Research in magnetism has a long, redoubtable history, and one fundamental reason for its continuing productivity is the fact that advances are very often coupled intimately with progress in materials science. New materials, preparation methods, and characterization technique s continu e to provide opportunities for exploring magnetic phenomena in a progressively broader array of systems with a growing arsenal of analytical tools. Challenges to theory are often presented by discoveries on the experimental side, engendering constructive interaction which enhances our basic understanding of the materials and the physics controlling their properties. Technology invariably benefits from that ongoing process. The following series of five articles in this issue of the MRS BULLETIN aims at conveying the excitement and vitality of current work in magnetism and magnetic materials. Each author or set of author s has endeavored to provide a succinct overview of a specific area, and in combination the papers offer an informative (although certainly not exhaustive!) survey of this very active field.

In 1907 Weiss postulated that a ferro-magnet has many small regions or domains that are spontaneously magnetized below the Curie temperature. In an unmagnetized ferromagnetic specimen the domains form so as to minimize the free energy, which includes contributions from the exchange energy, crystal and shape anisotropy energy, magnetostrictive energy, and so on. The actual size and shape of domains can only be calculated for the simplest geometries and for idealized materials. To investigate magnetic microstructure and how it determines macroscopic magnetic properties, we want to be able to see domain configurations and understand how the magnetic microstructure is changed by external influences and varying material properties. The observation of microscopic domain configurations can aid in engineering magnetic materials with desired magnetic properties.

Domains were first observed using the Bitter method in which fine magnetic particles collect on the surface of a specimen in the field gradients at domain walls. Domains can also be observed by electron microscopy, with scanning electron microscopy, or at higher resolution with transmission electron microscopy. Specimens must be thinned to about 100 nm for use in the TEM. This, in turn, changes their magnetic properties. The magneto-optic Kerr effect has the advantage that the signal is directly related to the magnetization of the specimen and not just to the leakage fields or internal magnetic fields as in the methods mentioned above. This technique has the disadvantage of being limited to an optical resolution of typically 1μm.

The most intriguing aspect of the new “high Tc” superconductors is that we still have no consensus about the nature of the basic mechanism for superconductivity in these materials, nor about why Tc is so much larger than in conventional superconductors. As is now well known, these materials include compounds of the type La2-xSrxCu2O4-y (where Sr can also be replaced by Ba) in which the maximum Tc attained is between 35 K and 40 K, and of the type YBa2Cu3O7-δ (where Y can also be replaced by most rare-earth atoms) in which the maximum Tc attained is about 98 K. Very recently, new compounds such as Bi2Sr2CaCu2O8+y have been discovered where Tc's of up to 120 K have been observed, but the physical properties of these at the time of writing are much less well known. The interesting thing about all these compounds is that they all result from doping of cupric oxide insulators, and they all contain planes of Cu-O atoms.

Perhaps not entirely by coincidence, high Tc superconductivity thus occurs in a class of materials, namely transition metal oxides, whose electronic ground states are currently least well understood in terms of conventional band theory. It is now generally accepted that the d-shells of transition metals ions (including Cu) have associated with them a fairly large Coulomb repulsion that makes it energetically unfavorable for two holes to be on the same d-shell simultaneously. For Cu, the energy U is estimated to be between 4 and 8 eV. This implies that Cu++ (d9 configuration) is favored, while Cu+++ (d8 configuration) is not. This is in accordance with spectroscopic measurements of these compounds.

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 range 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 e-beam 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.

Diluted magnetic semiconductors (DMS) are semiconducting alloys whose lattice is partly made of substitutional magnetic ions. The most extensively studied materials of this type are the alloys, in which a fraction of the group II sublattice is replaced at random by Mn. The entire family of ternary alloys, along with their crystal structure and corresponding ranges of composition, is listed in Table I. Over the past decade, these alloys have attracted a growing scientific interest because of new fundamental effects in semiconductor physics and magnetism in these materials and because of their potential applications in optical nonreciprocal devices, solid state lasers, flat panel displays, infrared detectors, and other optoelectronic applications.

