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

La Torre, E.; Smekhova, A.; Schmitz-Antoniak, C.; Ollefs, K.; Eggert, B.; Cöster, B.; Walecki, D.; Wilhelm, F.; Rogalev, A.; Lindner, J.; Bali, R.; Banerjee, R.; Sanyal, B.; Wende, H.


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 Fe60Al40 thin films of 40 nm thickness along the order-disorder (B2→A2) phase transition initiated by 20keV Ne+ ion-irradiation with fluences of (0.75 - 6) × 1014 ions cm-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 ~ 1% of volume expansion through the transition. The visualization of the Fe and Al nearest neighbors 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 XMCD spectra which show an increased orbital magnetic moment of Fe atoms and a pronounced magnetic multi-electronic excitations peak at ~60 eV above the edge. The amplitudes of both peaks demonstrated similar dependencies on the irradiation fluence. The results of self-consistent DFT calculations on relaxed Fe60Al40 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.

Keywords: Chemical disorder; ion-irradiation; magnetism; EXAFS; wavelet transform; XMCD; DFT

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