On the transition between thermally activated and dynamic magnetic reversal of fine particles

A method of calculating exactly the switching time t o of the magnetization of a uniaxially anisotropic single domain (SD) particle at the boundary between thermo-activated and damping-limited reversal is presented. The generalized free energy function of the system is used to deduce the intrinsic switching field marking the transition point and numerical integration of the stochastic Landau–Lifshitz–Gilbert (LLG) equation of motion is carried out to determine the switching statistics. We study the variation of t o with the tilt angle of the applied field with respect to the easy axis, the grain size and damping. At the cross-over transition the barrierless reversal is driven by thermal perturbations and the switching time in the presence of an axial field varies as t o ∝ K u V / k B T , where K u is the first order anisotropy constant and V is the volume of the particle. • Calculation of boundary between thermal and dynamic switching of SD particles. • Critical switching time is determined by the thermal field. • Analytic expression for the critical switching time. • Thermally induced symmetry breaking of SW astroid confirmed by simulations. • The induced asymmetry is invariant for long field pulses.