Activation Volumes of Magnetic Aftereffects: Role of the Magnon Chemical Potential in Nanomagnets

A non-Arrhenius temperature variation of the thermal magnetic aftereffect has been observed in some particulate media. This behavior could be understood by a quasilinear variation of the magnon chemical potential above some finite Bose-Einstein condensation temperature, and zero chemical potential below. Including the chemical potential variation into the thermal activation volumes derived from the Street and Brown formulation removes a nonphysical order of magnitude increase in the activation volume, but instead yields an activation volume that is essentially temperature independent, and is identified with the physical volume