Magnetization of Mn-12 acetate in a slowly varying magnetic field: A quantum mechanical study - art. no. 014408

Beginning with a Heisenberg: spin Hamiltonian for the manganese ions in the Mn12Ac molecule, we find a number of model exact low-energy states of the system whose spectrum corresponds to the ordering of the states proposed fr om experiments. We also obtain the spatial symmetry classification of the l ow-lying states and find that none of the excitations below about 100 K hav e the same symmetry as the S = 10 ground state. This allows us to restrict ourselves to the S = 10 ground-state manifold in a time-dependent formalism to find the magnetization of the molecule in the presence of a slowly vary ing magnetic field at zero temperature. In our study, we consider the field sweep rate, fourth-order anisotropic spin interactions, and a transverse f ield. We study the effects of these parameters on the magnetization steps w hich are known to occur in the hysteresis plots in this system. We also stu dy the weights of the different states at the magnetization plateaus as fun ctions of these parameters. We find that the magnetization plateaus are rep roduced even though there are a large number of states which can tunnel int o each other.