Magnetic properties of Fe0.9-qMn0.1Alq disordered alloys: Theory

By using the free-energy variational method based on the Bogoliubov inequal ity and a diluted and random-bond Ising model with nearest-neighbors intera ctions, we investigate the magnetic phase diagram and some of the magnetic properties of the disordered Fe0.9-qMn0.1Alq alloys system. Thus the mean m agnetization per site, and hence the average hyperfine magnetic field, are computed. The so-obtained results are compared with room-temperature experi mental data obtained by Fe-57 Mossbauer spectroscopy and vibrating sample m agnetometry, from which a very good agreement is achieved. Here, the occurr ence of a "critical concentration" at 40.0 at. % Al, for which the system p asses from a ferromagnetically ordered state to a paramagnetic one, is evid enced. The model allows obtaining an estimate of the exchange energy betwee n Fe-Fe pairs. How this energy depends on the Al concentration and the role of the manganese atoms is also presented and discussed.