Handbook of Magnetic Materials, Volume 3 North-Holland Publishing Company, 1982 Edited by: E.P Wohlfarth ISBN: 978-0-444-86378-2 by kmno4 PREFACE This Handbook on the Properties of Magnetically Ordered Substances, Ferro- magnetic Materials, is intended as a comprehensive work of reference and textbook at the same time. As such it aims to encompass the achievements both of earlier compilations of tables and of earlier monographs. In fact, one aim of those who have helped to prepare this work has been to produce a worthy successor to Bozorth's classical and monumental book on Ferromagnetism, published some 30 years ago. This older book contained a mass of information, some of which is still valuable and which has been used very widely as a work of reference. It also contained in its text a remarkably broad coverage of the scientific and tech- nological background. One man can no longer prepare a work of this nature and the only possibility was to produce several edited volumes containing review articles. The authors of these articles were intended to be those who are still active in research and development and sufficiently devoted to their calling and to their fellow scientists and technologists to be prepared to engage in the heavy tasks facing them. The reader and user of the Handbook will have to judge as to the success of the choice made. Each author had before him the task of producing a description of material properties in graphical and tabular form in a broad background of discussion of the physics, chemistry, metallurgy, structure and, to a lesser extent, engineering aspects of these properties. In this way, it was hoped to produce the required combined comprehensive work of reference and textbook. The success of the work will be judged perhaps more on the former than on the latter aspect. Ferromagnetic materials are used in remarkably many technological fields, but those engaged on research and development in this fascinating subject often feel themselves as if in strife for superiority against an opposition based on other physical phenomena such as semiconductivity. Let the present Handbook be a suitable and effective weapon in this strife! The publication of Volumes 1 and 2 took place in 1980 and produced entirely satisfactory results. Many of the articles have already been widely quoted in the scientific literature as giving authoritative accounts of the modern status of the iv PREFACE subject. One book reviewer paid us the compliment of calling the work a champion although with the proviso that the remaining two volumes be published within a reasonable time. The present Volume 3 goes halfway towards this event and contains articles on a variety of subjects. There is a certain degree of coherence in the topics treated here but this is not ideal due to the somewhat random arrival of articles. The same will be the case for the remaining Volume 4 as such, although this will then complete the work so as to finally produce a fully coherent account of all aspects of this subject. Three of the authors of Volume 3 are members of the Philips Research Laboratories, Eindhoven and, as already noted in the Preface to Volumes 1 and 2, this organization has been of immense help in making this enterprise possible. The North-Holland Publishing Company has continued to bring its profes- sionalism to bear on this project and Dr. W. Montgomery, in particular, has been of the greatest help with Volume 3. Finally, I would like to thank all the authors of Volume 3 for their co-operation, with the profoundest hope that those of Volume 4 will shortly do likewise! E.P. Wohlfarth Imperial College TABLE OF CONTENTS Preface . . . . . . . . . . . . . . . . . . . . . . v Table of Contents . . . . . . . . . . . . . . . . . . vii List of Contributors . . . . . . . . . . . . . . . . . . ix 1. Magnetism and Magnetic Materials: Historical Developments and Present Role in Industry and Technology U. ENZ . . . . . . . . . . . . . . . . . . . . . 1 2. Permanent Magnets; Theory H. ZIJLSTRA . . . . . . . . . . . . . . . . . . 37 3. The Structure and Properties of Alnico Permanent Magnet Alloys R.A. McCURRIE . . . . . . . . . . . . . . . . . . 107 4. Oxide Spinels S. KRUPICKA and P. NOVAK . . . . . . . . . . . . 189 5. Fundamental Properties of Hexagonal Ferrites with Magnetoplumbite Structure H. KOJIMA . . . . . . . . . . . . . . . . . . . 305 6. Properties of Ferroxplana-Type Hexagonal Ferrites M. SUGIMOTO . . . . . . . . . . . . . . . . . . 393 7. Hard Ferrites and Plastoferrites H. STJid~LEIN . . . . . . . . . . . . . . . . . . 441 8. Sulphospinels R.P. VAN STAPELE . . . . . . . . . . . . . . . . . 603 9. Transport Properties of Ferromagnets I.A. CAMPBELL and A. FERT . . . . . . . . . . . . 747 Author Index . . . . . . . . . . . . . . . . . . . . 805 Subject Index . . . . . . . . . . . . . . . . . . . . 833 Materials Index . . . . . . . . . . . . . . . . . . . 845 vii chapter 1 MAGNETISM AND MAGNETIC MATERIALS" HISTORICAL DEVELOPMENTS AND PRESENT ROLE NI INDUSTRY AND TECHNOLOGY U. ENZ Philips Research Laboratories Eindhoven The Netherlands Ferromagnetic Materials, Vol. 3 Edited by E.P. Wohlfarth © North-Holland Publishing Company, 1982 CONTENTS Introduction 3 1. From lodestone to ferrite: a survey of the history of magnetism . . . . . . . . . 3 2. The role of magnetism in present-day technology and industry . . . . . . . . . . 6 3. Development of some classes of magnetic materials . . . . . . . . . . . . . . 10 3.1. Iron-silicon alloys . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2. Ferrites . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.3. Garnets . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.4. Permanent magnets . . . . . . . . . . . . . . . . . . . . . . . 24 4. Trends in magnetism research and technology . . . . . . . . . . . . . . . . 