Phase Transformation and Magnetostriction in Fe100-xGax bulk alloys

Abstract

Large magnetostriction and magnetoelastic coupling are properties extensively sought after because of their exploitability in low power consumption devices, energy conversion, and spintronics. Many alloys displaying large magnetostriction coefficients contain rare earths, whereas Fe-Ga systems, being rare earths free, have attracted attention because of their significant magnetostrictive response, coupled with good corrosion resistance and mechanical hardness [1]. Magnetostriction is particularly significant in Fe-rich alloys, with Ga content approximately equal to 19% and 27%. However, in this range of compositions, the Fe-Ga binary system is characterised by the possible presence of multiple phases, whose complex interplay strongly affects the magnetostrictive response. The stabilisation of the A2 phase by means of suitable thermal treatments turns out to be an effective way to obtain remarkable magnetostrictive properties.In this work, Fe-Ga alloys, containing 18, 21 and 23 at.% of Ga, were prepared in bulk form by arc melting. In their as-cast state they display a small magnetostriction, that is strongly improved after annealing at 1000 °C for 24 h, and subsequent rapid cooling. Multiple characterisation techniques, such as X-ray diffraction, Differential scanning calorimetry, Mössbauer spectroscopy, temperature-dependent magnetisation curves, hysteresis loops, magnetic force microscopy and magnetostriction measurements were exploited in synergy to gain a deep understanding of the structure-properties relationships in the studied alloys, before and after annealing. The A2 phase, which is favoured in the lower range of compositions and is promoted at the expense of the D03 one by the annealing, is responsible for characteristic dendritic and maze magnetic domains, shown in Figure 1, reporting domains patterns acquired with a magnetic force microscope on as-cast and annealed samples of the three compositions. Conversely, the lower-contrast regions, especially in the Ga23 as-cast sample, are due to the D03 phase.The transformation of the D03 phase into the A2 one with annealing, followed through extensive microstructure investigations, is responsible for the strong improvement of the magnetostriction, which almost reaches 240 ppm (transverse configuration) in the alloys with 18 at.% of Ga, after annealing (Figure 2). **