Handbook of Magnetic Materials (eBook)

Preface to Volume 15

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 Materials 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 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 15 of this Handbook series.

Advanced ultra-high vacuum deposition methods, make it possible to manufacture highly perfect artificial layered magnetic materials. The investigations performed in the last two decades on nanometer-scale thin film and artificial multilayers with well defined layer thickness and interface flatness have led to the discovery of novel and most interesting effects. A general overview of the giant magnetoresistance effect in magnetic multilayers was already presented in chapter 1 of volume 12 of this Handbook. A prominent role among the layered magnetic materials is played by the so-called exchange biased spin valves and their excellent magnetoresistive properties. The advent of the exchange biased spin valves has led to many sensor applications, including those in hard disk read heads and applications in position and velocity sensors. In chapter 1 of the present volume, an application-oriented overview is presented of the extensive research efforts made on spin valves during the last decade. This overview includes work dealing with the magnetoresistance ratio, the thermal and field stability and the micromagnetic stability. The magnetic interactions and their interplay are discussed together with theoretical understanding and modeling of the magnetoresistance. Because of the high application relevance in spin valves and spin-electronic devices and because of the involved novel physics and materials science, special attention is paid to the exchange bias effect. Special emphasis is placed also on work dealing with magnetic tunnel junctions, which are presently considered as excellent candidates for storage elements in non-volatile magnetic random access memories.

A further novel field of interest in magnetism is that of transition metal nanostructures. It has largely kept pace with microelectronics, forming the core of information technology. Current research efforts include the preparation of thin films for improved data storage, the exploitation of the electron spin rather than its charge for device switching (“spintronics”), and the development of new materials for lightweight and low-cost applications. Generally, there has been a need for adequate theoretical descriptions able to explain most of the experimental phenomena and results. The many-body aspect of magnetic systems makes the task of calculating low-energy configurations of spin ensembles a formidable one. Because a full quantum mechanical description is actually intractable, various approximations have been used. The concentrated effort and the enthusiasm of a large number of scientists have resulted in an impressive display of new ideas and truly new discoveries. Theoretical work has already played and still plays a most important role in the process of active feedback between theories and experiments which has helped and speeded up the occurrence of novel accomplishments. Indeed, all magnetic properties of a solid are attributable to its electrons. In a free atom, there are two contributions to the magnetic moment. First, every electron has intrinsic spin and its associated magnetic moment. Second, there is the magnetic moment associated with the electron's orbital angular momentum. In a free atom these contributions are typically comparable in magnitude. For transition metals Hund's rules predict the ground state configuration, but the situation is quite different for solids in which a restricted number of these atoms have condensed into low-dimensional arrangements. In chapter 2 of the present volume a survey is given of the electronic structure of low-dimensional transition metals. It comprises not only thin films and multilayers but also clusters of transition metal atoms and nanowires. Results of novel experimental techniques are discussed hand in hand with theoretical approaches proposed to describe the electronic structure of these low-dimensional systems.

Diluted magnetic semiconductors (DMS) can be characterized as substitutional mixed crystals with some of the cations of the semiconductor host lattice replaced by magnetic ions such as Mn, Fe or Eu. These materials encompass a large number of different compounds. A review covering the field of bulk II–VI compounds has been presented already in chapter 4 of volume 7 of the Handbook. In the last decade many new materials (e.g., IV–VI compounds) have been investigated. This is true in particular for low-dimensional quantum structures based on diluted magnetic semiconductors. Therefore, the present chapter is a logical extension of the earlier chapter presented in volume 7. It gives an overview of the research activities on low-dimensional structures of II–VI diluted magnetic semiconductors with manganese, including new DMS materials in which the magnetic components are different from Mn. Special emphasis is put on results obtained with IV–VI materials containing a magnetic component. In a way it can be regarded as a complement to the chapter on III–V Ferromagnetic Semiconductors that has appeared in volume 14 of the Handbook in 2002. Chapter 3 of the present volume, like the chapter in volume 14, will be of use in particular to the numerous scientists who have recently been attracted to the field of DMS by the prospect of incorporating diluted magnetic semiconductors in “spintronic” devices including those for quantum information applications. The current interest in spintronics has given renewed impetus to studies of diluted magnetic materials. As a result, we are witnessing now a vast increase of the number of contributions to the field of DMS, which has led to a new type of topical conference (Physics and Applications of Spin Related Phenomena in Semiconductors, or PASPS) organized already twice (in Sendai in 2000, and in Würzburg in 2002).

High-Tc superconductors are prominent examples of novel materials that are not only interesting because of their surprisingly high superconducting transition temperatures but also because of their unusual magnetic properties and the interplay between antiferromagnetism and superconductivity. After the discovery of the first high-Tc superconductor La2−xBaxCuO4 by Bednorz and Müller in 1986 tremendous efforts have been spent world-wide in raising Tc even further and to interpret the rich phase diagrams displayed by the cuprates and nickelates for various doping levels. A thorough discussion of the two-dimensional antiferromagnetism of the cuprates was already presented in chapter 1 of volume 10. In chapter 4 of the present volume an account is given of the enormous progress made more recently. This is in particular true with regard to statical and dynamical stripes and the collective magnetic mode, the so-called resonance peak. Results of novel experimental techniques, like ARPES, STM and μSR are presented together with results obtained from neutron scattering, NMR and NQR. These and many other experimental result are discussed in the light of the corresponding theoretical framework.

Magnetotransport properties of materials have become of quite substantial importance in the competitive market of technological devices. This is true in particular for devices dealing with the storage and reading of information in magnetic recording media. In the last decade we have seen concentrated efforts to search for new giant magnetoresistive (GMR) materials and to fully uncover its origin. Nowadays, GMR based devices are already a reality in commercial hard disks, and they are responsible for a considerable increase in the recording areal density. A chapter on Giant Magnetoresistance in Magnetic Multilayers has appeared already in volume 12 of the Handbook. Of almost equal technological importance is the so-called giant magnetoimpedance effect, GMI. Initially, its observation and the concomitant research accomplishments were received with only modest enthusiasm, probably because of the envisaged modest technological expectations and an apparent lack of intrinsically new magnetic effects related to its origin. Nevertheless, it soon became clear that its interpretation requires a deep understanding of the micromagnetic characteristics of soft magnetic materials and its dependence on dynamic magnetism. With the vast increase of the number of scientists all over the world investigating GMI and its technological applications, GMI has actually opened a new branch of research linking micromagnetics of soft magnets with classical electrodynamics. From the applications...