Are superparamagnetic spins classical?
Abstract
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 ->∞. In particular, the prefactor Gamma0 in the Arrhenius escape rate over the barrier Gamma =Gamma0 exp[-Delta U/(kB T)] has an anomalously large sensitivity to symmetry-breaking interactions such as transverse field
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