Analysis and interpretation of time dependent magnetic phenomena (invited)

Time dependence of intensity of magnetization is the result of thermal activation of metastable domain processes. The simple (M,H) representation of magnetic behavior may be extended to include time dependent effects using a constitutive equation. Magnetic processes may be described by the movement of a point over a three-dimensional surface with coordinates M, H, and Ṁ. The parameters required to define the surface, Λ (or fluctuation field H _f) and χ, may be derived from measurements of the time dependence of magnetization. A review of the utilization of the constitutive equation is given and simple methods for obtaining the material parameters from conventional magnetic viscosity measurements (under constant applied field conditions) are described. Measurements of Λ in particular provide additional quantitative information on magnetization processes which may be compared with predictions of proposed models of magnetization. The results of measurements of Λ and susceptibility χ for CrO₂ powders and sintered and melt-quenched rare earth-transition metal borides are presented. Conclusions about the magnetization mechanisms of these materials are discussed.