Modelling order-disorder and magnetic transitions in iron-aluminium alloys

Information from recent X-ray and neutron diffuse scattering investigations of Fe-Al alloys and the resulting effective interaction parameters are used to construct a model in which the lattice configurational degrees of freedom of these alloys are described by Ising spins, and the magnetic moments of iron atoms are described by classical Heisenberg spins. Starting from the exact ground state of the model, information on the phase diagram at non-zero temperature is obtained from both spin wave theory, mean-field approximations and extensive Monte Carlo simulations. It is shown that use of the unrenormalized interaction parameters from the various experiments yields a phase diagram which disagrees severely with experiment even at the qualitative level. However, with a suitable renormalization of the ratio between magnetic and non-magnetic interactions, a qualitatively reasonable phase diagram of the model is obtained. The consequences for the authors' understanding of effective interaction parameters in Fe-Al alloys are spelt out, and the implications for experimental work are discussed.