Theory of far infrared properties of magnetic surfaces, films and superlattices

Many magnetic materials, particularly antiferromagnets and ferrimagnets, have resonance frequencies in the far infrared, say between 10 and 100 cm−1. The first experimental studies of these were by absorption spectroscopy but in the last 15 years attention has shifted to a more ‘optical’ point of view in the study of reflection and transmission. Experimental techniques that are now available should lead to a rapid expansion of such studies. This expansion is likely to include the application of far-infrared spectroscopy to characterisation, that is, the determination of the basic magnetic parameters of samples. Particular interest will attach to thin films and superlattices. We review the theoretical framework necessary for the interpretation of such experiments. The first part of the review is devoted to existing studies, particularly resonance, reflectivity and transmissivity studies of antiferromagnets and ferrimagnets. We include a brief section on growth of thin films and superlattices. The underlying quantity is the dynamic permeability tensor μ(ω). In the second part we discuss derivations of μ(ω) for various magnetic materials and then give an account of derivations of reflectivity from μ(ω). Experimental data often exhibit the property of non-reciprocity, that is, the reflectivity, for example, changes when the static magnetic field is reversed. In some cases non-reciprocity occurs only when the sample is dissipative while in others it is intrinsic. We pay attention to non-reciprocity throughout the review and in a final section we review the results of discussions based on thermodynamics.