MAGNETIC VISCOSITY EFFECT IN AC SUSCEPTIBILITY MEASUREMENTS

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