Influence of dipolar interactions on the Neel superparamagnetic relaxation time, r, of an assembly of ultrafine ferromagnetic particles (γ-Fe2O3) with uniaxial anisotropy and of different sizes has been widely studied using Mossbauer technique. These studies, based on different analytical approaches, have shown that r decreases with increasing interactions between particles. To interpret these results, we propose a model where interaction effects are considered as being due to a constant and external randomly oriented magnetic field B(ψ,Φ). The model is based on the resolution of the Fokker-Planck equation (FPE), generalizes previous calculations and gives satisfactory interpretation of the relaxation phenomenon in such systems.
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