Local magnetovolume effects in Fe65Ni35 alloys

A systematic ab initio study of static ionic displacements in a face-centered-cubic Fe65Ni35 alloy has been carried out. Theoretical results for magnitudes of average Fe-Fe, Fe-Ni, and Ni-Ni < 110 > bond vectors agree well with experimental measurements. In addition, we have observed that in collinear ferrimagnetic states, iron-iron nearest-neighbor pairs with antiparallel local magnetic moments are shorter on average than those with parallel moments. Furthermore, having considered different states (ferromagnetic, nonmagnetic, and collinear ferrimagnetic states) for the same lattice spacing, we have shown that the magnetic structure strongly influences local geometrical properties of the alloy. For example, a transition from a ferromagnetic state to a collinear ferrimagnetic state induces a significant contraction of the volume associated with an iron site where the moment flips. A model system based on a Hamiltonian written as the sum of Lennard-Jones energies and a classical Heisenberg Hamiltonian has been introduced. It yields structural properties which are qualitatively similar to those obtained ab initio. We have found that some of the phenomena can be classified as magnetovolume effects.