  The relaxation time of weakly interacting classical spins is calculated by
introducing the averages of the local dipolar field, obtained by thermodynamic
perturbation theory, in a rigorous expression for the single-spin
thermoactivation rate in a weak but arbitrarily oriented field. At low
temperatures the non-trivial dependence of the superparamagnetic blocking on
the damping coefficient, numerically found by Berkov and Gorn, is reproduced by
our model and interpreted in terms of the deviations from uniaxial anisotropy
associated to the transversal component of the dipolar field acting on each
spin.
