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About this book

This book is based on the contributions to a course, entitled Applied Magnetism, which was the 25th Course of the International School of Materials Science and Technology. The Course was held as a NATO Advanced Study Institute at the Ettore Majorana Centre in Erice, Sicily, Italy between the 1st and 12th July 1992, and attracted almost 70 participants from 15 different countries. The book deals with the theory, experiments and applications of the main topical areas of applied magnetism. These selected areas include the physics of magnetic recording, magnetic and magneto-optic recording devices, systems and media, magnetic fine particles, magnetic separation, domains and domain walls in soft magnetic materials, permanent magnets, magnetoresistance, thin film magneto-optics, and finally, microwave, optical and computational magnetics. The material is organised into I 0 self-contained chapters which together provide a comprehensive coverage of the subject of applied magnetism. The aim is to emphasise the connection between the fundamental theoretical concepts, key experiments and the important technological developments which have been achieved in this field up to the present time. Moreover, when and where possible, pointers to future trends are indicated which hopefully, together with the background material, will promote further advancement of research. The organizing committee would like to acknowledge the sponsorship of the NATO Scientific Affairs Division, the National Science Foundation of the USA, the Science and Engineering Research Council of the UK, the Italian Ministry of Education, the Italian Ministry of University and Scientific Research and the Sicilian Regional Government.

Table of Contents

Frontmatter

The Physics of Magnetic Recording

Abstract
A review of the essential elements of magnetic recording is presented. Emphasis is placed on physical processes that occur in the writing of a data pattern onto magnetic media. The discussion here is focused on thin metallic films that are currently in use in high density disk recording. Nonetheless, concepts are presented in a fundamental manner so that applications to different materials or recording systems can easily be generalized. It is assumed that the reader has an introductory acquaintance with magnetism and magnetic materials [e.g. 1].
H. Neal Bertram

Magnetic Information Storage

Abstract
The advances in magnetic recording which have enabled storage density to increase by more than 50, 000 times in 35 years are described. The fundamental principles of magnetic and magneto-optic recording are discussed. Current implementations of these technologies are presented, and future trends are projected. Storage densities of 1 Gbit/in2 are likely before the year 2000, and storage densities of 10 Gbit/in2 are expected in the first decade of the next century.
Mark H. Kryder

The Magnetic Properties of Fine Particles

Abstract
Magnetic fine particles find a large number of applications, perhaps most notably as particulate recording media. In addition, the ideas of fine particle magnetism are applicable in a number of areas as diverse as geomagnetism and biomagnetism. The purpose of this chapter is to introduce the fundamental aspects of fine particle magnets, including magnetisation reversal phenomena, interaction effects and time dependence, and to describe applications to geomagnetism and recording media.
R. W. Chantrell, K. O’Grady

Magnetic Separation

Abstract
Magnetic separation is based upon a competition between a number of forces. To explain its principle [1, 2] it is convenient to refer to the diagram shown in Fig. l.
Richard Gerber

