses were applied overthe probed area of 30}100 mm2, going "rst to nega-tive values [187]. In a second experiment, the dy-namics of magnetization reversal at surfaces hasbeen studied using soft X-rays emitted from onlyone electron bunch. This novel time-resolved sur-face magnetometry measures the spin polarizationof the total photoejected electron yield by means ofMott scattering. The "rst results obtained with thistechnique are concerned with the magnetizationreversal of iron ultrathin "lms deposited asamorphous low-coercivity ferromagnetic ribbons[188].

4.5. Magnetic spectromicroscopy

The ultimate goal in magnetic microscopy wouldbe the direct observation of atomic spins and theirmotions and the correlation of the orientation andsize of the atomic spin and orbital magnetic mo-ments with the atomic structure and composi-tion of the sample. The accomplishment ofthis goal would revolutionize our fundamentalunderstanding and lead to new designs of techno-logical devices. In many ways, these goals ofmagnetic microscopy underlie many of the researchfrontiers summarized in Table 1. It is for thisvery reason that much e!ort has been exerted topush the magnetic resolution towards the atomiclimit.

To put VUV/soft X-ray studies in proper per-spective requires noting that magnetic microscopycan be carried out with several di!erent and com-petitive approaches such as Lorentz microscopy,Kerr microscopy, secondary electron microscopywith polarization analysis (SEMPA), spin-polari-zed low-energy electron microscopy (SPLEEM),and magnetic force microscopy, to name a few[189]. To date the best methods o!er magneticresolutions around 10 nm. In this environment, thequestion arises as to whether synchrotron-radi-ation-based magnetic microscopy can make uniqueand important contributions.

It is already clear from the discussion in Section 3that, in principle, magnetic microscopy based oncore-level spectroscopies (spectromicroscopy/micro-spectroscopy techniques) o!er several unique capa-bilities, most important of which are elemental andchemical- state speci"city, the ability to separatethe spin and orbital magnetic moments, depth sen-sitivity varying from surface to bulk, the ability toimage antiferromagnetic domains (for "rst resultssee Ref. [190]), and the ability to image in thepresence of large magnetic "elds [191]. An exampleis shown in Fig. 11 where the "eld-dependent do-main structure is observed at a lateral resolutionof 30}40 nm [191]. Despite these various uniquecapabilities, X-ray-based spectromicroscopy hashad little impact on research in magnetism andmagnetic materials in part because its resolutionhas historically lagged behind the leadingtechniques. Also, &home based' techniques are


Fig. 11. Magnetic domain pattern of a Gd/Fe multilayer "lm measured with circularly polarized X-rays at the iron L3

edge as a functionof an applied magnetic "eld, using a transmission X-ray microscope. The domains have magnetization directions perpendicular to the"lm. The magnetic resolution is between 30 and 40 nm (from Ref. [191]).

more accessible and the instrumentation is lessexpensive.

Looking ahead, in order for magnetic spectro-microscopy to have an impact on magnetic mater-ials research, unique capabilities of the X-raytechniques must be brought to bear on importantproblems. One technical improvement here is topush the lateral resolution below 10 nm. In fact,resolutions of about 2 nm appears possible with anaberration-compensated PEEM microscope [164].Such a microscope, if available in a timely fashion,could have a signi"cant impact on magnetic-mater-ials research. For example, it would help in under-standing the stability of the magnetic bits and themagnetic structure of the transition regions be-tween the bits in magnetic-recording media, asshown in Fig. 12. Increases in storage density re-quire the design of new materials with domains thatare stable to smaller dimensions (i.e., by somehow

avoiding the superparamagnetic limit throughmaterials engineering) and that have reducedtransition widths (about 10 nm). Such materialswill most likely consist of grains of reduced size((10 nm) that are separated from each other at thegrain boundaries by a magnetically passive mater-ial. The ability to obtain element-speci"c magneticimages of such materials will be of great import-ance.

A second area of opportunity is to make use ofthe elemental sensitivity, penetrating power, andlarge contrast to directly image domains and mag-netization correlations into the depth of multilayerstructures. It appears that only X-ray microscopytechniques can directly image magnetization in dif-ferent layers (including possibly buried interfacelayers) by tuning to core levels of elements dis-tinct to each layer. So these techniques shouldhave impact an characterizing three-dimensional


Fig. 12. Advanced computer disks consisting of granular magnetic materials like CoPtCr with admixtures of boron or tantalum in orderto minimize the transition width between the magnetic domains. In the disk material, the grains are believed to be coated bya nonmagnetic shell that reduces the magnetic coupling between the grains. A small transition width is required in order to achievea high magnetic-#ux density in the direction perpendicular to the disk surface, as shown. The #ux from the spinning disk is sensed by thespin-valve magnetic read head shown in Fig. 2 ("gure courtesy of J. StoK hr, IBM Almaden Research Center).

magnetization distributions in nanostructures ofboth fundamental and applied interest. Many ques-tions relating to the interaction of one magneticlayer with another can now be studied with spatialresolution of the magnetization in the individuallayers. The transmission geometry facilitates quant-itative magnetization imaging, which will be valu-able in testing micromagnetic theories. Imagingin applied "elds then enables observation of howthe reversal process proceeds in di!erent layers, oralternatively allows spatially resolved hysteresisloops of buried layers to be measured. Such capa-bilities may be important in the development ofmagnetic memory elements.

