Journal of Statistical Physics, Vol. 60, Nos. 3/4, 1990. p. 473-784.
We consider the irreversible dynamics of the magnetization vector M in a single-domain particle. The dynamics is given by a stochastic phenomenological equation due to Gilbert. It contains a damping field proportional to M and a corresponding white noise field component. The probability distribution function satisfies a Fokker-Planck equation derived by Brown. We give the overbarrier decay rate k out of a metastable minimum. First we rederive the well-known expression for k for an axially symmetric model. We argue that this result is unphysical. For systems of general point symmetry of the magnetic anisotropy energy we give k in both the low-damping and intermediate- to high-damping limits.