Effect of a randomly oriented field on superparamagnetic ultrafine particles

The effect of a constant and a randomly oriented magnetic field H(ψ, ?), on the Néel relaxation time τ, of an assembly of ultrafine ferromagnetic particles with uniaxial anisotropy, is investigated. In this regard, the smallest non-vanishing eigenvalue of the Fokker-Planck equation is calculated as a function of ψ, ? and the parameter h = M_svH / k_BT, where M_s and v are denoting the saturation magnetization and the volume of a particle, respectively. The results obtained bring noticeable changes compared to previous calculations, and give satisfactory interpretation of the relaxation phenomenon in such systems.