Journal of Magnetism and Magnetic Materials 200 (1999) 571}582 Magnetic superlattices and multilayers Ivan K. Schuller, S. Kim*, C. Leighton Physics Department 0319, University of California } San Diego, La Jolla, CA 92093-0319, USA Received 1 February 1999 Abstract We brie#y review the active areas of current research in magnetic superlattices, emphasizing later years. With recent widening use of advanced technologies, more emphasis has been made on quantitative atomic level chemical and structural characterization. Examples where the multilayer structure has been controlled, characterized and correlated with the physical properties are discussed. The physical properties are categorized according to the complexity of a structure needed to observe a particular e!ect. We outline a number of general important unsolved problems, which could considerably bene"t from theoretical and experimental input. An extensive list of magnetic multilayer materials is provided, with references to recent publications. 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Magnetic superlattices; Multilayer materials; Physical properties 1. Introduction not to have signi"cant impact on magnetism re- search until the 1980s. Advances in vacuum tech- Much of modern condensed matter materials nologies in the 1970s resulted in major discoveries physics, basic and applied research relies on the in magnetic multilayers in 1980s, and we have wit- development of new materials in unusual con"g- nessed an explosion of the number of publications urations. Magnetic materials in particular provide in magnetic multilayers in the 1990s (Fig. 1). There- the underpinning science for a number of technolo- fore, it is impossible to review properly the vast gies. Basic research in magnetism has been con- available literature [3}8] in this short article. We siderably revitalized recently by the preparation apologize for any omissions which are solely our and discovery of novel magnetic materials as well oversight. as the exploitation of known materials in unusual The term superlattice was coined originally to geometries. The interest in arti"cially layered sys- describe multilayers in which long range (larger tems in particular, increased tremendously since than one bilayer thickness) structural coherence the discovery of giant magneto-resistance (GMR) exists along the growth direction, but the two terms [1]. have been frequently used interchangeably [4]. It is Metallic superlattices and multilayers have been this peculiar geometry that can modify their phys- studied for more than 60 years [2]. However, it was ical properties. Therefore, the amount of structural disorder which can be tolerated depends on the * Corresponding author. Tel.: #1-619-5347-161; fax: #1- length scale which governs the physical properties 619-5340-173. being investigated. A comparison of the length E-mail address: sikim@ucsd.edu (S. Kim) scales relevant for structural characterization tools 0304-8853/99/$ - see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 3 3 6 - 4 572 I.K. Schuller et al. / Journal of Magnetism and Magnetic Materials 200 (1999) 571}582 Fig. 1. Number of publications on metallic and/or magnetic multilayers, extracted from INSPEC database. and physical properties is shown in Fig. 2. Charac- ropy, or GMR, a consequence of antiferromagnetic teristic lengths vary widely, from interatomic coupling across non-magnetic spacer layer, were distances (direct exchange or Ruderman}Kittel} "rst observed in superlattices. On the other hand, Kasuya}Yosida (RKKY)) to several hundreds to superlattice e!ects which were the main motivators thousands of As (magnetic dipolar coupling or spin in the original stages of this "eld, have been ob- di!usion length). In general, the physical phe- served in only a few circumstances for metals in; the nomena in superlattices can be classi"ed as single structure [10], the magnon bands [13], energy "lm, interface, proximity, coupling and superlattice bands [24], and transport properties [25]. Table e!ects in increasing order of sample complexity. 