Thermal relaxation over a barrier in single domain ferromagnetic particles

Abstract

We present a summary of our general results on the theory of magnetization switching via thermal decay (the classical case) in a small, single domain ferromagnetic (superparamagnetic) particle. Our results allow us to calculate the prefactor of the thermal decay rate and apply them to realistic models, viz., to systems of cubic symmetry (both the Fe and Ni type) and to uniaxial systems with tetragonal, triclinic, and hexagonal symmetry in the basis plane. In each case an external magnetic field parallel to an easy axis can be present. We compare our results with the strictly axially symmetric model often quoted in the literature. This model is singular in that it lacks a saddle point, i.e., it is qualitatively different from any realistic system, which always exhibits a saddle. The singular model results in a temperature-dependent prefactor in the entire range of dissipation while any regular model leads to a temperature-independent prefactor for not too low dissipation.