Theoretical and experimental comparative study about magnetic behavior of Fe-Al alloys in the high Fe concentration region

Abstract

In this research, annealing samples of Fe100-qAlq (with q ≤ 600/32; Δq = 100/32) were obtained by arc-melting. These were heat treated at 1000 °C during a week followed by quenching in ice water. The ferromagnetic iron matrix was modified with the non-magnetic Al atom. These samples were analyzed by X-ray diffraction and Mössbauer spectroscopy at room temperature. All alloys present disordered body centered cubic (BCC) structure or A2 with mean crystallite sizes of about 34 nm. It was observed that when q = 3.125% the iron's hardness falls over 80%. Furthermore, Mössbauer spectroscopy shows that all alloys are disordered and ferromagnetic. The spectra were fitted using a hyperfine magnetic field distribution with ΔH = 1T. Statistical and hyperfine parameters of the distributions were obtained as a function of q. After fixing the Mössbauer spectra, it was found that the mean hyperfine magnetic field (MHMF) presents a linear decrease with the Al concentration. This behavior is strongly related with the diluted character of the impurity. Through Mössbauer spectroscopy and/or X-ray diffraction, an anomalous behavior was observed around q = 12.500. Moreover, by using the Monte Carlo method and the local environment model, the long-range order parameter of the samples was possible to obtain. Additionally, only 2 samples reached the perfect disorder, which indicated that the method of preparation does not guarantee the state of perfect disorder. The local environment model allowed finding the variations in the hyperfine field of the Fe when one atom of Fe is replaced by one Al atom in the first or second coordination shell of the 57Fe. Finally, the Monte Carlo method with Metropolis dynamics and the ½ Ising model were used to simulate the magnetic behavior of the alloys, used in this study. The results were compared with experimental phase diagrams of these alloys, allowing us to find the functionality of the exchange energy in the system as a function of Al concentration. For this system, the phase diagram was fitted using a cubic function of the JFeFe exchange energy with the aluminum concentration.