1 lists some of the major achievements in multi- Single "lm e!ects are due to the restriction in ge- layers which are particularly relevant for magnet- ometry. Proximity e!ects occur due to the contact ism. Magnetic superlattices and multilayers between two unlike materials. Magnetic coupling encompass almost every combination of transition across normal materials [1,11,13] has been exten- metals, and to a lesser extent, rare-earth elements. sively investigated. The phenomena described An extensive list of magnetic superlattice systems is above require at most three layers, i.e., a superla- provided in Table 2 with references to recent publi- ttice structure is not needed. It is easier to observe cations. these phenomena in superlattices because they are enhanced by the increased number of layers or because most interfaces are well protected from 2. Preparation and structural properties surface contamination. For example, perpendicular magnetic anisotropy (PMA) [14], where surface Sputtering (DC or RF) and molecular beam anisotropy overcomes the stronger shape anisot- epitaxy (MBE) are the main techniques used to I.K. Schuller et al. / Journal of Magnetism and Magnetic Materials 200 (1999) 571}582 573 Fig. 2. Comparison of structural characterization techniques used for metallic superlattices with relevant length scales. Shaded areas represent regions of uncertainty. d "exchange length, d"screening length, d0))7"RKKY length, d+"magnetic dipolar length, d"spin di!usion length, l"mean free path, "superconducting coherence length, "superconducting penetration length, XRD"X-ray Di!raction, IMSA"Ion Mill Surface Analysis, TEM"Transmission Electron Microscopy, SEM"Scanning Electron Microscopy, SPM"Scanning Probe Microscopy. fabricate metallic superlattices, while ion beam sput- growth, far from thermodynamic equilibrium, is tering (IBS) [26] and pulsed laser deposition (PLD) governed mainly by the surface kinetics occurring [27] have been used less frequently. Growth by both when the impinging atoms encounter the substrate. MBE and sputtering followed by detailed character- UHV MBE has the unique advantage that it allows ization can yield complementary information. in situ surface characterization by sensitive diag- Ultrahigh vacuum (UHV) MBE uses atomic be- nostic tools such as re#ection high energy electron ams to deposit epitaxial "lms on a substrate at an di!raction (RHEED) and Auger electron spectro- elevated temperature. Low growth rates, typically scopy (AES). submonolayer per second, combined with surface Sputtering permits higher throughput, is easy to migration enable layer-by-layer growth. Film rate-control and allows tunability of the energy 574 I.K. Schuller et al. / Journal of Magnetism and Magnetic Materials 200 (1999) 571}582 Table 1 distribution of particles arriving at the substrate. Chronology of selected achievements in multilayers The presence of sputtering gas generally excludes the use of in situ structural characterization tech- Year Ref. niques and is more susceptible to contamination. 1935 Fabrication of metallic superlattices and [2] However, it is fair to state that the structural and multilayers physical properties of metallic superlattices pre- 1978 Anomalous magnetization in Cu/Ni [9] pared by both techniques are comparable, if the 1980 Lattice mismatched superlattices [10] same care is taken in the growth process. Probably 1981 RKKY coupling in Cu/Ni [11] 1982 Absence of 2D magnetism in Cu/Ni [12] the reason for this is that, contrary to semiconduc- 1983 Magnon bands in magnetic superlattices [13] tors, most properties of metals are relatively insen- 1985 Perpendicular magnetic anisotropy in Co/Pd [14] sitive to small amounts of contamination. Spin injection experiment [15] Metallic superlattices have been grown from 1986 Spiral coupling in Dy/Y [18] a large variety of combinations of metallic ele- Oscillatory coupling in Gd/Y [19] Anti-ferromagnetic (AF) coupling in Fe/Cr [16] ments, without consideration regarding their crys- sandwiches tallographic structures. On one hand, elements that Inequivalence of magnetic and structural [17] are closely lattice matched and have the same crys- roughness tal