Transient spin dynamics in a single-molecule magnet

We explore the limitations and validity of semiclassically formulated spin equations of motion. Using a single-molecule magnet as a test model, we employ three qualitatively different approximation schemes. From a microscopic model, we derive a generalized spin equation of motion in which the parameters have a nonlocal time dependence. This dynamical equation is simplified to the Landau-Lifshitz-Gilbert equation with (i) time-dependent and (ii) time-independent parameters. We show that transient dynamics is essentially nonexisting in the latter approximation, while the former breaks down in the regime of strong coupling between the spin and the itinerant electrons.