Domains and Domain Walls in Soft Magnetic Materials, Mostly

Abstract
These lecture notes aim at introducing the main concepts underlying magnetic domain and wall physics. Emphasis is laid on soft magnetic materials, i.e. materials which, in the bulk state, are essentially magnetically isotropic, although most of the phenomenology remains general. Besides, magnetization dynamics and its consequences on domain wall state transformations are not treated in this chapter. The text is divided into nine sections. Section 2 depicts the various energy terms to be considered within the micromagnetic approach which treats the magnetization as a continuous Maxwell field whereas energy minimization in the general case is treated in Section 5, leading to the basic static equilibrium equations governing the distribution of magnetization inside a magnetic body. The question of the motive force behind decomposition into domains and domain geometry is addressed in Sections 4, 6 and 7. The former briefly pinpoints the magnetostatic energy reduction stemming from a decomposition of the magnetized volume into domains. The latter describes in detail the domain configuration to be expected from anisotropy-free soft magnetic elements with a basically two-dimensional in-plane magnetization distribution, and offers several illustrations of the elegant geometrical construction due to Van den Berg. It also provides an introduction to quasi-singular magnetization distributions in soft ferromagnets. Section 6, devoted to the nucleation of stripe domains, provides one example of the use of the general equilibrium equations in the vicinity of saturation. The rather involved issue of domain wall structures is considered in Sections 8, 9 and 10. A comprehensive treatment of one-dimensional wall structures expected to occur in bulk materials will be found in the first of these three sections, whereas the second is concerned with those specific domain wall structures to be found at intermediate thicknesses and in the half-space. Wall sub-structures, namely lines, are briefly alluded to. Section 10 is also devoted to the fast growing field of magnetic artificial structures with dipolar and exchange inter-layer interactions. Lastly, intrinsic coercivity mechanisms are described in Sections 3 and 11, starting with Stoner-Wohlfarth hysteresis in thin films and ending with some manifestations of the more intricate topological hysteresis, i.e. those hysteretic phenomena associated with noncontinuous transformations of the magnetization distribution.
J. Miltat

Permanent Magnets

Abstract
This is an overview of the present status of permanent magnet technology with special emphasis on the rare-earth intermetallic compounds, which represent the most advanced development in this area. The first part deals with the general principles of magnetism that form the basis of permanent magnet techniques and applications, focusing attention on the stored energy and the energy exchanged with the external world. Important intrinsic properties of hard magnetic materials for the manufacture of permanent magnets are an intense saturation magnetization, a high Curie temperature and a strong magnetic anisotropy. The latter is a key property in the development of coercivity, a necessary condition for a ferromagnetic material to be useful as a permanent magnet. Modern high energy density materials (rare-earth intermetallics) are especially characterized by a very elevated coercive field, and show an improvement of an order of magnitude in the energy product (BH)max compared to traditional high moment materials (Alnico alloys). Besides the SmCo5 and NdFeB type compounds, the most recent achievements in this class of materials include interstitial 2:17 compounds, which at present suffer from the difficulty of achieving sufficient densification and stability. However, application of these compounds to bonded type materials seems to be very promising. Other possibilities are presented by a series of different preparation routes, among which rapid solidification methods offer a wide range of opportunities. In this category, increasing attention is being paid to special nanostructured materials because of the possibility of obtaining enhanced remanence by profiting from a favorable exchange interaction between adjacent grains.
G. Asti, M. Solzi

Magnetoresistance

Abstract
The phenomenon of magnetoresistance has been known for more than a century but it has only been found to be useful in engineering application in more recent times.
D. J. Mapps

Thin Film Magneto-Optics

Abstract
In this chapter an account will be given of how electromagnetic radiation interacts with magnetic films either deposited singly on a substrate or as part of a multilayer structure designed to enhance the interaction. The account will be mainly at a phenomenological and macroscopic level, although the atomic origins of the macroscopic magneto-optical constants of materials will be considered in general terms in connection with theories of dispersion. The relevance of well-established principles of thin film optics will be emphasised with specific reference to the topical application of magneto-optical interactions in the “read-out” process of magneto-optical recording.
P. H. Lissberger

Microwave and Optical Magnetics

Abstract
An overview of microwave and optical magnetics is given with emphasis on physical models for the magnetic susceptibility, the properties of electromagnetic waves in ferrites, the magnetostatic approximation, and devices based upon dynamic and magnetostatic modes.
Particular attention is given to the microstrip circulator as an example of one of the most widely used ferrite devices based on dynamic modes. Field theory as well as S-parameter treatments are given.
Finally, the theory and applications of magnetostatic waves in thin films are discussed. A particularly promising area of research is the interaction of magnetostatic waves with optical guided modes. This interaction could lead to a new class of optical and microwave signal processing devices.
Daniel D. Stancil

A Scientific Basis for Computational Magnetics

Abstract
Throughout this chapter the notation for fields and field variables generally adheres to the IEEE recommended standards. Rationalized MKS units are assumed so that in all equations, ∈0 represents the permeability, μ0 the permittivity, of free space.
Peter P. Silvester

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