Handbook of Magnetic Materials (eBook)

Preface to Volume 20

The Handbook of Magnetic Materials is a continuation of the Handbook series on Ferromagnetic Materials. When Peter Wohlfarth started the latter series, his original aim was to combine new developments in magnetism with the achievements of earlier compilations of monographs, producing a worthy successor to Bozorth’s classical and monumental book Ferromagnetism. This is the main reason that Ferromagnetic Materials was initially chosen as the title for the Handbook series, although the latter aimed at giving a more complete cross section of magnetism than Bozorth’s book. In the past few decades, magnetism has seen an enormous expansion into a variety of different areas of research, comprising the magnetism of several classes of novel materials that share with truly ferromagnetic materials only the presence of magnetic moments. For this reason, the Editor and Publisher of this Handbook series carefully reconsidered the title of the Handbook series and changed it into the Handbook of Magnetic Materials. It is with much pleasure that I introduce to you now Volume 20 of this series.

Ferrite materials were known already long ago to ancient cultures. The value of ferrites as materials for ultrahigh frequency applications was not recognized until around 1940 when ferrites were systematically studied by Snoek and coworkers for applications in devices that send, receive, and manipulate electromagnetic signals at rf, microwave, and millimeter wave frequencies. Early work on microwave ferrites has been reviewed in Chapter 4 of Volume 2 of this series. As will be discussed in Chapter 1 of this volume, modern high-frequency magnetic materials operate either near ferromagnetic resonance, as absorbers or electromagnetic interference shielding materials. Above or below resonance they operate as low-loss, high-permeable materials in microwave passive devices such as circulators, isolators, phase shifters, filters, inductor cores, etc. The focus of Chapter 1 is principally in the area of off-resonance materials and their applications. As the operational frequency of these materials increases, the requirement for low-loss properties concomitantly increases, which in turn leads to the need of suitable insulating magnetic materials. Ideal materials possess high magnetization, high permeability, high electrical resistivity, and subsequently very low conduction loss. Insulating magnetic materials that fit these criteria include ferrites and related structures. Ferrite materials are unique because they are part of a few classes of insulating magnetic oxides that display high permeability, moderate to high permittivity, and low losses at frequencies ranging from dc to submillimeter wavelengths. These properties lend them value in high-frequency devices that require strong coupling to electromagnetic signals while giving rise to only low losses. Moreover, because of their intrinsic magnetism, these materials also exhibit nonreciprocal behavior, being of prime importance for many device applications in radar and communications systems (i.e., isolators, circulators, etc.). All these properties and applications will be discussed in detail in Chapter 1, where the author presents recent advances in ferrite materials in various forms, focusing his review toward high-frequency properties and applications ranging from 0.1 to 100 GHz. A discussion is presented of the latest trends in processing, composition, theory, and the utility of ferrite films, crystals, compacts, metamaterials, and other unique heterostructures.

Magnetic data storage on disks and tapes has been the omnipresent technology for about 60 years already, and most likely it will keep this position in the future. Over the years, both magnetic data storage technologies have shown enormous growths due to constant innovations and technical breakthroughs. Hard disk drives are now able to record data with areal density that are larger by 8 orders of magnitude when compared to the first hard disk drives. Also magnetic tape recording has seen a steady growth though somewhat less impressive than that of hard disk drives. Notwithstanding the lower areal density, magnetic tape recording remains the technology preferred for archival data storage in the information technology industry. Its advantages include higher volumetric density, lower media costs, media removability, and high recording reliability. These features make magnetic tape recording ideally suited for applications not requiring rapid access. The most important components of magnetic recording devices are the recording media. For tapes, there are two competing technologies, namely, particulate media and metal evaporated media. The former consists of a thin polymer layer in which small magnetic particles are embedded. The metal evaporated media are obtained by the evaporation of magnetic alloys onto a plastic substrate in the presence of oxygen. Chapter 2 is devoted to such metal evaporated media. Metal evaporated media technology has been used in magnetic recording devices for more than 20 years now, and it has demonstrated some of the highest areal densities on tape. This is the result of a significant amount of research and development efforts that have been carried out by several groups around the world in the last 30 years. The authors of Chapter 2 review part of this work, with emphasis on the material and magnetic aspects of the metal evaporated media technology. A detailed description is given of the evaporation process leading to the production of obliquely evaporated media and their complex microstructures. The recording physics of tilted media is explained including the important role played by the medium recording asymmetry. Special attention is devoted to recording performance and media durability. The authors also review early work and very recent developments on perpendicular metal evaporated media.

It has been known for a long time already that some of the rare-earth metals possess extraordinary magnetostrictive properties. Technological application of these metals was, however, hampered by the fact that their Curie temperatures are below room temperature. As described in Chapter 7 of the first volume of this Handbook series, an intense search for both efficient and practical magnetostrictive materials led to the discovery of high magnetostriction in RFe2, the so-called Laves phase alloys. These alloys have the advantage that their Curie temperatures are far above room temperature. Eventually, the best performance was reached for Tb0.27Dy0.73Fe2 which became the principal magnetostrictive material employed in engineering applications. As described in Chapter 3 of this volume, continuous search for novel materials led in 1999 to the discovery of large magnetostrictive strains in iron-gallium alloys. Although the measured strains were lower than those found in the series of rare-earth alloys, some other advantageous properties of the newly discovered iron-gallium alloys have contributed much to expanding the applicability of magnetostrictive materials in modern sensor/actuator and energy harvesting industries. As described in detail in Chapter 3, included in those properties are high strains at moderate fields, high permeability, low hysteresis, ductility, and shock resistance. Compared to the rare-earth alloys, machining and welding can be done with ordinary techniques. Further, there is a substantial cost reduction associated with the absence of rare earths in these alloys which adds to their marketability. The authors give a detailed account of the physical principles forming the basis of their magnetostrictive properties. At the same time, they show that the understanding of structural aspects and phase relationships are of at least equal importance for reaching the highest performance.

The Mössbauer effect, discovered in 1958, has become a powerful hyperfine field tool not only in different branches of physics and chemistry but also in biology and geology. Nuclei embedded in solids can show recoilless absorption and emission of radiation. The bonding of the nuclei to the hosting solid results in quantization of the recoil energy, and therefore a part of the nuclei, the recoilless fraction, shows a zero recoil energy. This phenomenon made resonant absorption of nuclear radiation possible and opened the possibility of obtaining information on small variations in the nuclear levels caused by various types of interaction such as magnetic interactions due to the presence of magnetic ions in the solid or electric interactions originating from so-called crystal fields. In Chapter 4, a survey is presented of the results of rare-earth intermetallic compounds studied by rare-earth Mössbauer spectroscopy. The emphasis is mainly on the nuclei of 141Pr and 169Tm, but much attention is also paid to important nuclei such as 155Gd, 161Dy, 166Er, and 170Yb. This chapter includes a discussion of the different magnetic aspects that can be found in the various types of rare-earth compounds. Many examples are presented of how the various Mössbauer spectra can be analyzed in terms of hyperfine splitting and quadrupole splitting and how from the temperature dependences of these quantities experimental information can be obtained on the crystal-field splitting and the exchange splitting of the (2J + 1) fold degenerate ground manifold of the 4f electron system. Further elaboration of the data in terms of crystal-field theory and mean field models leads then to information on such important quantities as the crystal-field-induced magnetic anisotropy and...