First- and Second-order Crossover of the Escape Rate for the Biaxial Spin Model with the Field Applied along the Hard Axis at Resonance

We have studied numerically the quantum-classical crossover of the escape rate in the biaxial spin system with an external field along the hard axis. Using the simple quantum transition state theory, we have obtained the crossover boundary separating the first- and the second-order crossovers and found that the first-order region is greatly suppressed in a moderate spin system, compared with the quasiclassical results. Applying an external longitudinal field, we have presented the peculiar crossover boundaries at resonant fields. The results can apply to a molecular nanomagnet.