The high-frequency switching of the magnetization in particulate materials

A theoretical treatment of the dependence of the remanent magnetization and coercivity on the sweep-rate of an applied field is developed. The high-speed thermo-activated switching which arises, when the rise time of the applied field is very fast, is also considered. Assuming perfect alignment of the magnetic moments, the effective time of the sweep-rate process is shown to be given by the relation t(R) = partial derivative H/partial derivative R\(M),. The theory is applied to single-domain particles, using Brown's asymptotic solution for the over-barrier escape rate. In the high-frequency regime, the rate dependence of the magnetic properties is modified. A relative enhancement of the rate dependence of the magnetization and of the coercivity and a reduction of the irreversible susceptibility is predicted. These results appear to be consistent with indirect experimental evidence from fast pulsed-field magnetometry measurements on more complex materials such as magnetic recording media. Analytic expressions are derived for the effective time tR and the switching speed limit by thermal activation is discussed. (C) 1999 Elsevier Science B.V. All rights reserved.