Relaxation of superparamagnetic spins: Classical vs large-spin description

Effective giant spins of magnetic nanoparticles are considered classically in the conventional theory of superparamagnetism based on the Landau-Lifshitz-Langevin equation. However, microscopic calculations for a large spin with uniaxial anisotropy, coupled to the lattice via the simplest generic mechanism, show that the results of the conventional theory are not reproduced in the limit S ->infinity. In particular, the prefactor Gamma(0) in the Arrhenius escape rate over the barrier Gamma=Gamma(0) exp[-Delta U/(k(B)T)] has an anomalously large sensitivity to symmetry-breaking interactions such as transverse field.