Ordering processes in Fe-Ga alloys studied by positron annihilation lifetime spectroscopy

To investigate the nature of defects in Galfenols (Fe100−xGax) with content near the local minimum (x=20.5 and x=22.4) and the local maximum of magnetostriction (x=26.3), the positron annihilation lifetime spectroscopy (PALS) and the X-ray diffraction analysis (XRD) are applied. It is found that the behavior of vacancies during ordering processes in the alloy with content higher than in stoichiometric composition of Fe3Ga (x>25) differs significantly from the other two alloys (x<25). All the studied alloys quenched from 1000 °C exhibit a positron lifetime of about 175 ps, which corresponds to high concentration of single vacancies (>2·10−4 at−1). The growth of the bcc-born D03 ordered phase with relatively low concentration of vacancies in Fe79.5Ga20.5 and Fe77.6Ga22.4 samples causes a decrease of the average positron lifetime. The formation of the D03 phase in Fe73.7Ga26.3 does not reduce average positron lifetime, which indicates a high concentration of constitutional vacancies in the ordered D03 phase of Fe73.7Ga26.3. The D03→L12 phase transition is always accompanied by additional vacancy formation.