Magnetization relaxation of Mn12-acetate via thermally assisted tunneling and quantum tunneling: a perturbative approach

We study the magnetization relaxation of a Mn12-acetate molecular magnet in the presence of fields both parallel and perpendicular to the easy axis. Employing perturbation theory in the fourth order transverse anisotropy and a perpendicular field, we derive a tunneling splitting formula, by which we investigate the crossover between thermally assisted relaxation and quantum tunneling. It is found that the tunneling splitting formula can be expressed as a polynomial in the transverse anisotropy and the perpendicular field. We also observe that the fourth-order transverse anisotropy enhances the ground-state quantum tunneling to give a crossover temperature higher than the value predicted in previous theories, which agrees well with the experimental result.