Escape rate of a biaxial nanospin system in a magnetic field: First- and second-order transitions between quantum and classical regimes

We investigate the escape rate of the biaxial nanospin particle with a magn etic field applied along the easy axis. The model studied here is described by the Hamiltonian H = -AS(z)(2)-BSx2-HSz (A > B > 0). By reducing this Ha miltonian to a particle one, we derive an effective particle potential for this model and find an analytical form of the phase boundary line between f irst- and second-order transitions, from which a complete phase diagram can be obtained. We also derive an analytical form of the crossover temperatur e as a function of the applied field at the phase boundary.