First-Principles Calculations of the Instabilities in Fe-(Ni,Co, Pt) Alloys

First-principles calculations using the Korringa-Kohn-Rostocker method and the Coherent-Potential Approximation for Fe-Ni, Fe-Ni-Co and Fe-Pt alloys show that several features are responsible for the Invar anomalies. Atomic short range ordering in the alloys is responsible for the appearance of antiferromagnetic and non-colinear magnetic moments. The antiferromagnetic contributions are responsible for two effects, the negative anharmonicity due to the tendency of the alloys to have a smaller lattice constant, as well as the tendency to have a larger lattice constant because of additional density of states of antibonding majority-spin orbitals at the Fermi level, which simultaneously stabilizes the antiferromagnetic moments.