The time dependence of materials for magnetic recording media, both particl
es and thin films, is studied. The stability, as deduced from the magnetiza
tion decay, magnetic viscosity and fluctuation field, is greater in undoped
materials with good morphological and structural order. The activation vol
ume is smaller than the physical size in all the particulate materials, and
related to the magnetization switching: it shows relative decrease when th
e reversal does not take place by coherent rotation, as it occurs in doped
particles, or when the particles are composed of segregated sub-units. (C)
1999 Elsevier Science B.V. All rights reserved.