Simulation of a spin-wave instability from atomistic spin dynamics

We study the spin dynamics of a Heisenberg model at finite temperature in the presence of an external field or a uniaxial anisotropy. For the case of the uniaxial anisotropy, our simulations show that the macromoment picture breaks down. An effect which we refer to as a spin-wave instability results in a nondissipative Bloch-Bloembergen-type relaxation of the macromoment where the size of the macromoment changes and can even be made to disappear. This relaxation mechanism is studied in detail by means of atomistic spin dynamics simulations.