Applied Magnetism

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].

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.

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.

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.

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.

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.

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.

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.

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.

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.