  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 ->\infty. 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
