The coefficient of magnetic viscosity. II. The time dependence of the magnetisation of interacting fine-particle magnetic materials

For pt.I see ibid., vol.14, L.155 (1984). The time dependence of the magnetisation of a system of fine spherical cobalt particles in dipole interaction is computed by using an appropriate Monte Carlo algorithm. For weak interactions, with volume packing factor p up to about 10%, the time dependence follows an exponential law but with a corresponding particle volume v eff which exceeds the actual volume v by an amount proportional to p. For strong interactions (p up to about 50%), the time dependence was found to be logarithmic due to the interaction-induced distribution of relaxation times. The corresponding time t 1/2 which elapses before the magnetisation falls to half its original value follows a Vogel-Fulcher law (ln t 1/2 ) -1 approximately (T-T 0 ) with a characteristic temperature T 0 , at high temperatures but deviates from this law at low temperatures so that the system never experiences any phase transitions. The temperature T 0 depends linearly on the volume packing factor, thus confirming that the Vogel-Fulcher law arises from short-range cooperative phenomena.