  We study the classical dynamics of resonantly modulated large-spin systems in a strong magnetic field. We show that these systems have special symmetry. It leads to characteristic nonlinear effects. They include abrupt switching between magnetization branches with varying modulating field without hysteresis and a specific pattern of switching in the presence of multistability and hysteresis. Along with steady forced vibrations the transverse spin components can display transient vibrations at a combination of the Larmor frequency and a slower frequency determined by the anisotropy energy. The analysis is based on a microscopic theory that takes into account relaxation mechanisms important for single-molecule magnets and other large-spin systems. We find how the Landau-Lifshitz model should be modified in order to describe the classical spin dynamics. The occurrence of transient oscillations depends on the interrelation between the relaxation parameters. 
Hysteresis, transient oscillations, and nonhysteretic switching in   resonantly modulated large-spin systems