Ising model for disordered ferromagnetic Fe-Al alloys

A simple site-diluted Ising model is proposed to study the magnetic properties of Fe pAlq alloys ~with p 1q51) in the structural disordered phase. It is assumed that Al atoms, which are nonmagnetic, can induce an extra ferromagnetic interaction between second-neighbor Fe atoms. It is further assumed that this secondneighbor interaction, as well as the nearest-neighbor one, decreases as q increases. The critical properties are obtained by the variational approach based on Bogoliubov inequality for the free energy in the pair approximation. Quite good fittings to the experimental results of the 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 obtained. It is argued that this negative exchange can drive a spin-glass-like phase in these compounds, a fact that should be sought experimentally. The study of the effects of disorder on the critical behavior of magnetic systems has been the subject of a great amount of investigations during the last few decades, both theoretically and experimentally. 1,2 From the theoretical point of view it is known that Ising-like models are well suitable to describe the critical behavior of insulating anisotropic magnetic systems. 1 However, for band magnets 3 the situation is not so clear due to the lack of theoretical results on models that could be applicable to real experimental systems.