30 4.1. Magnetism research between physics, chemistry and electronics . . . . . . . . 30 4.2. Trends in applied magnetism . . . . . . . . . . . . . . . . . . . . 31 4.3. Outlook and acknowledgement . . . . . . . . . . . . . . . . . : . 34 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Introduction In this contribution we attempt to trace a few main developments of the history of magnetism and to give an account of the present role of ferromagnetic materials in industry and technology. The treatment of a subject as broad as the present one must necessarily be limited and incomplete; nevertheless, we may give an im- pression how the large body of knowledge on magnetism accumulated in the past, and how important it is at present. The first section gives a short sketch of some early historical developments and inventions. Such a flash back to history may be useful to place the modern activities and achievements in a wider context. The next section deals with the role of magnetism and magnetic materials in modern technology, especially in the context of power generation and distribution, tele- communication and data storage. Some statistical figures on the economic significance of magnetic materials are included. The third-section gives a some- what more detailed account of the development lines~i~f a few selected classes of materials, whereas in the last section an attempt is made to indicate the trends in applied magnetism. 1. From lodestone to ferrite: a survey of the history of magnetism The notion of magnetism dates back to the Ancient World, where magnets were known in the form of lodestone, consisting of the ore magnetite. The name of the ore, and hence that of the whole science of magnetism, is said to be derived from the Greek province of Magnesia in Thessaly, where magnetite was found as a natural mineral. It seems very likely that the early observers were fascinated by the attractive and repulsive force between lodestones. Thales of Miletus (624-547 BC) reports that the interaction at a distance between magnets was known before 800 BC. Another, probably more apocryphal account is due to Pliny the Elder, who ascribes the name magnet to its discoverer, the shepherd Magnes "the nails of whose shoes and the tip of whose staff stuck fast in a magnetic field while he pastored his flocks". From such modest beginnings grew the science of mag- netism, which may be represented as a tree on whose growing trunk new shoots and branches continuously appeared (see fig. 1). The trunk represents the mag- 4 u. ENZ netic materials such as metals, alloys or oxides, because history shows that the use and study of materials have been the main sources of discoveries and progress. The new branches which developed in the course of time formed scientific fields in themselves. A brief account of some of these developments is given in the following pages (Encyclopedia Britannica: Magnetism; see also, Mattis (1965)). Magnets found their first application in compasses, which were made from a lodestone buoyant on a disc of cork. We know that the compass was used by Vikings and, of course, by Columbus, but the art of navigation guided by compasses may be much older. The invention is probably of Italian or Arabic origin. The earliest extant European reference to the compass is attributed to the English scholar Alexander Neckam (died 1217). The influence of this simple device was far-reaching in every respect: it made it possible to navigate on the high seas. The principle of the compass has remained unchanged, the device is still in full use. The invention of the compass is characteristic of many later develop- ments in magnetism: seemingly marginal effects turned out to be very important and to have had a tremendous impact on later technological developments. A milestone in the history of magnetism was William Gilbert's De Magnete, electro- high neutron- critical materials spin structures leo- magnetic fields diffr, phenom metals semi- amorphous micromagnetism cam netism radiation ~oitI cudni , tnemommagn. M6ssbauer R~IN; para- alloys ;sp;e'Ig° ne i cond. mogn. J I ss~lsgpin I I doI mains bb,es '~ stellar ;mogn. magnetism or ~ ?F Curi~ J I alloys ~- I oxides L ~ ~ Maxwell I Faraday, ,ere~A 180Or~ ert' tengoeml[2rdet e 1269 Peregrinus de Maricourt J 1200 Neckarn describes compass J J 008 b ~ r l d : magnesian stones .giF .1 Development of the modern branches of magnetism from a common root. A few names and dates are indicated to mark some otfh e most crucial moments in this development. The modern sdleif of magnetism, ranging from basic entities like magnetic fields and particles to more complex ensembles, emanate in quite a straightforward way from a few basic branches. A central position si reserved to the various classes of materials, reflecting the central position of materials in magnetism research. MAGNETISM AND MAGNETIC MATERIALS 5 Magneticisque ,subiroproC et de Magno Magnete Tellure (1600, "Concerning Magnetism, Magnetic Bodies and the Great Magnet Earth") which summarized all the available knowledge of magnetism up to that time, notably that of Petrus Peregrinus de Maricour (1269). In addition Gilbert describes his own experiments: he measured the direction of the magnetic field and its strength around spheres of magnetite with the aid of small compass needles. For this purpose he introduced notions like magnetic poles and lines of force. Gilbert found that the distribution of the magnetic field on the surface of his sphere or terella ("microworld") was much like that of the earth as a whole and concluded that the earth is a giant magnet with its two magnetic poles situated in regions near the geographical poles. This observation made him the founder of geomagnetism. Gilbert's work not only strongly