  The magnetic relaxation and hysteresis of a system of single domain particles
with dipolar interactions are studied by Monte Carlo simulations. We model the
system by a chain of Heisenberg classical spins with randomly oriented
easy-axis and log-normal distribution of anisotropy constants interacting
through dipole-dipole interactions. Extending the so-called $T\ln(t/\tau_0)$
method to interacting systems, we show how to relate the simulated relaxation
curves to the effective energy barrier distributions responsible for the
long-time relaxation. We find that the relaxation law changes from
quasi-logarithmic to power-law when increasing the interaction strength. This
fact is shown to be due to the appearence of an increasing number of small
energy barriers caused by the reduction of the anisotropy energy barriers as
the local dipolar fields increase.
