Effect of incommensurate transverse magnetic anisotropy on tunneling rate in Mn12-acetate

Recent density-functional calculations on the single-molecule magnet Mn$_{12}$-acetate have revealed that there are two types of transverse magnetic anisotropy such as that induced by disordered solvent and the coupling between electronic and vibrational degrees of freedom. The former breaks fourfold symmetry of an isolated molecule so that it contributes to the second-order transverse anisotropy, while the latter contributes to the fourth-order transverse anisotropy. The hard axis of the second-order anisotropy induced by solvent disorder does not necessarily align with that of the fourth-order anisotropy caused by the coupling, which complicates the tunneling rate. In this talk, we determine the hard axes of the second-order and the fourth-order anisotropy from first-principles calculations. Using our calculated hard axes and anisotropy parameters, we calculate the tunneling rates as a function of the energy levels and the direction and magnitude of the transverse magnetic field, in order to examine the revival or suppression of the tunneling-rate oscillations. We also explore how small isotope defects influence the tunneling rates.