Effects of the magnetic field on the relaxation of small particle systems

We study the effect of a magnetic field on the thermal relaxation of non-interacting small monodomain particles with a distribution of anisotropy constants and random easy-axes directions. Numerical calculations of the relaxation curves for different distribution widths, and under different magnetic fields H and temperatures T, have been performed in the framework of a two-state approximation. We show how the obtained data can be analyzed in terms of an modified Tln(t/τ0) scaling from which the field dependence of the mean relaxing energy barriers can be extracted, a microscopic information which is not easily obtainable by other methods.