Showing papers in "Handbook of Magnetic Materials in 1993"

Chapter 5 Magnetic properties of binary rare-earth 3d-transition-metal intermetallic compounds

Abstract: Publisher Summary This chapter discusses the magnetic properties of binary rare-earth 3d-transition-metal intermetallic compounds The basic concepts related to the intrinsic magnetic properties of the 3d-rich R n T m (R = rare earths; T = 3d heavy transition metals Mn, Fe, Co, Ni) intermetallic compounds are discussed The study of rare-earth transition-metal intermetallic compounds has a number of interesting aspects Owing to the wide range of intermetallics and their different stoichiometries and variable rare-earth elements, modifications of magnetic properties of 3d transition-metal and 4f rare-earth ions can be investigated systematically These investigations illuminate the complex interactions in which the 3d and 4f electrons are involved The rare-earth metals in their ‘normal’ state where the magnetic properties of the ion cores are well defined The 4f electrons are positioned within the ion cores and hybridization with the conduction-band-electron states is negligible This situation is realized for most iron- and cobalt-based compounds with 4f elements The Ce and Yb ions, which sometimes demonstrate unusual properties connected with valence fluctuations, tend to behave quite normally in the compounds with iron or cobalt Nevertheless, there are some anomalies in the Ce compounds that can be ascribed to a mixed-valence state of the cerium ion

Chapter 1 Magnetism in ultrathin transition metal films

Abstract: Publisher Summary This chapter discusses the magnetism transition in ultrathin metal films. In discussing magnetic thin metal films, intrinsic from defect-induced magnetic thin film phenomena is distinguished. The first group includes phenomena that are connected with the finite thickness and the existence of two surfaces. Because films can be prepared only on substrates, the interaction with these substrates and their magnetic implications must be considered as intrinsic film phenomena. Thin films are usually prepared by condensation from a highly supersaturated atomic beam and therefore contain a high level of defects. Depending on the vacuum level used in the preparation, a high level of impurities must be considered, too. A rich variety of interesting magnetic properties is connected with the resulting defect- and impurity-structure of thin magnetic films, in particular polycrystalline ones. Because of their connexion with basic theoretical models, intrinsic magnetic film phenomena are of considerable interest for the understanding of fundamental magnetic phenomena, like magnetic order in lower dimensions. High-resolution diffraction and microscopical methods, which became available during the last years, will promote the progress towards a straightforward understanding of magnetic film phenomena in well defined film structures, based on a full atomistic structural analysis.

Chapter 4 Diluted magnetic semiconductors

Abstract: Publisher Summary This chapter discusses the diluted magnetic semiconductors (DMS). New aspects of DMS physics are reviewed and an update of various numerical data that are relevant to DMS materials is provided. The specific nature of DMS makes it impossible to leave out those properties that are typical for semiconductors. The information concerning crystal structures and lattice constants of diluted magnetic semiconductors is presented in the chapter. The properties of these semiconducting mixed crystals in the absence of an external magnetic field are described. The quantities typically considered when discussing semiconducting materials, such as, energy gaps, separating valence, and conduction bands are also focused. Magnetic properties are presented, which is followed by where properties that single out DMS as a separate group of materials are discussed, namely those that are connected with a mutual dependence of the magnetic and semiconducting properties. This interdependence stems from strong exchange coupling between the conduction band electrons and/or valence band holes with the localized electrons from 3d shells of ‘magnetic’ ions, which form localized magnetic moments. The materials that are most commonly considered as the members of the DMS family are substitutional solid solutions of II—VI semiconductors and transition-metal monochalcogenides such as CdTe and MnTe.

Chapter 3 Density functional theory of the ground-state magnetic properties of rare earths and actinides

Abstract: Publisher Summary This chapter discusses the density functional (DF) theory of the ground-state magnetic properties of rare earths and actinides. The results of calculations made using DF theory are used, this is the theory used for nearly all electronic structure calculations in solids. DF theory is a ground-state theory—a variational principle for the ground-state energy in terms of the electron density, or electron spin density. The eigenvalues that are obtained in this theory are the functional derivatives of the total energy with respect to the density or spin density. They are not in general simply related to measured quantities, although the temptation to compare the two is rarely resisted. DF theory contains an unknown functional for the exchange and correlation energy into which most of the lack of knowledge of the contribution of many-body interactions to the total energy is placed. The functional is replaced by an approximate one, which is derived for a homogeneous electron gas with constant density. The functional and its functional derivatives are used to obtain the potential in terms of the density; the constant density is then replaced by the real density the local density or spin density approximation (LDA or LSDA).

