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 particle systems 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 $Tln(t/\tau_0)$ 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.