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

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Abstract
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 veff 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 t1/2 which elapses before the magnetisation falls to half its original value follows a Vogel-Fulcher law (ln t1/2)-1 approximately (T-T0) with a characteristic temperature T0, at high temperatures but deviates from this law at low temperatures so that the system never experiences any phase transitions. The temperature T0 depends linearly on the volume packing factor, thus confirming that the Vogel-Fulcher law arises from short-range cooperative phenomena.

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