Chapter 6 Neutron scattering on heavy fermion and valence fluctuation 4f-systems

Abstract: Summary This chapter deals with valence fluctuation and heavy fermion 4f-systems such as Ce, Sm, Eu, Tm and Yb alloys and compounds. In sections 1 and 3 we summarize the most important physical properties of these systems, based on the main theoretical results achieved so far and on representative data. In the main part (section 2) we summarize neutron scattering data of HF and VF systems. By means of neutrons one can measure the magnetic structure of a system (elastic magnetic scattering) and magnetic and nonmagnetic excitations such as relaxational modes (quasielastic excitations), magnons, crystal field and spin-orbit transitions and phonons. Some of these excitations have been observed in all HF or VF systems. Both HF and VF systems exhibit a quasielastic line due to spin fluctuations or the Kondo effect. Both expressions are used synonymously, but in VF systems the quasielastic line is very broad, corresponding to a high characteristic energy, and is fairly temperature independent. Here one talks in general about spin fluctuations. In HF systems the quasielastic line is narrower, temperature dependent, and has at low temperatures the halfwidth Γ/2 ≈ K B T N K . A typical quasielastic line can be fitted by a Lorentzian and corresponds to a single relaxation time. In most HF compounds the line becomes Q -dependent at low temperatures indicating spin correlations that often lead at still lower temperatures to magnetic order. These spin correlations, on the other hand, lead already to deviations from the Lorentz shape above the spin ordering temperature. In V F compounds spin correlations do not play a role, but the high-temperature quasielastic line also changes its shape and becomes narrower and inelastic at low temperatures. Ce-based HF systems have been investigated by many groups, whereas few data exist for the corresponding VF systems and for Yb-, Sm-, Eu- and Tm-based VF and HF systems. As a consequence, our survey over Ce-based VF and Yb, Sm, Eu and Tm systems is rather complete, whereas for Ce-based HF systems we could only select representative examples. Part of this strong research activity on Ce compounds is due to the fact that CeCu 2 Si 2 becomes superconducting, and it is an enormous challenge to find other superconducting HF systems. In addition, one has in these systems a variety of magnetic structures such as ferromagnetic, antiferromagnetic and spiral order and metamagnetism. In many cases the detection of these structures is hampered by moments that are strongly reduced by the Kondo effect. In addition to the Kondo effect one has in HF systems strong crystal field effects, and the corresponding 4f-levels can be determined by neutron scattering experiments. The interplay between Kondo and CF effects varies from system to system, and the corresponding information is part of the content of this chapter. Since in HF systems the Kondo temperature is typically rather small, one observes at low temperatures mainly the properties of the lowest CF level. Valence fluctuations couple to phonons, and our review gives a rather complete survey of the corresponding anomalies. In addition, one has phonon anomalies due to the CF-phonon interaction, since a lattice deformation around an R ion modifies the crystalline electric field. Finally, we mention the spin-orbit interaction, which is modified in VF and HF systems due to the hybridization between f and conduction electrons. The few neutron scattering experiments on spin-orbit transitions in VF and HF systems show unexpected splittings, line shifts, and line broadenings if compared to the free ion case.

Chapter 2 Energy band theory of metallic magnetism in the elements

Abstract: Publisher Summary This chapter discusses the energy band theory of metallic magnetism in the elements. Some of these magnetic phases are produced by epitaxial growth, which can expand a lattice, but the greatest number is found by application of the band theory of magnetism. This theory has a sound basis and a computationally practicable form to obtain reliable ground-state properties of crystalline elemental solids. The theory makes use of a set of Schrodinger-like one-electron equations to describe the ground state of a system with given nuclear positions. The equations contain an effective one-electron potential such as a function of electron position, which adds exchange and correlation effects to Hartree-type Coulomb terms to describe electron electron interactions. Several forms of the effective potential are deduced from various physical approximations; among these forms are the X α form based on a statistical treatment of the exchange interaction and the local-density approximation (LDA) in the density functional theory of the ground state. The LDA results agree better with experiment and have a sounder physical basis, so that recent work is mostly with the LDA. The computation of ground-state magnetic properties by the augmented spherical wave method is described, including the procedure for locating magnetic phases and their stability limits. The useful simple Stoner formulation is given and related to the general theory.