Emission and absorption spectroscopy of magnetic materials

In recent years, optical methods have been accepted as a useful means for studying the magnetic properties in solids. In this paper it will be shown, from an introductory point of view, what kind of information optical studies can provide. The most numerous investigations have been performed with insulating materials. The observed phenomena can be divided into four types: first those where spectra can be described by a simple molecular field picture; second those where one incorporates small quantities of magnetic ions into a diamagnetic host and studies the spectra of various neighbour configurations; third those where one deals with the simultaneous excitation of states in the electronic system (excitons) and the spin system (magnons) which gives rise to the so-called magnon sidebands in absorption and emission spectra; and fourth those which deal with the exchange of energy between various magnetic sublattices by the magnetic interactions. Much less information is available on magnetic semiconductors. Yet here a very striking and perhaps technologically important effect has been observed, namely a considerable red shift of the absorption edge in going through the magnetic transition temperature, e.g., in EuO and EuS. In metals, so far only a few experiments have been performed, and only those on Ni and MnAg are mentioned here. In addition, the question of short-range correlations, which can be observed via optical spectra will be discussed. Finally, recent reviews in this field will be compiled to facilitate a deeper understanding.