Theoretical aspects of the ${\mathrm{Fe}}_{\mathit{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{c}}$ Invar alloy

We present first-principles coherent-potential-approximation calculations for the fcc FeNi alloy. We have found that this system is characterized by a competition between a low-spin (LS) and a high-spin (HS) state, and we have calculated this energy difference as a function of alloy concentration. For fcc Fe we find that several magnetic states are locally stable whereas for the FeNi alloys only one magnetic state is stable. In agremeent with previous theories for explaining the Invar effect in the ${\mathrm{Fe}}_{65}$${\mathrm{Ni}}_{35}$ alloy, we find that at this concentration the LS and HS states are very similar in energy. The calculated magnetic moment, equilibrium volume, and bulk modulus agree well with experimental data, and we predict that the Grüneisen constant exhibits an anomaly at a Ni concentration around 25%.