Evidence for lattice softening of the Fe-Ga magnetostrictive alloy: Stress-induced local martensites

The over tenfold magnetostriction enhancement by soluting nonmagnetic Ga into body centered cubic (BCC) Fe has been ascribed to crystal lattice softening, which indicates that the Fe-Ga solid solution lies in the crossover of martensitic transformation. Identifying the martensitic phase then becomes essential. Here we report the stress-induced local six-layer modulated monoclinic (6 M) martensites in a Fe₇₉Ga₂₁ alloy solution-treated at 1373 K. Although X-ray diffraction (XRD) and magnetic measurements reveal that the bulk sample exhibits BCC average structure, the transmission electronic microscopy (TEM) characterizations however show that the mechanically grinded foil sample contains local 6 M phase. First principal calculations suggest that low formation energy barrier between the ordered D0₃ and the 6 M phases are responsible for the stress-induced structural transformation. Our work provides evidence for the lattice softening of Fe-Ga solid solution, adding important insight into understanding the origin of its extraordinary magnetostriction enhancement.