This book represents Volume 2 in a series on the use of Mossbauer spectroscopy in the study of magnetism and materials. However, the perceptive reader will notice some differences from Volume 1. Specifically, in order to market the book at a more affordable price for most universities and research laboratories, the book has been prepared in camera ready format The editors and the authors agreed to do this because there is a demand for such a book in the Mossbauer community. This format has placed an extra burden on the editors and the authors and we hope we have overcome all the difficulties generated by the transfer of files between different computers. In order to make the book more attractive to materials scientists who are not experts in Mossbauer spectroscopy, this volume is particularly oriented towards the study of materials by Mossbauer spectroscopy and related complementary techniques, such as neutron scattering and a variety of surface scattering techniques. The authors of this volume can be proud of the high quality professional effort they have devoted to clearly presenting their specific topics. As a result we very much enjoyed working with the authors on this volume. We hope that their effort will help to educate the next generation of Mossbauer effect spectroscopists, a generation which will face the challenge of maintaining equally high scientific and professional standards in their research work."

The book you are now holding represents the final step in a long process for the editors and organizers of the Advanced Study Institute on hard magnetic materials. The editors interest in hard magnetic materials began in 1985 with an attempt to better understand the moments associated with the different iron sites in Nd Fe B. These 14 moments can be obtained from neutron diffraction studies, but we qUickly realized that iron-57 Mossbauer spectroscopy should lead to a better determination of these moments. However, it was also realized that the complex Mossbauer spectra obtained for these hard magnetic materials could not be easily understood without a broad knowledge of their various structural, electronic, and magnetic properties. Hence it seemed useful to the editors to bring together scientists and engineers to discuss, in a tutorial setting, the various properties of these and future hard magnetic materials. We believe the inclusion of engineers as well as scientists in these discussions was essential because the design of new magnetic materials depends very much upon the mode in which they are used in practical devices.

In 1988 the Mossbauer effect community completed 30 years of continual contribution to the fields of nuclear physics, solid state science, and a variety of related disciplines. To celebrate this anniversary, Professor Gonser of the Universitat des Saarlandes has contributed a chapter to this volume on the history of the effect. Although Mossbauer spectroscopy has reached its mature years, the chapters in this volume illustrate that it is still a dynamic field of science with applications to topics ranging from permanent magnets to biologi cal mineralization. During the discussion of a possible chapter for this volume, a potential author asked, "Do we really need another Mossbauer book?" The editors responded in the affirmative because they believe that a volume of this type offers several advantages. First, it provides the author with an opportunity to write a personal view of the subject, either with or without extensive pedagogic content. Second, there is no artificially imposed restriction on length. In response to the question, "How long should my chapter be?," we have responded that it should be as long as is necessary to clearly present, explain, and evaluate the topic. In this type of book, it is not necessary to condense the topic into two, four, or eight pages as is now so often a requirement for publication in the research literature."

During the past 30 years materials science has developed into a full-fledged field for basic and applied scientific enquiry. Indeed, materials scientists have devoted their efforts to creating new materials with improved electronic, magnetic, thermal, mechanical, and optical properties. Often unnoticed, these new materials are rapidly invading our homes and automobiles, and may be found in our utensils, electronic equipment, textiles, home appliances, and electric motors. Even though they may go unnoticed, these new materials have either improved the efficiency and lifetime of these items or have reduced their weight or cost. In particular, magnetically ordered materials are useful in various applications, such as motors, magnetic imaging, magnetic recording, and magnetic levitation. Hence, much effort has been devoted to the development of better hard magnetic materials, magnetic thin films, and molecular magnets. During the same period of time, Mossbauer-effect spectroscopy has grown from a laboratory curiosity to a mature spectroscopic technique, a technique that probes solid-state materials at specific atomic sites and yields microscopic information on the magnetic and electronic properties of these materials. Iron-57 is the most commonly and easily used Mossbauer-effect isotope and, of course, is particularly relevant for the study of magnetic materials. Various applications of Mossbauer spectroscopy to magnetic materials are discussed in the first six chapters of this volume. Other isotopes such as zinc-67 and gadolinium-ISS have recently been used to study the electronic properties of zinc compounds and the electronic and magnetic properties of rare-earth transition metal compounds.

This book represents Volume 2 in a series on the use of Mossbauer spectroscopy in the study of magnetism and materials. However, the perceptive reader will notice some differences from Volume 1. Specifically, in order to market the book at a more affordable price for most universities and research laboratories, the book has been prepared in camera ready format The editors and the authors agreed to do this because there is a demand for such a book in the Mossbauer community. This format has placed an extra burden on the editors and the authors and we hope we have overcome all the difficulties generated by the transfer of files between different computers. In order to make the book more attractive to materials scientists who are not experts in Mossbauer spectroscopy, this volume is particularly oriented towards the study of materials by Mossbauer spectroscopy and related complementary techniques, such as neutron scattering and a variety of surface scattering techniques. The authors of this volume can be proud of the high quality professional effort they have devoted to clearly presenting their specific topics. As a result we very much enjoyed working with the authors on this volume. We hope that their effort will help to educate the next generation of Mossbauer effect spectroscopists, a generation which will face the challenge of maintaining equally high scientific and professional standards in their research work.

