Magnetic viscosity effect in ac susceptibility measurements

The linear and nonlinear initial ac susceptibilities of thermally relaxing particulate media an derived perturbatively from a master equation. A distribution of activation energies endows the real part of linear susceptibility with a logarithmic dependence on frequency in analogy to magnetic viscosity in de fields, the proportionality constant is found to be only weakly dependent on temperature via its dependence on thermal stability of the equilibrium state. The temperature and frequency range of this ac viscosity effect are enhanced by ferromagnetic coupling and the predicted behavior is compared with experimental data for Ni-x(SiO2)(1-x) granular films.