structure, generally have equilibrium thermo- 1988 Giant Magneto-Resistance (GMR) [1] dynamic phase diagrams forming continuous sets 1989 AF coupling in Co/Cu [20] 1990 Oscillatory coupling in GMR materials [21] of solid solutions [28]. Therefore, they are driven 1991 Perpendicular transport in multilayers [22] thermodynamically towards interdi!usion, al- Phase diagram of AF coupled ferromagnetic [23] though thin "lm growth is kinetically limited. On layers the other hand, as known for many years, lattice 1992 Superlattice energy bands in Ag/Au by [24] matching is not a necessary condition for epitaxy photoemission 1994 Superlattice e!ect in transport in Co/Ni [25] [29]. Therefore, if the superlattice components form no alloys, it may be expected that they will be Table 2 Characterization on metallic magnetic multilayers. Code: Sp"Sputtering, MBE"Molecular Beam Epitaxy, Ev"Evaporation, IBS"Ion Beam Sputtering, PLD"Pulsed Laser Deposition, Ed"Electrodeposition, XRD"X-Ray Di!raction, ND"Neutron Di!raction, IS"medium/low energy Ion Scattering, RBS"Ratherford Back Scattering, TEM"cross sectional Transmission Elec- tron Microscopy, SPSEE"Spin Polarized Secondary Electron Emission, PE"Photoemission, XMCD"X-ray Magnetic Circular Dichroism, XFS"X-ray Fluorescence Spectroscopy, XES"X-ray Emission Spectroscopy, XAS"X-ray Absorption Spectroscopy, DAFS"Di!raction Anomalous Fine Structure Spectroscopy, XRMS"X-ray Resonance Magnetic Scattering, VSM"Vibrating Sample Magnetometer, SQUID"Superconducting Quantum Interference Device, AGFM"Alternating Gradient Force Mag- netometer, MOKE"Magneto Optical Kerr E!ect, MO"Magneto-Optical, O"Optical, KM"Kerr Microscopy, MS"Conver- sion Electron MoKssbauer E!ect, FMR"Ferro Magnetic Resonance, TM"Torq Magnetometer, NMR"Nuclear Magnetic Resonance, MC"Magnetic Coupling, MR"Magneto-Resistance, GMR"Giant Magneto-Resistance, AMR"Anomalous Mag- neto-Resistance, MA"Magnetoic Anisotropy, PMA"Perpendicular Magnetic Anisotropy, M"Magnetic Moment, DOS"Den- sity of States System [Ref.] Deposition Characterization Properties Fe/Ti [35,36] Sp XRD TEM VSM MS MA MR Fe/V [37}39] Sp [0 0 1] XRD XES SQUID GMR AMR DOS [40] Sp [0 0 1][2 1 1 ][1 1 0] MOKE XMCD M Fe/Cr [1,41,42,45}48] MBE [0 0 1][1 1 0] poly XRD ND MS MOKE GMR MA MC [49}56] Sp [0 0 1][2 1 1] poly XRD ND SQUID MOKE GMR MA MC Fe/Cu [57,58] PLD [0 0 1] MOKE PE XMCD M DOS Fe/CuZr [59] MBE MOKE KM MC Fe/Zr [60] Sp XRD MS M Fe/Nb [61] Sp XRD GMR I.K. Schuller et al. / Journal of Magnetism and Magnetic Materials 200 (1999) 571}582 575 Table 2 (continued) System [Ref.] Deposition Characterization Properties Fe/Mo [62,63] Sp XRD VSM SQUID GMR Fe/Pd [64] MBE [0 0 1] TEM SQUID AGFM KM MA Fe/Ag [65] Sp XRD TEM VSM GMR Fe/Ir [66] MBE [0 0 1] DAFS Fe/Pt [67,68] Sp Ev [0 0 1] poly XRD VSM TM MS MA Fe/Au [69}72] Sp Ev MBE [0 0 1][1 1 1] XRD IS VSM TM MS FMR PMA MR Fe/Si [73,26] IBS [0 0 1][1 1 0] XRD TEM XFS VSM MC Fe/Ge [74] MBE SPSEE MC Fe/Gd [75] Ev MR Co/Ti [76,77] Sp XRD TEM VSM MA Co/V [78] Sp FMR MA Co/Cr [81,79,80] MBE [0 0 1][1 1 0] MOKE PMA MC MR Co/Cu [82,83,30] MBE [111] XRD IS FMR GMR MA [84}91] Sp XRD XAS XES VSM AGFM GMR MR [92] Ev XRD TEM RBS MOKE GMR [93] IBS XRD VSM FMR MC [94}96] Ed XRD TEM GMR Co/Zr [97] Sp XRD FMR MO O Co/Pd [98] SP FMR PMA Co/Ag [99,100] SP GMR Co/Ir [101] IBS XRD GMR Co/Pt [102}105] Sp XRD MOKE VSM MO O DOS Co/Au [106,107] Ev MBE [111] XRD MOKE MR PMA M Ni/Ag [108,109] SP VSM FMR XRMS MA MC Ni/Pt [110] Ev XRD TEM MOKE PMA NiFe/Cu [111,112] Ev Sp XRD TEM GMR NiFe/Mo [113] SP XRD NiFe/Ag [114] SP XRD MS M MR NiFe/Au [115] Sp XRD CoNi/Cu [116,117] Ed GMR Fe/Ni [118,119] SP [0 0 1] XRD SQUID M Fe/Tb [120,121] Sp XRD ND SQUID MOKE MS PMA M Co/Ni [122,123] Sp AMR [25,124}126] MBE [111] XRD MR Co/SmCo [127] Sp XRD TEM SQUID M Dy/Sc [128] MBE [0 0 1] XRD MR Dy/Lu [129] MBE [0 0 1] XRD Se/Eu [130] MBE XRD M Y/Tb [131] MBE [0 0 1] NMR Ho/Gd [132] MBE [0 0 1] XRD ND M Ho/Tm [133] MBE [0 0 1] XRD ND M Ho/Lu [134] MBE [0 0 1] XRD ND M Er/Lu [135] MBE [0 0 1] XRD ND M more segregated. Since the "rst growth of lattice Another important issue is that the growth of mismatched metallic superlattices from the