Spectroscopic Determination of the Magnon Density of States of GdCl3, a Ferromagnetic Rare-Earth Salt

The magnon density of states of ferromagnetic Gd${\mathrm{Cl}}_3$ in an external magnetic field has been observed by means of high-resolution optical spectroscopy. The relevant transitions arise from a single-ion transition mechanism, but the initial (magnon) and in some cases the final (exciton) states of the transitions are shown to have measurable dispersion, contrary to the usual description of rare-earth salts.