Localized-magnetic-moment theory of Fe-Ni Invar

Abstract

A Green's function technique for localized magnetic moments coupled to lattice degrees of freedom is used to study the moment-volume instability in Invar alloys. We present calculations of the total energy as function of volume, magnetic moment, and alloy concentration incorporating both longitudinal and transverse spin fluctuations. For ordered ${\mathrm{Fe}}_{3}$Ni, the total energy as function of volume consists of two separate but crossing branches corresponding to the low-spin (LS) and high-spin (HS) states, with a discontinuous magnetic moment at the crossing. With increasing temperature we find that the LS and HS states come closer and finally merge at a critical temperature. In addition, the temperature dependence of the magnetic contribution to the relative volume change ${\mathrm{\ensuremath{\omega}}}_{\mathrm{m}}$ has been calculated in the presence of an external magnetic field. We have extended the calculations to off-stoichiometric concentrations of the alloy system ${\mathrm{Fe}}_{\mathrm{x}}$${\mathrm{Ni}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ and present results for the temperature and concentration variation of the thermal expansion coefficient ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{m}}$ and of the energy difference between the LS and the HS state. Theoretical results compare fairly well with experimental data in spite of using a nonitinerant electron model.