Ising model for disordered ferromagnetic Fe-Al alloys

A simple site-diluted Ising model is proposed to study the magnetic propert ies of Fe,AI, alloys (with p +q = 1) in the structural disordered phase. It is assumed that Al atoms, which are nonmagnetic, can induce an extra ferro magnetic interaction between second-neighbor Fe atoms. It is further assume d that this second-neighbor interaction, as well as the nearest-neighbor on e, decreases as q increases. The critical properties are obtained by the va riational approach based on Bogoliubov inequality for the free energy in th e pair approximation. Quite good fittings to the experimental results of th e ordering temperature are obtained as a function of q. A negative value of the extra exchange interaction for some range of the Al concentration is o btained. It is argued that this negative exchange can drive a spin-glass-li ke phase in these compounds, a fact that should be sought experimentally.