Nature of the ferromagnetic ground state in the Mn4 molecular magnet

Using ab initio band-structure and model calculations, we studied magnetic properties of one of the Mn-4 molecular magnets [Mn-4(hmp)(6)], where two types of the Mn ions exist: Mn3+ and Mn2+. The direct calculation of the exchange constants in the GGA+U approximation shows that in contrast to a common belief, the strongest exchange coupling is not between two Mn3+ ions (J(bb)), but along two out of four exchange paths connecting Mn3+ and Mn2+ ions (J(wb)). Within the perturbation theory, we performed the microscopic analysis of different contributions to the exchange constants, which allows us to establish the mechanism for the largest ferromagnetic exchange. In the presence of the charge order, the lowest in energy virtual excitations, contributing to the superexchange, will not be those across the Hubbard gap similar to U, but will be those between the Mn3+ and Mn4+ ions, which cost much smaller energy V (<< U). Together with strong Hund's rule coupling and specific orbital order, this leads to large ferromagnetic exchange interaction for two out of four Mn2+-Mn3+ pairs.