Low-frequency modes in rare-earth magnets with degenerate crystal-field levels

Abstract

We develop a general theory of the low-frequency spin dynamics in rare-earth magnets with degenerate crystal-field levels. The analysis applies to the paramagnetic phase of those systems which have a well-defined separation between high- and low-frequency response. We calculate the low-frequency part of the zone-center dynamic susceptibility. Treating the static correlations in the molecular-field approximation we use the theory to interpret electron-paramagnetic-resonance studies of TbP. Critical anomalies in the electron paramagnetic resonance in singlet-ground-state ferromagnets are pointed out. The temperature dependence of the intensity and width of the central peak in ${\mathrm{Pr}}_3$Tl is calculated assuming the broadening comes from the first-order interaction with the conduction electrons. Contact is made with other treatments of the low-frequency dynamics.