In 1988 the Mossbauer effect community completed 30 years of continual contribution to the fields of nuclear physics, solid state science, and a variety of related disciplines. To celebrate this anniversary, Professor Gonser of the Universitat des Saarlandes has contributed a chapter to this volume on the history of the effect. Although Mossbauer spectroscopy has reached its mature years, the chapters in this volume illustrate that it is still a dynamic field of science with applications to topics ranging from permanent magnets to biologi cal mineralization. During the discussion of a possible chapter for this volume, a potential author asked, "Do we really need another Mossbauer book?" The editors responded in the affirmative because they believe that a volume of this type offers several advantages. First, it provides the author with an opportunity to write a personal view of the subject, either with or without extensive pedagogic content. Second, there is no artificially imposed restriction on length. In response to the question, "How long should my chapter be?," we have responded that it should be as long as is necessary to clearly present, explain, and evaluate the topic. In this type of book, it is not necessary to condense the topic into two, four, or eight pages as is now so often a requirement for publication in the research literature."

High density digital magnetic and magneto-optical storage devices are widely used in audio, video, and data processing information technology, as well as in CAD/CAM computer systems. These widespread uses generate a continually increasing demand for both increased information storage densities and capacities, and for reduced access times. Hence, the materials engineering of high density storage media, with a high signal to noise ratio, and the associated design of sophisticated read and write heads, form the basis of major technological research. This research is especially complex because, ideally, the recorded information should be both erasable and, at the same time, secure and accessible over periods of many decades. As a result, research on these complex problems requires a multidisciplinary approach which utilizes the expertise in such widely differing fields as organic, inorganic, and solid state chemistry, metallurgy, solid state physics, electrical and mechanical engineering, and systems analysis. Often, further research specialization is necessary in each of these different disciplines. For instance, solid state physics and chemistry address the problems of crystallographic structure and phase diagram determination, magnetism, and optics, but more advanced research methods, such as high resolution electron microscopy and electronic band structure calculations, are necessary to understand the microstructure of particulate recording media or the electronic spectra of magneto-optical recording media.

It is well known that the density of molecular hydrogen can be increased by compression and/or cooling, the ultimate limit in density being that of liquid hydrogen. It is less well known that hydrogen densities of twice that of liquid hydrogen can be obtained by intercalating hydrogen gas into metals. The explanation of this unusual paradox is that the absorption of molecular hydrogen, which in TiFe and LaNis is reversible and occurs at ambient temperature and pressure, involves the formation of hydrogen atoms at the surface of a metal. The adsorbed hydrogen atom then donates its electron to the metal conduction band and migrates into the metal as the much smaller proton. These protons are easily accomodated in interstitial sites in the metal lattice, and the resulting metal hydrides can be thought of as compounds formed by the reaction of hydrogen with metals, alloys, and intermetallic compounds. The practical applications of metal hydrides span a wide range of technologies, a range which may be subdivided on the basis of the hydride property on which the application is based. The capacity of the metal hydrides for hydrogen absorption is the basis for batteries as well as for hydrogen storage, gettering, and purification. The temperature-pressure characteristics of metal hydrides are the basis for hydrogen compressors, sensors, and actuators. The latent heat of the hydride formation is the basis for heat storage, heat pumps, and refrigerators.

About two years ago, while studying the dynamic properties of Fe (CO)IJ, we realized that there 3 was virtually no single source of infonnation on the structural dynamics of materials. The time domain of different dynamic structural processes covers many orders of magnitude and may be in vestigated by numerous, vastly different, experimental techniques. Indeed, the subject seemed ap propriate for a NATO Advanced Study Institute at which we could bring together chemists, physicists, metallurgists, and bioscientists using the various techniques for the study of sundry time sensitive materials. The actual Advanced Study Institute, which met in II Ciocco, Italy, from 14 to 26 June 1987, was, in fact, a dynamic experience for those of us involved. Now we have come to the final phase, the communication of the results of this Advanced Study Institute to the general scientific community. In so doing, we hope to provide in one place a convenient source of infor mation on dynamics at the surface and within a solid state material. The beautiful mountainous setting of Tuscany and especially the idyllic surroundings of II Ciocco provided an ideal venue for the Advanced Study Institute. Our field trip to Pisa linked our topic to the history of time measurement through a visit to the Pisa cathedral where, presumably, Galileo first conceived the isochronism of the simple pendulum and its use in time measurement."