eutectic a superlattice is somewhat di!erent from that of Nb}Cu system [10], many more systems have been a bilayer. The structure is a!ected by the momen- fabricated. However, atomic level interdi!usion is tary substrate and the temperature on which a layer found in even immiscible systems [30]. is growing, i.e., di!erent interfaces and layers have 576 I.K. Schuller et al. / Journal of Magnetism and Magnetic Materials 200 (1999) 571}582 di!erent growth conditions. At elevated growth Superlattices are routinely checked using labor- temperatures, annealing and interdi!usion may oc- atory X-ray di!ractometers (Table 2), while syn- cur in the buried layers. Therefore, it is important chrotron sources provide tunability, polarization to characterize the structure once the whole super- and increased intensity, to improve di!raction lattice is grown. quality [39,41,84,109] or provide di!use scattering For relatively thick multilayers, detailed know- [82,87,115] data. Conventional di!raction (specu- ledge of the interface structure is not important lar) and di!use scattering (o! specular) data con- because physical properties are not signi"cantly tains complementary information. The specular a!ected by interface quality. On the other hand, peaks contain information on defect structures multilayers with constituents approaching single along the growth direction, while the lateral length monolayer (ML) level, are routinely fabricated scale being probed is rather uncertain, whereas these days. In such cases, structural character- di!use scattering data shed light on lateral correla- ization is crucial. Non-destructive di!raction tion lengths. Quantitative disorder parameters can techniques, such as X-ray di!raction (XRD), are be extracted from the data by detailed re"nement commonly used to analyze multilayered struc- techniques [33]. There are exploratory reports on ture [10]. Powerful tunable photon sources are the use of ion scattering to investigate interface capable of element speci"c characterization [31] roughness by low [30] and medium [69] energy ion and polarized photons or neutrons are available to scattering (LEIS, MEIS). probe the magnetic structure [31,51]. Since quant- Powerful tunable photon sources become more itative di!raction studies require modeling and important in spectroscopic areas to probe the su- a priori knowledge of the probed length scale, com- perlattice electronic structure, i.e., X-ray #uores- plementary techniques, such as cross-sectional cence spectroscopy (XFS) [73], X-ray emission transmission electron (TEM) or scanning probe spectroscopy (XES) [39], X-ray absorption spec- microscopies (SPM) are helpful. troscopy (XAS) [88], di!raction anomalous "ne The major types of structural imperfection pres- structure spectroscopy (DAFS) [66], X-ray reson- ent in superlattices are interfacial roughness, inter- ance magnetic scattering (XRMS) [109] and near di!usion, imperfect crystallinity, and crystalline edge X-ray absorption "ne structure (NEXAFS) orientation. The distinction between interdi!usion [66]. The magnetic pro"le could be di!erent from and roughness is arti"cial, since at the atomic level the chemical pro"le of the superlattice [17,34]. the concept of interdi!usion is somewhat meaning- Magnetic structure of interfaces can be probed by less. At short length scales, smaller than the lateral X-ray magnetic circular dichroism (XMCD) and coherence length of a particular probe, an interface neutron di!raction techniques, which are reviewed with roughness &looks' like a homogeneous inter- by other authors (Stirling, X-Ray Magnetic Scatter- face with an average scattering function given by ing; StoKhr, X-Ray Magnetic Dichroism Studies of the relative proportion of the constituents. In Magnetic Anisotropies; Felcher and Ankner, Polar- a naive interpretation, interdi!usion a!ects only ized Neutron Reyectivity). the peak intensities, while layer thickness #uctu- ations broaden the peaks [32]. Rocking curve widths are a!ected by the angular distribution of 3. Physical properties crystallites and crystalline orientation, while vari- ations in interatomic spacing