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-speci fic near-neighbour exchange constants are the only adjustable paramete rs, 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 c omposition, spontaneous magnetizations versus temperature at different compositions, and high-field (paraprocess) susceptibility at T=0 K ve rsus composition give good qualitative agreement with J(NiNi) = 405 K, J(NiFe) = 280 K, and J(FeFe) = - 20 K. This shows that purely local m oment models should be considered strong contenders to explain Invar b ehaviour and suggests that the fee Fe-Ni alloys are strong local momen t systems.