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抯
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抯
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
vi Preface to Volume 17
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 toxiPreface
to Volume 17 vii
city, 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 microscopy, conversion electron
M鰏sbauer spectroscopy and to some extent susceptibility measurements. The results
of such investigations are discussed in Chapter 5 together with results of NMR
and ESR and surface insensitive experiments, where only the bulk properties can be
studied (magnetic measurements, neutron and X-ray diffraction, X-ray absorption,
transmission M鰏sbauer spectroscopy).
viii Preface to Volume 17
It is well known that there have been many new developments in the field of
magnetic sensing and actuation, including new forms of magnetic material. Apart
from this has been much progress in the development of microelectromechanical
systems (MEMS). Hand in hand with this has gone the advance in density of electronic
components on a chip, expressed by the so-called Moore抯 Law, where areal
density has doubled every eighteen months. Much of MEMS technology is silicon
based, with three-dimensional structures being manufactured from a silicon
platform by means of various lithographic techniques. It is common practice to
include functionality in to MEMS, opening the possibility of sensing or actuation.
Frequently piezoresistive materials are used which requires current and voltage connections
to the sensor element, the measured quantity being the strain dependence
of electrical resistivity in the active film. Of special interest is the incorporation of
magnetic materials in to MEMS making it possible to use inductive coupling for
sensing or activation. The major advantage to be gained from this is the possibility
to avoid the requirement for connections, and that it allows packaging and deployment
in remote or hostile environments. In Chapter 6 the authors address the
integration of magnetic components into MEMS as a way of providing additional
functionality. They present an overview of advances in thin film magnetic materials
that make the use of MagMEMS a viable option.
Volume 17 of the Handbook on the Properties of Magnetic Materials, as the
preceding volumes, has a dual purpose. As a textbook it is intended to be of assistance
to those who wish to be introduced to a given topic in the field of magnetism
without the need to read the vast amount of literature published. As a work of
reference it is intended for scientists active in magnetism research. To this dual purpose,
Volume 17 of the Handbook is composed of topical review articles written
by leading authorities. In each of these articles an extensive description is given in
graphical as well as in tabular form, much emphasis being placed on the discussion
of the experimental material in the framework of physics, chemistry and material
science.
The task to provide the readership with novel trends and achievements in magnetism
would have been extremely difficult without the professionalism of the
North Holland Physics Division of Elsevier Science B.V.
K.H.J. BUSCHOW
VAN DER WAALS-ZEEMAN INSTITUTE
UNIVERSITY OF AMSTERDAM
新书Handbook of Magnetic Materials [Vol 17]-K. Buschow(Elsevier, 2008)
新书Handbook of Magnetic Materials [Vol 17]-K. Buschow(Elsevier, 2008)
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