Crossover from quantum tunneling to classical hopping of barrier transition in nano-magnets

Crossover from quantum tunneling to thermal hopping of barrier transition in biaxial nano-magnets with external magnetic field is studied as a phase transition process. We obtain the phase diagram in terms of the nonlinear perturbation expansion of small oscillation around the saddle point of the first-order Euclidean (imaginary-time) differential equations for polar and azimuthal angles theta and theta of the nano-magnet without approximation and so that the phase diagram can be extended to the whole range of anisotropy constants of the magnet as well as the magnitude of the external magnetic field. New phase diagrams are observed and are compared with the result in the literature. (C) 2002 Elsevier Science B.V. All rights reserved.