Observation of a composition-controlled high-moment/low-moment transition in the face centered cubic Fe–Ni system: Invar effect is an expansion, not a contraction

We report the first conclusive observation of a high-moment (HM)/low-moment (LM) transition occurring in face centered cubic Fe–Ni alloys. 57Fe Mössbauer isomer shifts give local electronic densities that exhibit a large discontinuity of ∼0.4 el./ a 0 3 at the transition that spans the concentration range ∼60–80 at% Fe, in agreement with ab initio predictions. Our electronic structure calculations give an isomer shift discontinuity at a comparable composition and of the same magnitude as the observed one. This identification of the HM/LM transition in Fe–Ni allows an interpretation of the compositional dependence of the lattice parameter (at room temperature or extrapolated to T=0 K) in which it is seen that the Invar effect is an expansion, relative to normal HM non-magnetovolume active behavior, not a contraction as is required in all two-γ-state-like interpretations. Indeed, the Invar effect and the HM/LM transition are seen as two distinct and competing phenomena that dominate at different compositions and that arise from different features of the electronic structure: a large inter-atomic separation dependence of the magnetic exchange interaction between large local moments versus instability of the local moment magnitude, respectively. In the Fe-rich alloys including Invar (Fe 65Ni 35), we observe temperature-induced changes in electronic density that follow the spontaneous magnetization curves and that are both consistent with the associated loss of local moment orientation order and inconsistent with a significant loss of local moment magnitude. This establishes that Invar is predominantly a HM phase at all temperatures where an Invar effect occurs. In the most Fe-rich alloys that have LM ground states (including γ-Fe), we find that thermal stabilization of the HM phase occurs at high temperatures (i.e., increase of local moment magnitude with increasing temperature), along a continuum of homogeneous phases between the LM and HM extremes, in a manner that is consistent with the expected local moment entropic contribution to the free energy. The latter effect, reported here for the first time, gives rise to the unusually large thermal expansion coefficients of these alloys, known as the anti-Invar phenomenon.