Local probe of irradiation-induced structural changes and orbital magnetism in Fe60Al40 thin films via an order-disorder phase transition

Abstract

Hard x-ray absorption and magnetic circular dichroism spectroscopy have been applied to study the consequential changes of the local environment around Fe atoms and their orbital polarizations in 40 nm thick ${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$ thin films along the order-disorder $(B2\ensuremath{\rightarrow}A2)$ phase transition initiated by 20-keV ${\mathrm{Ne}}^{+}$ ion irradiation with fluences of $(0.75--6)\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{4pt}{0ex}}\text{ions}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. The analysis of the extended x-ray absorption fine structure spectra measured at the Fe K edge at room temperature revealed an increased number of Fe-Fe nearest neighbors from 3.47(7) to 5.0(1) and $\ensuremath{\sim}1%$ of volume expansion through the transition. The visualization of the Fe and Al nearest-neighbor rearrangement in the first coordination shell of Fe absorbers via the transition was carried out by wavelet transformations. The obtained changes in Fe coordination are evidently reflected in the x-ray magnetic circular dichroism spectra which show an increased orbital magnetic moment of Fe atoms and a pronounced magnetic multielectronic excitations peak at $\ensuremath{\sim}60$ eV above the edge. The amplitudes of both peaks demonstrated similar dependencies on the irradiation fluence. The results of self-consistent density functional calculations on relaxed ${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$ model structures for the ordered $(B2)$ and the disordered $(A2)$ phases are consistent with the experimental findings and point to the formation of Fe-rich regions in the films studied.