Nanomagnetism (eBook)

Volume Preface

During the past two decades, we have witnessed marvelous advances in our ability to synthesize nanoscale structures of all sorts, as well as the development of novel experimental methods that allow us to explore their physical properties. This is exciting for two reasons. First, new forms of matter with no counterpart in nature have been fabricated, and these have unique physical properties not found in bulk materials. This is so because either a large fraction of their atomic constituents reside in surface or interface sites of low symmetry, or their physical size is so small they are completely quantum dominated. Second, we have now realized nanostructures that open new avenues for the development of very small devices. This has already had a remarkable, qualitative impact on technology, as we will see from remarks in the next paragraph. It is our view that soon nanoscience and the nanotechnology derived from it will have an impact on human affairs comparable to the industrial revolution, when combined with the submicron technology of the past 10–15 years. We can appreciate this from the remarkable influence of modern information technology on our lives. This volume is devoted to the exposition of new structures and the associated physics in an important area of modern nanoscience.

This is the field of nanomagnetism, where very small-scale structures such as ultrathin (few atomic layer) films of ferromagnetic material, often incorporated into superlattices or multilayer structures have been found to have magnetic and transport properties qualitatively and dramatically different than realized in bulk magnetic matter. More recently, we have seen a new generation of studies of the magnetism of patterned arrays ranging from micron-sized discs and wires down to dimers and single atoms adsorbed on substrates. Physicists have been intrigued by the new phenomena uncovered as these novel materials have been fabricated and their unique properties explored, materials scientists continue to present us with new structures, and by the time of this writing we have witnessed the enormous impact of ultra high-density magnetic data storage on computer technology. Here it is the remarkable phenomenon of giant magnetoresistance (GMR) of magnetic multilayers that has been exploited to increase the capacity of hard discs by over a factor of a hundred in a small number of years. Other exciting applications are envisioned, through the use of the systems and new concepts discussed in this volume. In this regard, we have in hand as well unique effects such as giant tunneling magnetoresistance (TMR), the phenomenon of the spin blockade, and other fascinating new effects that operate only in nanoscale magnetic systems. New methodologies developed by both theorists and experimentalists drive the field. Examples are the use of spin sensitive atomic force microscopy, and the development of large-scale computer simulations of real structures. The material in this volume is directed toward a broad audience of readers with backgrounds in condensed matter science who may not be experts in the field of nanomagnetism. It is our hope as well that the pedagogical nature of the discussions will also provide some experts with deeper insights in to the fundamental physics associated with areas of the field that have not been the focus of their own research. We comment next on the material discussed in the various chapters.

The first chapter, written by the undersigned volume editors, contains a broad overview. We discuss the unique and special aspects of the magnetism of ultrathin ferromagnetic films. Spin ordering is a long-ranged phenomenon, and the excitations which control both the response characteristics and thermodynamics of ultrathin films are influenced by long ranged couplings as well. Thus, as we shrink magnetic structures down to nanometer length scales, we find fundamental differences in all aspects of their physics. In addition, a large fraction of the moment bearing ions sit in interface or surface sites, with qualitative consequences for both their magnetic and chemical properties. Ultrathin films are the “building blocks” of the magnetic multilayers, spin valves and related structures that have been explored and discussed intensively in recent years. We also introduce materials with lateral structure (nanodot, nanodisc and nanowire arrays), and then turn our attention to the experimental methods, which have proved central to the elucidation of the properties of very small magnetic structures.

The nature of the magnetic moments found in ultrasmall structures can differ dramatically from their bulk counterparts, by virtue of the fact that a large fraction reside on surfaces, at interfaces as noted above. These also can be affected by the chemisorption of selected molecules. It follows that one realizes magnetic anisotropies one or two orders larger than that found in the bulk, and their strength and character are subject to design. Thus, we have spin engineering. R. Wu provides us with a broad survey of the ground state properties of diverse nanoscale magnetic structures. It is impressive to see the success of modern density functional theory in its ability to provide reliable quantitative accounts of the properties of these often-complex systems with low symmetry.

When ultrathin ferromagnetic films are assembled into multilayers or superlattices, weak interactions of exchange character act between the constituent films. These are mediated by the spin polarization induced in nonferromagnetic spacer layers inserted between them. These weak exchange couplings, tunable both in sign and magnitude by varying structural details, lead us to magnetic entities whose underlying structure can be manipulated by very modest or weak applied fields, in contrast to bulk materials whose magnetic ions are tightly coupled by very strong interatomic exchange. This key property allows us to fabricate new, artificial materials whose magnetic properties differ qualitatively from those of bulk materials, and which can be varied over a wide range by design. M.D. Stiles provides us with discussions of the physical origin and nature of these interfilm exchange couplings. Arrays of ultrathin ferromagnetic films coupled by the exchange interactions just discussed display a rich variety of spin structures, where the total (macroscopic) magnetic moment of each ultrathin film plays the role of a large, and consequently fully classical “spin.” R.E. Camley discusses examples of these structures, and the collective excitations they support. We have here the opportunity to develop new materials with microwave response tunable by the application of very modest magnetic fields.

When one wishes to exploit magnetic multilayers in devices, the signal detected has its origin in the rotation of the magnetization vector in one selected film relative to that in a neighboring film, and the resulting influence on properties of the structure such as its electrical resistance, as discussed in the next paragraph. A commonly used structure is the “spin valve,” which consists of two ferromagnetic films separated by a nonmagnetic spacer which gives rise to the weak interfilm interactions described in the previous paragraph. A question is then how one may use an applied magnetic field to rotate the magnetization of one of the two films, while the second remains pinned in place. A phenomenon referred to as “exchange bias,” discovered many decades ago, allows one to selectively “pin” the magnetization of one layer. A.E. Berkowitz and R.H. Kodama introduce us to this central topic, whose origin is only very recently appreciated.

Transport properties of magnetic multilayers have excited many researchers, since A. Fert and his colleagues reported the discovery of the astonishing phenomenon of GMR in 1988. In parallel with their work, P. Grunberg and collaborators observed this phenomenon as well at very close to the same time. The origin of GMR has stimulated efforts by many experimentalists and theorists for some years now, since it is not only a spectacular physical phenomenon, but has provided us with the basis for ultra high-density magnetic storage and its enormous impact on computer technology. A. Fert, A. Barthelemy and F. Petroff present us with a discussion of this most important phenomenon in their chapter, and cover tunneling magnetoresistance as well.

Physicists, materials scientists and engineers actively discuss a new field called “spintronics,” wherein it is the spin of the electron rather than its charge that is exploited and manipulated. This has led to the exploration of new physics that must be understood before the field can prosper. As we have known for decades, we can inject electrons from semiconductors into metals. But if the electrons are highly spin polarized, is the spin polarization preserved or destroyed in the injection process, and if it is the latter how do we make...