ALTERNATING-FIELD STABILITY OF HIGH-TEMPERATURE VISCOUS REMANENT MAGNETIZATION

We have investigated the resistance of viscous remanent magnetization (VRM) acquired at high temperature by single-domain (SD) magnetite to alternating field (AF) demagnetization at room temperature. In our experiments, VRM was produced by exposing a fresh vacuum-sealed sample for 2.5 h to a field of 50-mu-T at a fixed temperature T. Nine different samples were studied, with T ranging from 100-degrees-C to 552-degrees-C. The overall AF stability increased as T increased. VRM produced at 552-degrees-C had coercivities comparable to those of total thermoremanent magnetization (TRM). Using Neel SD theory, we have successfully predicted remanent intensities and median destructive fields of VRM produced at various temperatures, using the known grain-size distribution of this magnetite and assuming a constant average microscopic coercivity of 30-35 mT. Total TRM intensity was also reasonably well predicted by summing observed and interpolated VRM in non-overlapping intervals of activation volume. Inversion of the AF demagnetization curves yielded a fairly stationary spectrum of microcoercivities, as required for certain theories of VRM.