Lattice dynamics and structural stability of ordered Fe₃Ni, Fe₃Pd and Fe₃Pt alloys using density functional theory

We investigate the binding surface along the Bain path and phonon dispersion relations for the cubic phase of the ferromagnetic binary alloys Fe₃X(X=Ni,Pd,Pt) for L1₂ and D0₂₂ ordered phases from first principles by means of density functional theory. The phonon dispersion relations exhibit a softening of the transverse acoustic mode at the M point in the L1₂ phase in accordance with experiments for ordered Fe₃Pt. This instability can be associated with a rotational movement of the Fe atoms around the Ni-group element in the neighboring layers and is accompanied by an extensive reconstruction of the Fermi surface. In addition, we find an incomplete softening in [111] direction which is strongest for Fe₃Ni. We conclude that besides the valence electron density also the specific Fe-content and the masses of the alloying partners should be considered as parameters for the design of Fe-based functional magnetic materials.