A third area of opportunity lies in the ability toimage antiferromagnetic domains by means oflinear-magnetic-dichroism microscopy and, bya change to circular X-ray polarization, ferro-magnetic domains in the same sample. Such studies

may well hold the key to solving the importantexchange-anisotropy problem discussed in Section2.1.2. Here and in many other applications, theability of X-ray dichroism microscopy to probeburied layers and interfaces is essential.

5. Conclusions and recommendations

The importance of magnetism and magnetic ma-terials to society through technological and eco-nomic factors is considerable. The magnetic storageindustry alone is about one-third as large as thesemiconductor industry. In addition, the possiblerevolution in permanent-magnet materials with theadvent of &spring'magnets will have overwhelming-ly positive repercussions throughout society.Evolutionary growth in materials-synthesis capa-bilities is currently linked with vigorous basic


research into wide-ranging areas of magnetismand magnetic materials, with clear potential tocontinue to impact science, technology, andsociety. The increasing complexity of thesematerials and the novel phenomena they exhibitrequire increasingly sophisticated experimentalprobes to gain fundamental understanding. Of par-ticular importance is the reduction in size of thefundamental elements to the nanometer or evenatomic scales.

It is clear that a third-generation VUV/softX-ray synchrotron source such as the ALS o!ersnew opportunities to positively in#uence the "eldsof magnetism, magnetic materials and their ap-plication in many ways. Both established andemerging VUV/soft X-ray synchrotron-radiationspectroscopic techniques couple directly and withlarge signals to the electronic and spin states con-taining the physics responsible for these uniqueproperties. These techniques can directly probe thevalence electrons that lead to magnetism, andthrough core electronic excitations they provide anatomic-scale sensitivity to magnetism. Togetherthey provide, with spatial sensitivity at the sub-angstrom level (e.g., through photoelectron andX-ray di!raction methods) and at the nanometerlevel and above (e.g., through scattering and spec-tromicroscopy), clear opportunities to move be-yond classical models of magnetism (e.g., thosebased upon the usual Landau}Lifshitz}Gilbertmethods) and to establish an understanding ofmagnetism at a more microscopic and quantum-mechanical level. If time resolution down to thenano- and picosecond domains can also be incorp-orated into these techniques, it will dramaticallyenhance their ability to impact the understandingof magnetism at this fundamental level.

Of crucial importance for increased impact is anenhanced coupling of the ALS to the magnetismand magnetic-materials community. This is espe-cially important precisely because magnetism andmagnetic materials are very well-established "elds,with many groups actively engaged in such re-search at their home institutions. On the otherhand, the unique capabilities o!ered by third-gen-eration sources of VUV/soft X-ray synchrotronradiation require dedicated development of instru-mentation and techniques, as well as on-site sup-

port for outside users by experts who may be separ-ated from mainstream magnetism research groups.This working group thus recommends the estab-lishment of an ALS outreach program to bringthese groups together in order to help en-sure that the synchrotron-radiation resources arebrought to bear on problems that will have amaximum impact in the true frontiers of researchin magnetism and magnetic materials. Suchinteractions are encouraged with the magnetic-storage industry, the permanent-magnet industry,other industries making use of magnetic materials,and university and government laboratorygroups.

The ALS should work closely with user groupsto push the promising magnetic-spectromicroscopye!orts towards their ultimate nanometer-scale res-olutions and to develop other unique spectro-scopic, di!raction, and magnetooptic capabilitiesmade possible by the brightness of the ALS.Because of the ALS brightness, spectromicroscopyis a unique strength of the facility, and every e!ortshould be made to exploit this capability.Maximum impact in this area will require the es-tablishment of a user base with deep roots in themagnetics community. The ALS is encouraged toform and work with such a user group to raise thefunds necessary for the required instrumental ad-vances towards the ultimate lateral-resolutionlimits. It is also clear that state-of-the-art facilitiesfor various spectroscopic techniques involving bothphoton-in/electron-out (e.g., valence and corephotoemission with spin resolution and variablepolarization) and photon-in/photon-out (e.g., X-ray absorption, scattering, and emission) are neces-sary complements to maximize the unique impactsof the ALS in magnetism.

Acknowledgements

This paper is based on the report of the Magne-tism and Magnetic Materials Working Group ofthe Workshop for Scienti"c Directions at the Ad-vanced Light Source (23-25 March 1998, Berkeley,California) and bene"ts from input at and followingthe working group meetings by its participants:Juana Acrivos, Uwe Arp, C.T. Chen, Cylon Da


Silva, Deborah Dixon, Charles Fadley, Hsueh-Hsing Hung, Peter Johnson, Akito Kakizaki,Sang-Koog Kim, Kannan Krishnan, Ki Bong Lee,Steve Marks, William Oosterhuis, Z.-Q. Qiu,Bruno Reihl, Mike Scheinfein, Frank Schumann,Thomas Stammler, James Tobin, Christian Vettier,Dieter Weller, and Anthony Young. The entirereport of this workshop can be obtained as LBNL-42014 from the E.O. Lawrence Berkeley NationalLaboratory or at http://www-als.lbl.gov/als/work-shops/scidirect/. Work at UCSB is supported bythe National Science Foundation, the O$ce of Na-val Research, and the Air Force O$ce of Scienti"cResearch. Work at UCSD is supported by the USDepartment of Energy and the O$ce of NavalResearch. The U.S. Department of Energy, O$ceof Science, Materials Sciences Division supportswork at LBNL under Contract No. DE-AC03-76SF00098, and at Argonne under Contract No.W-31-109-ENG-38.

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