influenced the later development of magnetism, but also contributed to the development of the idea of universal gravitation: it was believed, for some period of time before Newton, that the planets were held in their orbits by magnetic forces in some form or other. Gilbert also discovered that lodestone, when heated to bright red heat, loses its magnetic properties, but regains them on cooling. In this way he anticipated the existence of the Curie temperature. For more than two centuries after Gilbert little progress was made in the understanding of magnetism, and its origin remained a mystery. The early nineteenth century marked the beginning of a series of major contributions. Hans Christian Orsted discovered in 1820 that an electric current flowing in a wire affected a nearby magnet. Andr6 Marie Amp6re established quantitative laws of the magnetic force between electric currents and demon- strated the equivalence of the field of a bar magnet and that of a current-carrying coil. Michael Faraday discovered magnetic induction in 1831, his most celebrated achievement, and introduced the concept of the magnetic field as an independent physical entity. Guided by his feeling for symmetry and harmony he suspected that an influence of magnetic fields on electric conduction should exist as a counterpart to Orsted's magnetic action of currents. After a long period of unsuccessful experiments with static fields and stationary magnets, he discovered the induction effects of changing fields and moving magnets. This line of in- vestigation culminated in Maxwell's equations, establishing the synthesis between electric and magnetic fields. Progress in the understanding of the microscopic origin of magnetism was initiated by Amp6re, who suggested that internal electric currents circulating on a molecular scale were responsible for the magnetic moment of a ferromagnetic material. Amp6re's hypothesis enabled Wilhelm Eduard Weber to explain how a substance may be in an unmagnetized state when the molecular magnets point in random directions, and how they are oriented by the action of an external field. This idea also explained the occurrence of saturation of a magnetic material, a state reached when all elementary magnets are oriented parallel to the applied field. This line of thinking led to the studies of Pierre Curie and Paul Langevin on paramagnetic substances, and also to the work of Pierre Weiss (1907) on ferro- magnetic materials. Pierre Curie described the paramagnetic substances as an en- 6 u. ZNE semble of uncoupled elementary magnetic dipoles subjected to thermal agita- tion, the orienting action of the external field being counteracted by the thermal agitation. Such a description was also applied successfully to ferromagnetic materials at temperatures higher than the Curie temperature. The modern dis- cipline of critical phenomena is, for the time being, the end point of this branch. Weiss, in turn, postulated the existence of a hypothetical internal magnetic field of great strength in ferromagnets, resulting in a spontaneous magnetization even in the absence of an external field. Amongst his other contributions is the notion of a magnetic domain, a small saturated region inside a ferromagnet, and the notion of domain walls. Weiss's work can be viewed as the starting point of the branch leading to the modern disciplines of micromagnetism and domain theory, and also as the point of departure of N6el's work on interactions, leading to the fields of ferrimagnetism and antiferromagnetism, including the actual disciplines of spin structures and spin glasses. A branch of its own, the study of the magnetic aspects of particles is perhaps a less obvious offshoot from the common source, but it nevertheless forms a very important part of magnetism. Indeed fields like electron spin resonance and nuclear spin resonance, M6ssbauer spectroscopy and structure analysis by neutron diffraction are at the same time indispensable tools and important disciplines of magnetism. The modern disciplines of magnetism as represented at the top of fig. 1 range from the fundamental entities like fields and particles on the left to more complex systems on the right. Critical phenomena, magnetic phase diagrams and spin structures in various materials including spin glasses are important fields of modern research. The classical discipline of domains, domain walls and micromagnetism is still being actively studied, and has even received renewed attention stimulated by the modern investigations on bubbles. The various materials appear in the centre of the three, thus confirming their central role in magnetism. The few materials that are named represent just a very small fraction of the magnetic materials known at the present time. The study of the magnetic properties of materials is the subject of the present handbook, and the present article is intended to give a general introduction to the remaining chapters of this work. 2. The role of magnetism in present-day technology and industry Having outlined the early developments of magnetism as well as the subsequent accumulation of knowledge on magnetic phenomena and materials, we now turn to the description of the role of magnetism in present-day technology and industry. Magnetic materials occupy a key position in many essential areas of interest to society. The most important of these, which depend in an essential way on magnetic materials, are the generation and distribution of electrical power, the storage and processing of information, and of course communication in all its forms, including telephony, radio and television. Apart from these major fields, many other industrial machines and devices, including motors for numerous applications, depend on magnetic materials or magnetic forces. Figure 2 gives a