LOCAL MOMENT MAGNETISM OF FCC FE-NI ALLOYS .1. CLUSTER-METHOD MEAN-FIELD THEORY

We use a recently developed cluster-method mean field theory to model the magnetism of face-centred cubic Fe-Ni alloys. The three pair-specific near-neighbour exchange constants are the only adjustable parameters, one set of which is used to calculate all the magnetic properties at all temperatures, compositions, and applied fields. The calculated Curie point versus composition, spontaneous saturation moment Versus composition, spontaneous magnetizations versus temperature at different compositions, and high-field (paraprocess) susceptibility at T=0 K versus composition give good qualitative agreement with J(NiNi) = 405 K, J(NiFe) = 280 K, and J(FeFe) = - 20 K. This shows that purely local moment models should be considered strong contenders to explain Invar behaviour and suggests that the fee Fe-Ni alloys are strong local moment systems.