change the peak posi- Magnetic superlattices composed of ferromag- tion. In realistic situations, however, there is no such netic/non-magnetic (F/N) materials have been clear distinction between the particular type of dis- studied for the e!ects of dimensionality, magnetic order and its e!ect on a particular feature; all di!rac- anisotropy associated with the F/N interface, tion features are a!ected to some degree. Therefore, magnetic coupling through the non-magnetic quantitative analysis of di!raction data requires spacer layer, and to a much lesser extent, for comparison to simulated di!raction patterns with superlattice electronic or spin structure e!ects. detailed modeling of defect structures [33]. Ferromagnetic/ferromagnetic (F/F) or rare-earth I.K. Schuller et al. / Journal of Magnetism and Magnetic Materials 200 (1999) 571}582 577 superlattices attracted much less attention. In this Co/SmCo [127] exchange spring magnets, section, we will review the physical properties in CoO/NiO [154,155] exchange biased superlattices increasing sample complexity and will give as an or FeF example the most outstanding recent development. /CoF [153] antiferromagnets. An area highly neglected is that of magnetic proximity e!ect. Although some theoretical e!ort 3.1. Interface/proximity ewect; perpendicular [138,139] was devoted to this in the early 1970s, magnetic anisotropy very little experimental work has emerged. Con- trary to superconductivity, investigating the short Metallic multilayers composed of alternating length scale spacial dependence of the magneti- layers of a ferromagnetic transition metal (FT"Fe, zation is not easy, although some experiments were Co, Ni) and noble metals (NM"Cu, Ag, Pd, Pt, performed using polarized neutron re#ectivity Au) exhibit perpendicular magnetic anisotropy [17,34]. With the advent of more powerful neutron (PMA) and maybe useful as magneto-optic record- sources and the development of novel synchrotron ing media. In these multilayers, interfacial magnetic techniques [140] the magnetic proximity e!ect can anisotropy may be perpendicular and is controlled "nally be tackled. by the nature of interface. The interfacial spin is well described [136] by the interface hybridization 3.2. Coupling ewect; giant magneto resistance of electronic d states between FT and NM and gives rise to this out-of-plane spin orientation. This Several types of magnetic coupling across non- PMA is an example of an interface and/or proxim- magnetic spacer layers were investigated in the ity e!ect, which does not require multilayer struc- 1980s (Table 1). The discovery of GMR in Fe/Cr ture but it is commonly investigated for [1], shortly after the discovery of antiferromagnetic convenience in multilayers. We will leave more (AF) coupling [16], together with the oscillatory detailed review on this subject to other authors coupling [21], had an enormous impact in the area. (Freeman, Wu, Surface Magnetic Anisotropy). We will only brie#y summarize the current status The use of Pd as the nonmagnetic element for better understanding of the rest of the manu- (Fe/Pd [64], Co/Pd [14,98]) is particularly interest- script, leaving the more detailed review to other ing. Although Pd is non-magnetic it is well known authors (Stiles, Interlayer Magnetic Coupling; to possess unusually high susceptibility due to Celotta, Stiles, Unguris, Pierce, Inyuence of Inter- a large Stoner enhancement factor. Ferromagnetic facial Roughness on Magnetic Coupling of Fe/Cr impurities or proximity to ferromagnetic materials Layered Structures; Bass, Pratt, Current Perpendicu- can produce a magnetic moment in otherwise non- lar Magnetoresistance in Magnetic Metallic Multi- magnetic Pd. Co/Pd, the "rst system showing PMA layers). There are many experimental (see Table 2) [14], is still investigated [98] and since the Pd and theoretical [141] investigations dedicated to polarization is sensitive to structural defects, con- this area because of technological implications in siderable emphasis is made on structural character- magnetoresistive devices. The basic mechanism re- ization [64]. Co/Pt [102}105] multilayers