Preface to Volume 22

K.H.J. Buschow    Van der Waals-Zeeman Institute, University of Amsterdam

The Handbook series Magnetic Materials is a continuation of the Handbook series 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 Materi als was initially chosen as title for the Handbook series, although the latter aimed at giving a more complete cross section of magnetism than Bozorth’s book. In the last 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 the publisher of this Handbook series have carefully reconsidered the title of the Handbook series and changed it into Magnetic Materials. It is with much pleasure that I can introduce to you now Volume 22 of this Handbook series.

The perovskite manganites are mixed-metal oxide compounds that form in the comparatively well-known cubic (or pseudocubic) perovskite crystal structure. The latter can be represented by the general formula ABO3, one of its main features being the possibility of substitution on the A site (ideally 12-coordinated by oxide ions) as well as on the Mn site (6-coordinated). The ferromagnetic mixed-valence perovskite manganites of the type Ln1 − xMxMnO3 (where Ln are rare-earth atoms and M alkaline earth atoms) form a subgroup that has attracted quite substantial research activities during the last few decades. One of the reasons is that these compounds display a metal–insulator transition, the corresponding transition temperature being intimately coupled to their Curie temperature. Under such circumstances, the application of an external magnetic field leads to a strong enhancement of the electrical conductivity near the metal to insulator, giving rise to the so-called colossal magnetoresistance. The substitutional freedom in the Ln1 − xMxMnO3 compounds and the concomitant manganese mixed-valence state are able to generate a plethora of magnetic ground states. The latter is the result of the competition between the magnetic interactions mediated by the itinerant charge carriers, comprising the double-exchange interaction and the superexchange interactions between the localized spins of manganese ions. The fact that also Jahn–Teller distortions and lattice and orbital degrees of freedom play an important role has made the Ln1 − xMxMnO3 compounds an unequalled reservoir of materials that can serve as testing ground in modern solid-state physics for models dealing with complex and interrelated interactions. In Chapter 1 of this volume, the authors review recent accomplishments made in the field of perovskite manganites, discussing novel experimental results in the framework of existing theoretical models. A detailed discussion is presented of magnetocaloric effects in manganites, showing that the substitutional freedom of the manganites turns out as a useful way of tuning the optimum magnetocaloric response. Special emphasis is given to pressure effects, which via modification of the unit cell volume and the local structure can substantially modify the magnetic properties of the manganites. Finally, the authors discuss the changes of the magnetic properties of manganite when the size of the magnetic particles is reduced to the nanometer scale, showing that the spontaneous magnetization, the magnetic transition temperature, and coercivity generally can differ significantly from the bulk values.

Designs of refrigeration systems involve many problems which often are of a complex nature. Such designs invariably require a critical evaluation of possible solutions by considering factors such as economics, safety, reliability, and environmental impact. The vapor compression cycle still dominates the refrigeration market to date because of its many advantages comprising high efficiency, low toxicity, low cost, and simple mechanical embodiments. However, at the end of the last century, environmental aspects have become an increasingly important issue in the design and development of refrigeration systems. Especially in vapor compression systems, the banning of CFCs and HCFCs because of their environmental disadvantages has opened the way for other refrigeration technologies which in the past have been largely ignored by the refrigeration market. As environmental concerns grow, alternative technologies which use either inert gasses or no fluid at all have become attractive solutions to the environment problem. A significant part of the refrigeration industry R&D expenditures worldwide is now oriented toward the development of such alternative technologies in order to be able achieve replacement of vapor compression systems in a mid- to long-term perspective. One of these alternatives is magnetic refrigeration based on magnetocaloric effects. Magnetocaloric effects in the vicinity of phase transitions were already discussed by A.M. Tishin in Volume 12 of this Handbook, published in 1999. Since then, much effort has been spent in research on magnetocaloric materials and their application, mainly dealing with the option of magnetocaloric refrigeration at ambient temperature. A comprehensive review dealing with this latter aspect was presented by E. Brück in Chapter 4 of Volume 17, published in 2008. The last few years have seen a strong proliferation in the research activities on magnetocaloric materials which is reviewed in Chapter 2 of this volume, concentrating mainly on achievements reached on alloys and intermetallic compounds in the period after 2008. This chapter is self-contained where the author emphasizes the many novel experimental results obtained on metallic magnetocaloric materials, placing them in the proper physical and thermodynamic background. Also, materials for gas liquefaction and magnetic regenerators are discussed.

Over the years, magnetic tape recording has seen a steady growth though being somewhat less impressive than that of hard disc 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. Looking at the cost advantages of tape over other data storage media and taking into account the increasing usability of tape provided by advances such as long-term file systems, tape will likely play an important role in the rapidly expanding market for archival data storage solutions. It is still an open question how important this role really will be and how long it will last. It will be clear that this largely depends on the continued scaling of tape to higher areal densities at a constant cost so that the cost per Gbyte advantage of tape over other technologies either remains constant or increases. For tapes, there are two competing technologies namely particulate media and metal evaporated media. The metal evaporated media are obtained by the evaporation of magnetic alloys onto a plastic substrate in the presence of oxygen, and these media were extensively described in Chapter 2 of Volume 20 of this Handbook. The particulate media consist of a thin polymer layer in which small magnetic particles are embedded. Chapter 3 of this Handbook volume is devoted to such particulate media. In this chapter, the authors explore the future scaling potential of magnetic tape technology based on low-cost particulate media. In the first section of Chapter 3, the authors discuss the state of the art of particulate media. In the following section, the authors review the accomplishments and the technologies behind the recent 29.5 Gbit/in2 areal density demonstration using low-cost particulate barium ferrite media. In the third section, a critical assessment is made as to the potential for further scaling, posing the questions what are the limits of this scaling and what technologies would be required to reach them. A brief summary and conclusions are presented in the final section.

Superconductivity in intermetallics composed of layers of tetrahedrally coordinated iron atoms was discovered only recently, in 2006. Since then, numerous investigations have focused on structurally and chemically related compounds. The proliferation of the research activity in this field led to the discovery of new superconducting compounds, some of them showing enhanced superconducting properties that often were reached by chemical substitution. In analogy with the cuprate high-temperature superconductors, there exists a competition between superconductivity and magnetism in these layered iron compounds. The origin of superconductivity in the layered iron compounds is the presence of extended, two-dimensional iron layers in their crystal structure. These layers consist of edge-sharing tetrahedra where the iron atoms are accommodated at the centers and where the pnictogen or chalcogen atoms are located at the vertices of the tetrahedra. In some of the layered...