TEMPERATURE-DEPENDENT PREFACTOR

The saturation magnetization and anisotropy constants of a ferromagnet are, below the Curie point T(C), decreasing functions of temperature T. This effect leads to a T-dependent barrier height for thermal activation and, also, to T-dependent prefactor f0 of the thermal decay rate kappa, which is here studied in the underdamped limit. For a uniaxial single domain ferromagnet f0 approximately tau-1(1 - tau)rho, where tau = T/T(C) and rho is-an-element-of [2,11/3) depending on the chosen model of dissipative coupling. Ohmic and superohmic couplings are considered that are either isotropic or anisotropic in space. Estimates for cubic systems are in the range rho is-an-element-of [13/3,32/3]. Measurements of the function f0(tau) may thus provide information about the dissipative mechanism in single domain particles. Its influence on measurable data is demonstrated on a model calculation of frequency-dependent coercivity.