Exchange interactions and magnetic anisotropy in the 'Ni4' magnetic molecule

The magnetic properties of the tetrahedral 'Ni-4' molecule are discussed in terms of the Heisenberg model, with magnetic anisotropy terms included, and on the basis of first-principle calculations within the density functional theory. It is shown that the isotropic Heisenberg model does not provide an adequate description of magnetization at low temperatures, and the inclusion of single-site anisotropy terms does not help to improve the situation. We assume that the magnetostriction of the molecule and, hence, the dependence of the interatomic coupling parameters on the magnetization may be important for an adequate description of the magnetic properties. The first-principle calculations confirm the system's general preference for antiferromagnetic coupling, as well as the failure of the isotropic Heisenberg model. A conjugated-gradient search for the relaxed structure of the 'Ni-4' molecule in ferromagnetic and antiferromagnetic configurations did not show any clear tendency to diversification of the interatomic distances. These calculations, however, have not yet included the spin-orbit coupling, which can be essential for analysing the effects of magnetostriction.