Phenomenologies of ferromagnetism in metals (invited) (abstract)

Abstract

Magnetism remains mysterious 100 years after Ewing made his initial models of interacting compasses. Many aspects are seen clearly, but at each step from the spinning electron to the rotor of a motor complexity obscures. At the macroscopic level the difficulties in ferromagnetism start with magnetostatics, which in principle calls for the evaluation of sixfold integrals. At the quantum level one needs fully relativistic spin-polarized calculations that remain beyond the current state of the art in order to have a full description of the ground state of a homogeneous piece of material with periodic boundary conditions. While it is known in principle how to carry out such calculations, in practice an experimentalist relies on phenomenology for the interpretation of data. The role of phenomenology is to create molds into which the experimental data is pressed. Theoretical analyses of critical phenomena are among the triumphs of modern magnetism. The picture is beautiful as long as one ignores anisotropies, dipole–dipole interactions, magnetostriction, and dynamic effects, all of which are the heart of micromagnetics, not to mention imperfections, inhomogeneities, and surface effects. Studies of the response, static and dynamic, of nearly perfect single crystals of Fe, grown from the decomposition of FeCl2 in the form of whiskers, provide examples where the experiments attempt to approach the neatness of the theories. Interpretation of data requires understanding of the dominant magnetostatics, the underlying micromagnetics and the resulting magnetization patterns, and the major role of eddy currents in obscuring intrinsic loss effects. One combines these aspects of Maxwell’s equations with parametric formulations of the equation of state and the Landau–Lifshitz equations of motion to extract magnetizations, susceptibilities, anisotropies, and damping parameters as functions of internal fields and frequency. The micromagnetics of the ideally soft ferromagnetic material serves as a foundation for the phenomenology. Fifty years of progress in understanding of phase transitions provides the critical phenomenology. The Landau–Lifshitz equations of motion developed for feromagnetism also describe the dynamic response in the transition from paramagnetism to ferromagnetism when allowance is made for the frequency, field, and temperature dependence of the damping. When the phenomenology and the data are not in full accord it is difficult to assign the source of the misfit. Yet one rejoices in the fact that they fit, sort of.