Superparamagnetic relaxation time under the effect of a randomly oriented magnetic field for weakly interacting ! ultrafine particles

The effect of a constant and a randomly oriented magnetic field H(ψ,φ), on the Néel relaxation time τ, of an assembly of weakly interacting ultrafine ferromagnetic particles with uniaxial anisotropy, is investigated. In this regard, the resolution of the Fokker- Panck equation (FPE) for the probability density W(θ,ϕ,t) of an assembly of ultrafine ferromagnetic particles, with uniaxial anisotropy in the presence of an applied field H(ψ,φ) was performed and published as a first part of this work; the results generalize previous calculations, and give satisfactory interpretation of the relaxation phenomenon in such system. In this part, calculation of the relaxation time, τ, is performed under the assumption of weak interactions between particles. The derived expression of τ is in good concordance with those previously obtained in the first part, and is consistent with those of Coeffey et al. for high values of the anisotropy parameter.