Abstract
By using the free-energy variational method based on the Bogoliubov inequality and a diluted and random-bond Ising model with nearest-neighbors interactions, we investigate the magnetic phase diagram and some of the magnetic properties of the disordered ${\mathrm{Fe}}_{0.9\ensuremath{-}q}{\mathrm{Mn}}_{0.1}{\mathrm{Al}}_{q}$ alloys system. Thus the mean magnetization per site, and hence the average hyperfine magnetic field, are computed. The so-obtained results are compared with room-temperature experimental data obtained by ${}^{57}\mathrm{Fe}$ M\ossbauer spectroscopy and vibrating sample magnetometry, from which a very good agreement is achieved. Here, the occurrence of a ``critical concentration'' at 40.0 at. % Al, for which the system passes from a ferromagnetically ordered state to a paramagnetic one, is evidenced. The model allows obtaining an estimate of the exchange energy between Fe-Fe pairs. How this energy depends on the Al concentration and the role of the manganese atoms is also presented and discussed.
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