The switching volume of materials for recording media is obtained from meas
urements of the time dependence of the magnetization, and related to the ma
gnetization reversal mode. The switching, or activation, volume is large an
d close to the physical size in particles with reversal similar to coherent
rotation, and it decreases when the reversal is incoherent or heterogeneou
s, or, with larger extent, in materials switching the magnetization by nucl
eation. (C) 1999 Elsevier Science B.V. All rights reserved.