Handbook of Magnetic Materials (eBook)

Preface to Volume 17

The Handbook series Magnetic Materials is a continuation of the Handbook series Ferromagnetic Materials. When Peter Wohlfarth started the latter series, his original aim was to combine new developments in magnetism with the achievements of earlier compilations of monographs, producing a worthy successor to Bozorth's classical and monumental book Ferromagnetism. This is the main reason that Ferromagnetic Materials was initially chosen as title for the Handbook series, although the latter aimed at giving a more complete cross-section of magnetism than Bozorth's book.

In the last few decades magnetism has seen an enormous expansion into a variety of different areas of research, comprising the magnetism of several classes of novel materials that share with truly ferromagnetic materials only the presence of magnetic moments. For this reason the Editor and Publisher of this Handbook series have carefully reconsidered the title of the Handbook series and changed it into Magnetic Materials. It is with much pleasure that I can introduce to you now Volume 17 of this Handbook series.

Magnetic tunnel junctions form part of the exciting field of spintronics. In this field, nanostructured magnetic materials are employed for functional devices where both the charge and the spin are explicitly exploited in electron transport. Magnetic junctions offer a number of unique opportunities for investigating novel effects in physics and have led to several new research directions in spintronics. Equally important is the fact that magnetic junctions represent excellent materials for exploring novel and superior types of devices. The physics of spin-dependent tunneling in magnetic tunnel junctions is reviewed in Chapter 1, concentrating on ferromagnetic layers separated by an ultrathin insulating barrier. The tunneling current between the ferromagnetic electrodes in these junctions depends strongly on an external magnetic field and as such lends itself to novel applications in the fields of magnetic media and data storage. Followed by a short introduction on the background and the elementary principles of magnetoresistance and spin polarization in magnetic tunnel junctions, the author discusses basic and magnetic transport phenomena, emphasizing the critical role of the preparation and properties of the tunnel barriers. Later on, key ingredients to understand tunneling spin polarization are introduced in relation to experiments using superconducting probe layers. The author also discusses a number of crucial results directly addressing the underlying physics of spin tunneling and the role played by the polarization of the ferromagnetic electrodes. Apart from Al2O3, the successful use of alternative crystalline barriers such as SrTiO3 and MgO is discussed.

With decreasing size of magnetic elements in magnetic storage media, read heads, and MRAM elements, the time and energy necessary for reading and writing magnetic domains have become of paramount importance and are studied intensively worldwide. A concept of substantial impact is that of spin-accumulation, i.e. a non-equilibrium magnetization that is injected electrically into a non-magnetic material from a ferromagnetic contact by an applied voltage. A breakthrough in magnetoelectronics is the observation of current-induced magnetization reversal in several types of layered structures. This effect finds its origin in the transfer of spin angular momentum by the applied current. On the other hand, magnetization dynamics induces spin currents into a conducting heterostructure. These novel effects couple the magnetization dynamics in hybrid devices with internal and applied spin and charge currents. The time-dependent properties become non-local, meaning that they are not a property of a single ferromagnetic element, but depend on the whole magnetically active region of the device. Recent progress in understanding the magnetization dynamics in ferromagnetic hybrid structures is presented in Chapter 2.

Magnetic properties of 3d-4f intermetallic compounds have been reviewed in several previous Volumes of this Handbook. This includes reviews on magnetically hard materials and related compounds (Volumes 6, 9 and 10). In these materials, the magnetocrystalline anisotropy invariably plays a central role. Somewhat apart stands the literature on experimental studies of the crystal field effects in intermetallics of rare earths. Results obtained by means of inelastic neutron scattering have been reviewed in Volume 11. The separation between the topics of magnetic anisotropy and crystal field effects seems somewhat artificial. In view of the general acceptance of the single-ion model, little doubt remains about the intimate connection between the two phenomena. The origin of the apparent splitting between the two topics mentioned can most likely be found in the fact that the theoretical activity in the area has been lagging behind experiments ever since the appearance of the last major review written four decades ago by Callen and Callen, in 1966. However, one has to realize that theoretical advance on magnetic anisotropy and crystal field effects did not cease in the meantime. These topics just progressed in different directions, stimulated by the advent of the density functional theory (Volume 13). As regards the single-ion model proper, work on it proceeded at a rather slow pace. Nonetheless, a fair amount of new results has been published between the late 1960s and more recent times. Chapter 3 reviews the progress made in the theory, filling the gap in the literature between the anisotropy and the crystal field effects. In this Chapter the authors aim at reasserting the statement that magnetocrystalline anisotropy is the most important manifestation of the crystal field effects.

Magnetocaloric effects in the vicinity of phase transitions were already discussed by Tishin in Volume 12 of this Handbook, published in 1999. Since then there has been a strong proliferation in research on magnetocaloric materials and their application, mainly dealing with the option of magnetocaloric refrigeration at ambient temperature. A comprehensive review dealing with this latter aspect is presented in Chapter 4 of the present Volume. The design of a refrigeration system involves many problems which are far from simple. Its design invariably requires a critical evaluation of possible solutions by considering factors such as economics, safety, reliability, and environmental impact. The vapor compression cycle has dominated the refrigeration market to date because of its advantages: high efficiency, low toxicity, low cost, and simple mechanical embodiments. Perhaps this is because as much as 90% of the worlds heat pumping power; i.e. refrigeration, water chilling, air conditioning, various industrial heating and cooling processes among others, is based on the vapor compression cycle principle. However, in recent years environmental aspects have become an increasingly important issue in the design and development of refrigeration systems. Especially in vapor compression systems, the banning of CFCs and HCFCs because of their environmental disadvantages has opened the way for other refrigeration technologies which until now have been largely ignored by the refrigeration market. As environmental concerns grow, alternative technologies which use either inert gasses or no fluid at all become attractive solutions to the environment problem. A significant part of the refrigeration industry R&D expenditures worldwide is now oriented towards the development of such alternative technologies in order to be able to achieve replacement of vapor compression systems in a mid- to long-term perspective. One of these alternatives is magnetic refrigeration which is discussed in Chapter 4. In this chapter the author emphasizes the many novel experimental results obtained on magnetocaloric materials, placing them in the proper physical and thermodynamic background. Also measuring systems as well as demonstrators and prototypes for magnetic refrigeration are discussed.

Intermetallic compounds in which 3d metals (particularly Mn, Fe, Co and Ni) are combined with rare earth elements exhibit a large variety of interesting physical properties. The magnetic properties of these intermetallics are a matter of interest for two main reasons: Firstly their study helps to elucidate some of the fundamental principles of magnetism. Secondly they are of technical interest, because several compounds were found to be a suitable basis for high performance permanent magnets. More recently the unique soft magnetic properties made amorphous metal-metalloid alloys to a further class of materials which has attained considerable importance with regard to industrial application. In Chapter 5 the hydrides of such compounds and alloys are discussed. In fact, this chapter can be regarded as an updating of Chapter 6 in Volume 6 of this Handbook, published in 1991. In order to reach a self-contained form of this chapter, the authors and the editor agreed to incorporate the most important results of the previous chapter into the present one. In this way the novel results can be viewed in the right perspective, not requiring the interested reader to go back to the previous chapter in Volume 6 at regular intervals. Here it should be mentioned that a large variety of novel techniques has been employed more recently in order to elucidate the mechanism and effects of hydrogen uptake which is particularly complex in intermetallic compounds. They can roughly be devided into surface sensitive methods such as photo emission and related spectroscopies, X-ray absorption (XANES, EXAFS), X-ray magnetic circular dichroism (XMCD), transmission electron...