The increasing popularity of this field can be attributed to the broad variety of fascinating problems offered by the study of the alloys. To begin with, there is an interest in the semiconducting properties per se — for instance, the understanding of the electronic band structure and its variation with alloy composition. As in other ternary alloys, the band parameters and the lattice constant can be “tuned” by controlling the alloy composition, opening the door to band-gap engineering and lattice matching in the context of epitaxially grown superlattices and het-erostructures. The random distribution of Mn atoms with a well-characterized antiferromagnetic Mn-Mn exchange interaction provides an ideal system for studying fundamental questions in disordered magnetism. The sp-d exchange interaction between the spins of band electrons and the localized moments of the Mn atoms constitutes a unique interplay between semiconductor physics and magnetism. This leads to unusual magneto-transport and magneto-optic phenomena such as an extremely large Faraday rotation, giant negative magneto-resistance, and a magnetic-field-induced metal-insulator transition. Finally, the potential technological importance of DMS is also being recognized. For example, the large Faraday rotation holds promise of DMS applications as optical isolators, modulators, and circulators. We will briefly introduce some of the exciting research problems offered by the study of DMS. More detailed information is available in several extensive reviews and compendia.

Permanent magnets have long occupied an important position in technology. Among the multitude of products using permanent magnets are televisions, telephones, computers, videocassette recorders, audio systems, household appliances, and perhaps surprisingly to many consumers, automobiles. Figure 1 illustrates the numerous magnet applications in a modern passenger vehicle. These applications include an array of dc electric motors such as the starter, heater and air conditioner blower, windshield wiper, window lift, door lock, and fuel pump motors. A fully equipped car can have more than 30 dc electric motors. Other uses include actuators, gauges, and sensors. In all these examples higher performance magnetic materials may afford the advantages of increased operating efficiency and reduction in size and weight.

One performance index or figure of merit for a permanent magnet is the energy product (BH)max, the maximum product of magnetic induction B and applied field H in the second quadrant of the B-H hysteresis curve. The so-called theoretical (BH)max, the highest energy product realizable in principle, is simply given by (4πMs)2/4, where Ms is the saturation magnetization. Progress in the development of technologically significant hard magnets has been monitored generally by improvements in (BH)max. For many years three types of materials were of commercial importance, namely, alnico, ferrite, and samarium-cobalt alloys based on either the SmCo5 or Sm2Co17 intermetallic compounds. Of these the Sm-Co magnets offer the largest energy products, on the order of 20 MGOe.

A marriage of the new and the old is under way at the Lawrence Berkeley Laboratory (LBL), Berkeley, California. The new is the Advanced Light Source (ALS), a facility for generating laser-like, partially coherent beams of x-ray and ultraviolet (collectively, the XUV) synchrotron radiation of unprecedented spectral brightness. (See Figure 1.) The old is the domed hall that has sheltered the historic 184-inch cyclotron, a nuclear accelerator built 46 years ago by the laboratory's founder E.O. Lawrence. The ALS is to reside in an enlarged version of the old hall, a familiar landmark in the Berkeley-San Francisco area whose architectural features are to be preserved.

Another sign of the marriage is an increased emphasis on materials research. Though by no means forsaking its nuclear science origins, LBL has seen its budget for materials research grow steadily in recent years, especially with the establishment of the Center for Advanced Materials and the construction of two new laboratories to house its activities. [See the April 1988 MRS BULLETIN, p. 54, for an Up Close article on LBL's National Center for Electron Microscopy.] With its focus on the use of electromagnetic radiation for structural and spectroscopic studies of physical, chemical, and biological systems, including those investigated by materials researchers, the ALS fits neatly into this new emphasis. In fact, funding for the $98.7 million ALS comes to LBL through the Division of Materials Sciences of the U.S. Department of Energy (DOE).