Estudo das propriedades magnéticas e estruturais dos sistemas de ligas Fe-Al, Fe-Mn e Fe-Mn-Al

Abstract

In this work we studied the magnetic properties of the alloy systems: Fe-Al binary alloys in the BCC phase, Fe-Mn binary alloys in the FCC phase and Fe-Mn-Al ternary alloys in the BCC and FCC phases. All these alloys were quenched from 1000°C and this heat treatment assures that the alloys are all disordered. In the Fe-Al alloys the ferromagnetic disordered phase (á-Fe) is extended to the critical Al concentration of qc=0.475. The ferro- to paramagnetic transition obtained as observed by Mössbauer effect, due to the increase of Al concentration or the temperature, is done in steps. These steps are a consequence of the random occupation of the Fe sites by Al. The magnetic properties of the alloys are well interpreted utilizing a site diluted Ising model using the variational method based in Bogolyubov inequality and na aproximation of linked spins pairs and free spins. In this model system we utilized a exchange constant J which depends of the Al concentration, due to the expansion of the lattice. The studies of ternary alloys in BCC phase show that the Mn atom penetrates in the lattice in a random way and that the magnetic properties of these alloys systems are similar to that Fe-Al alloys. This suggests that it may possible to apply the site diluted Ising model, but be possible to apply the site siluted Ising model, but with a constant exchange parameter J, to investigates its magnetic properties. The alloys are ferromagnetic and disordered with the Al and Mn atoms acting as dilutors of this interaction. The studies of ternary alloys in FCC phase show that they are antiferromagnetic for small Al concentration and paramagnetic for higher Al concentration. The Mn atom is responsible for the antiferromagnetic exchange and the Al and Fe dilute this interaction.