Relaxation time of weakly interacting superparamagnets

PACS. 76.20.+q – General theory of resonances and relaxations. PACS. 75.10.Hk – Classical spin models. PACS. 75.50.Tt – Fine-particle systems; nanocrystalline materials. Abstract. – The relaxation time of weakly interacting classical spins is calculated by intro-ducing 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 ori-ented 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. Introduction. – The study of single-domain magnetic particles has been an active field of research since the pioneer work of Stoner and Wohlfarth [1], who studied the hysteretic rotation of the magnetisation over the magnetic-anisotropy energy barrier under the influence of applied fields, and Néel [2] who predicted that at non-zero temperature the magnetisation can surmount the energy barrier as a result of thermal agitation. Important progress has bee