MBE of Magnetic Metallic Structures
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Epitaxial growth of magnetic metals actually pre-dates epitaxial growth of semiconductors. The earliest work (1936),' which reported single-crystal Fe growth on NaCl, exploited the fact that single-crystal Substrates of NaCl were easy to obtain, readily cleaved, and could be cleaned in a vacuum by heating. The good lattice match between the two Systems and lack of interfacial disruption upon growth permitted excellent quality single-crystal films of Fe to be grown in relatively modest vacuum. Improved vacuum techniques broadened the ränge of materials which could be studied, with respect to both the films and Substrates. The most recent ultrahigh vacuum (UHV) techniques developed for molecular beam epitaxial growth of semiconductors, including the large array of electron-based analytical tools, have also been exploited to grow and characterize magnetic metal films. Some requirements for these magnetic materials, such as the need for higher temperature effusion sources to generate useful fluxes of Fe, Co and Ni, and high vacuum in the presence of ebeam sources in order to avoid oxidation of the rare earths, served to stimulate new technical developments for the field in general. It is now possible to control growth to a fraction of a monolayer (ML) and even to know when one ML coverage is complete and another is beginning. The techniques have become so successful that a whole new subfield of magnetism has emerged — surface and interfacial magnetism — in which the work would be largely meaningless if one could not grow precisely characterized epitaxial magnetic metal films. Recent work has shown that it is now possible to grow single-crystal magnetic metal films on a wide variety of Substrates, including insulators (oxides and salts), semiconductors, and metals. Strained and metastable structures have
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been grown, as have metallic superlattices of both 3d and 4f metals. A word on why such films are of interest... they are important from both technological and fundamental points of view. Technologically, magnetic materials account for a greater financial market than semiconductors. This stems largely from their widespread use in recording media (disks, tapes, etc.), and also from the need for ferrites (i.e., magnetic insulators) in electronics and recording heads, and p e r m a n e n t m a g n e t s in e l e c t r o mechanical devices, motors, etc. Many of these applications require properties unique to magnetic materials, and any improvements in these properties can yield high financial returns. From a fundamental viewpoint, only magnetic materials and superconducting materials exhibit long-range cooperative phenomena. The rieh variety of forms of magnetic ordering provides the necessary laboratory examples for testing theoretical predictions and stimulating new theoretical work. This is particularly important—although magnetism may be the oldest diseipline in solid State physics, there still exists no fundamental theory which permits predicting the properties of a magnetic material from first principles. As will
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