Crossover between pure quantum and thermally assisted tunneling in a molecular magnet with tetragonal symmetry

Abstract We have studied the escape rate crossover between pure quantum and thermally assisted tunneling in Mn 12 -acetate molecular magnet with an external field applied perpendicular to the easy axis. Unlike previous model we include fourth-order anisotropy terms due to the tetragonal symmetry in the spin Hamiltonian. We find higher crossover temperature ( ∼1.0−∼1.2 K ) that agrees with experimental observation [12]. We also obtain a phase diagram for the crossover types, from which the first-order region is observed to be enlarged. More interestingly, when the field is applied along the hard axis there exist several small second-order regions inside the first-order region.