attracted sponsible for this e!ect is the low "eld antifer- recent attention because of the simultaneous large romagnetism of adjacent Fe layers with the high magneto-optical (MO) Kerr rotation and PMA "eld ferromagnetic alignment. This, together with which were interpreted using theoretical band spin-dependent scattering (not spin #ip) [142] gives structure calculations [103]. The high potential in rise to additional scattering in zero "eld compared MO applications motivated the e!ort to optimize to high "eld. GMR is expected to be bigger in physical properties such as lowering the Curie tem- the perpendicular transport which is di$cult to perature by introducing Ni in the Co layer or at the measure in thin "lms, although this geometry is interface [102,103]. PMA has also been observed in now being probed by several methods [22,143,144]. Fe/Au [71], Co/Au [107,137], Co/Cr [79], and Fe/Cr is one of the most extensively investigated Fe/Tb [120,121] superlattices. Another manifesta- superlattices and has also been studied in trilayer tion of interaction at interfaces is found in &spin valves' [147]. Oscillations in the AF coupling 578 I.K. Schuller et al. / Journal of Magnetism and Magnetic Materials 200 (1999) 571}582 as a function of spacer layer thickness have been probably due to interdi!usion. This is consistent reported in wedged samples (short period &2 ML) with low-energy ion scattering experiments show- [145,43] or in superlattices (long period ing signi"cant surface di!usion for Co/Cu(1 1 1) &11}18 As) [21]. The magnitude of GMR varies even at room temperature [30]. As in Fe/Cr, re- greatly regardless of deposition method or crystal- duced GMR was reported with increasing Co line orientation (being as high as 220% [44]), even thickness [9]. In addition to the well-studied GMR, for a "xed con"guration. The discrepancies have other properties, such as in-plane magnetic anisot- not yet been clearly understood, although it has ropy [83], spin wave resonance [93] and electronic been implied that details of the structure are at the density of states (DOS) near buried interfaces [88], root of the problem. Although signi"cant e!ort was and interference of quantum well states due to dedicated to characterize the interface disorder, ferromagnetic layer [148], were reported. Even mostly by XRD, the results seem apparently in- nanowire fabrication by electrodeposition through consistent. Superlattices with di!erent interface nanopore membranes has been reported for uncon- roughness were fabricated by changing the growth ventional current perpendicular to the plane (CPP) temperature or bu!er layers in MBE or by varying measurement [94,96]. the deposition pressure in sputtering. As interface GMR and AF coupling has also been reported roughness increased (as extracted from quantitative in many other systems [37,63,65,71,99}101,107, XRD analysis), GMR decreased in MBE samples 112,116] although the GMR is small and/or only [41], but either increase [54] or decrease [146] in observed at relatively thin ferromagnetic layer sputtered polycrystalline samples. Another contro- thickness. It is interesting to note that antifer- versial claim was GMR oscillation with Fe layers romagnetic coupling, an underlying mechanism for thickness [46], although relatively big GMR GMR, has been observed also through amorphous has been consistently reported for thinner Fe layers (Fe/CuZr [59]) and semiconducting (Fe/Si [26,73], (less than 15 As). These reiterate our incomplete Fe/Ge [74]) materials. This suggests that GMR understanding of characterization tools and defect and AF coupling seem to be universal phenomena, structure in superlattices with a few MLs of alter- not speci"c to a particular material system. nating elements. Cr spacer layers in Fe/Cr deserve A number of other coupling e!ects are present, special attention because of the possible connection which, however, have received considerably to the antiferromagnetism of bulk Cr. Although the less attention. These include magnetic-structure magnetic structure of the Cr and Fe layers have investigation in rare-earth superlattices [128}135, been studied extensively [49}52], the existence of 149,150], and magnetic investigation of rare-earth antiferromagnetism in thin Cr spacer layers is not transition metal superlattices [151]. yet clearly identi"ed. Moreover, GMR is observed in superlattices with a normal metal spacer. 