The thermodynamics of magnetic aftereffect

Abstract

Thermal magnetic aftereffect is due to the interaction between magnons (spin waves) and the change of equilibrium magnetization states. This non-equilibrium process is controlled by (i) the height of the energy barriers between magnetization states and (ii) by the chemical potential that determines how many magnons (thermal quanta) are available. Non-Arrhenius behavior is observed in a non-monotonic variation of decay rate with temperature observed in nanoparticulate media at low temperature. We attribute this behavior to quantum statistics and the nonuniform density of states in energy space. At low temperatures, Bose–Einstein condensation of the magnons is observed in nanoparticulate systems. In these materials, the decay rate increases to a maximum at low temperatures above the condensation temperature. A standard method of evaluating permanent magnet materials is to accelerate the decay rate, by applying a reverse holding field of the order of coercivity. We show that the problem with this technique is the ratio of the decay rate in zero field to the decay rate at the coercivity is a function of the material properties. This model shows excellent agreement with measurements of the variation of the thermal magnetic aftereffect at low temperatures in both nanograin iron powders and MP tapes.