Application of a mean field theory of magnetic alloys to fcc iron-nickel.

Abstract

We present a local moment mean field theory for atomically disordered magnetic alloys. In this model, the level of approximation regarding atomic environment can be decreased gradually. The method applied to one-dimensional alloys at several levels of approximation. The results for the sample average spin indicate a convergence as the level of approximation is decreased. Other interesting results for 1D alloys are obtained and discussed. The method is also applied to the fcc Fe-Ni alloy series, with only three free parameters, the exchange integrals $\rm J\sb{Fe-Fe},\ J\sb{Fe-Ni}$ and $\rm J\sb{Ni-Ni}.$ Although the most extreme behavior of the purely magnetic properties of the Fe rich alloys is not reproduced, deviations from the behavior of normal ferromagnetic alloys are predicted, and the order of magnitude of the calculated paraprocess susceptibility is in agreement with experimental data. Considering the simplicity of the model, our results indicate that a more realistic local moment model (as opposed to a more complicated model based on itinerant electron ferromagnetism) could reproduce the purely magnetic properties of fcc Fe-Ni. Some suggestions to render the model more realistic for fcc Fe-Ni are provided. Finally, a simple approach to include magnetocrystalline coupling to the model is suggested, which could be used to explain Invar behavior in fcc Fe-Ni alloys.