Theoretical approach for computing magnetic anisotropy in single molecule magnets

We present a theoretical approach to calculate the molecular magnetic anisotropy parameters, D-M and E-M for single molecule magnets in any eigenstate of the exchange Hamiltonian, treating the anisotropy Hamiltonian as a perturbation. Neglecting intersite dipolar interactions, we calculate molecular magnetic anisotropy in a given total spin state from the known single-ion anisotropies of the transition metal centers. The method is applied to Mn12Ac and Fe-8 in their ground and first few excited eigenstates, as an illustration. We have also studied the effect of orientation of local anisotropies on the molecular anisotropy in various eigenstates of the exchange Hamiltonian. We find that, in case of Mn12Ac, the molecular anisotropy depends strongly on the orientation of the local anisotropies and the spin of the state. The D-M value of Mn12Ac is almost independent of the orientation of the local anisotropy of the core Mn(IV) ions. In the case of Fe-8, the dependence of molecular anisotropy on the spin of the state in question is weaker. We have also calculated the anisotropy constants for several sets of exchange parameters and found that in Mn12Ac the anisotropy increases with spin excitation gap, while in Fe-8, the anisotropy is almost independent of the gap.