3.3. Superlattice ewects Co/Cu, another well studied superlattice exhibi- ting similar oscillatory AF coupling and GMR has Superlattices alternating a few MLs with sharp a normal metal spacer. The higher room temper- atomic level interfaces could provide a new chal- ature GMR and lower saturation "eld make it lenge to fundamental physics. Superlattice energy more attractive for application. Co/Cu superlatti- bands have already been observed in Ag/Au [24] ces were fabricated by many techniques (MBE by photoemission and X-ray L-emission spectra of [30,82,83], sputtering [84}91], evaporation [92], the 3d band of Fe/V [39] seemed to be in agree- IBS [93], and electrodeposition [94}96]) and the ment with "rst-principle band structure calcu- structure was analyzed quantitatively by combin- lations. In Fe(1 ML)/Cu(1 ML) [58] superlattices it ing specular, o!-specular, and anomalous X-ray was claimed from spin-resolved photoemission, scattering [84,87]. The interface roughness ma- that the dispersion of Fe-type majority bands along nipulated by changing substrate temperature [85], the } axis indicates the presence of bands as or the interface width by codeposition [89] de- a consequence of the unit cell doubling in creases GMR with increasing interface roughness, the growth direction. With indications that the I.K. Schuller et al. / Journal of Magnetism and Magnetic Materials 200 (1999) 571}582 579 interface roughness may be controlled at the interfacial tunning is used to modify naturally oc- atomic level and that the electronic structure cha- curring structures holds much promise. Investigat- nges accordingly, it is interesting to investigate pos- ing the competition between di!erent magnetic sible new physical phenomena associated with this. phases holds the promise of development of un- Finally, it should be noted that observation of usual magnetic materials. Small, low-dimensional superlattice e!ects in the physical properties such structure have magnetic energies which are compa- as electrical transport is unusual. Oscillations in the rable to the temperature and possibly the large residual resitivity and magnetoresistance of Co/Ni magnetic "elds currently being developed. This will [124,125] were interpreted in terms of superlattice allow the exploration of completely novel thermo- e!ects [126,152] due to the fact that they occur as dynamic phase diagrams. a function of the individual layer thicknesses, the Finally, the question of how (or even if ) superla- total multilayer period, and depend on the number ttice e!ects manifest themselves in the measurement of bilayers. of the physical properties of superlattices is an interesting one. For instance, do extended wave functions exist in the perpendicular direction or are 4. Open questions they localized due to the unavoidable introduction of defects and disorder? The role played by energy It is perhaps "tting to highlight that these mater- gaps and localized states is yet to be clari"ed. Al- ials are in need of new theoretical and experimental though much of the motivation for this type of paradigms. Disorder is a key ingredient in all these work is basic in nature, important applications materials. Although striving for even higher perfec- which have moved into the commercial market in tion is a commendable e!ort, it is safe to state that a short time period have already emerged. Clearly, absolute perfection will not be achieved in the near many interesting phenomena are envisioned and future. As suggested above [17,34] disorder a!ects although it is hard to predict, applications are like- at di!erent length scale di!erent physical proper- ly to emerge. ties. Therefore, developing theories and experi- mental situations which probe the e!ect of disorder at varying length scale is of crucial importance. 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