Local magnetovolume effects in Invar Fe65Ni35
F. Liot and I. A. Abrikosov
Invar Fe65Ni35 alloy is one of the most fascinating magnetic materials. It exhibits a very small thermal expansion coefficient (less than 1.2*10−6 K−1) at room temperature in a wide temperature interval, and reveals the existence of other anomalous physical properties such as a downward deviation of the ideal lattice constant from the value predicted by Vegard’s law, a departure of the saturation magnetic moment from the Slater-Pauling curve, and an unusual temperature dependence of the magnetization.
Although the origin of the Invar effect has been extensively theoretically investigated, it is still controversial. Here 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. 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. In particular, 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 (see the Figure below). 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 local magnetovolume effects.
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Last updated: 09/18/09