Magnetic and Magneto-Optical Properties of Interfaces and Superlattices
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film on an insulator, or the free surface of a bulk crystal, are examples of the first category. However, the magnetic atoms at an interface between two différent materials are affected both by the reduced number of similar neighboring atoms (i.e., the dimensionality) and by the crystal and electronic structure of the neighboring layer (i.e., the substrate). By creating multilayer stacks of independent magnetic layers spaced by nonmagnetic materials, thèse interfacial properties can be magnified to a point that they are within the sensitivity of conventional bulk measurement techniques. An additional benefit of the layered structure is that the internai interfaces in thèse multilayers are protected from oxidation and contamination and, therefore, can be studied in air. Multilayers can also display additional effects due to interactions of the magnetic layers across the nonmagnetic layers. Thèse interlayer interactions are responsible for such phenomena as the existence of new superlattice collective modes and the préservation of longrange magnetic order across several superlattice bilayers. In addition, the design parameters of thèse artificial structures can be manipulated to tailor their physical properties for desired technological applications.2 Préparation and Characterization Most research on the magnetic properties of single interfaces has been carried out on clean surfaces or monolayers
deposited in ultrahigh vacuum (UHV) to avoid contaminating the freshly prepared samples (at lxlO"10 torr, 10% of a monolayer of impurities forms in =30 minutes). Such UHV conditions are necessary to prépare samples at low déposition rates (e.g., =0.05-0.5 Â/s) and to study them for reasonably long periods of time before their intrinsic properties are altered by oxidation or contamination. Today, several laboratories hâve UHV déposition Systems with both in situ structural and magnetic characterization. In such Systems, the crystal structure and growth mode of monolayer films can be determined by both reflection high- and low-energy électron diffraction (RHEED and LEED) as well as Auger électron spectroscopy (AES). Thèse well-characterized films can then be examined in vacuum with very sensitive magnetic probes, such as Kerr rotation, ferromagnetic résonance (FMR), and various spin-polarized électron techniques.1 In contrast, the majority of work to date on magnetic multilayers has been on samples prepared in vacuum Systems whose base pressures are several orders of magnitude higher (e.g., «5X10-8 torr), but at déposition rates that also are much higher (e.g., 20-50 Â/s). Thus, the "équivalent contamination" level at the internai interfaces of a sputtered multilayer, or at the surface of a UHV-prepared ultrathin film, can be comparable. More recently, MBE techniques (=10"10 -10~12 torr) hâve been successfully applied to produce magnetic metallic superlattices with high structural quality and very interesting physical properties.1 For example, single-crystal, epitaxial superlattices of Gd/Y3 and Dy/Y4 hâve been grown by MBE, a
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