Magnetic origin of the lattice instability of FePd alloys

It is shown that the lattice instability of FePd Invars is magnetic in origin and can be understood based on the Stoner band magnetism model by assuming the strain dependence of the exchange parameter, I. In the limit of weak itinerant magnetism, an explicit expression for the parent lattice lastability temperature, T_i, is derived in the context of a rigid band model. It is suggested that the drastic concentration dependence of T_i is a direct consequence of the rapid change of the density of states at the Fermi level with the degree of band filling. The values of the first and second derivatives of the exchange integral as found for FePd alloys are consistent with those from the analysis of the temperature dependence of the elastic constants using Alers et al. and Hausch's localized moment (Heisenberg) model.