Activation volumes of magnetic aftereffects: role of the chemical potential in nanomagnets

摘要

The thermal activation volumes derived from the Street and Brown formulation for thermal magnetic aftereffects often display a strong, albeit nonphysical, function of temperature. We have shown recently that the non-Arrhenius temperature variation of the thermal decay coefficient with temperature could be explained by a linear variation of the magnon chemical potential above some finite Bose-Einstein condensation temperature. The aftereffect is due to the thermal activation of metastable magnetic states over energy barriers. Traditionally, our understanding of energy barriers involves the introduction of an activation volume for magnetic reversal, which is computed from a magnetic aftereffect experiment. Following this approach, conventional estimates of such a volume often show an unphysically large temperature dependency, and so it has been stated that "The activation volume is an ill-defined concept" [1]. This paper shows how to correct the activation volume of a nanomagnet derived from the aftereffect measurement for this chemical potential variation, after which the volume is essentially temperature independent, and is identified with the physical volume.