Magnetic Excitations in Insulators and Metals

Abstract

Recently, we have seen a rapid development, using the dynamics to cast light on very fundamental problems in disordered or unknown ground state cases. Aspects to be discussed of this interesting development of both theory and experiment include the following topics. Spin waves in ordered structures depending on lattice dimensionality and frustration effects, the sinusoidal ordered structure and crystal field or anisotropy effects. The disordered ground state problems involve excitations in low dimensions, in the paramagnetic phase and in singlet ground state systems, with soft mode and central peak phenomena. The itinerant magnets are similar to the singlet ground state because in both cases a moment must be induced before spin wave excitations can propagate. The recent situation will be commented on. The task of the theory has been to include correlation effects and to treat non-linear phenomena. In some cases these are so prominent that new non-linear excitation modes can be identified, such as solitons and the Villain mode. The future holds a number of further problems for the study of magnetic excitations. For example in two dimensions (or surface physics) where so far mainly ground state properties have been investigated.