Crystal structure and local ordering in epitaxial Fe100−xGax/MgO(001) films

In this work we present a study of the structural properties of Fe100-xGax(x < 30) films grown by Molecular Beam Epitaxy on Mg0(100). We combine long range and local/chemically selective X-ray probes (X-ray Diffraction and X-ray absorption spectroscopy) together with real space imaging by means of Transmission Electron Microscopy and surface sensitive in situ Reflected High Energy Electron Diffraction. For substrate temperature T-s below 400 degrees C we obtain bcc films while, for x approximate to 24 and T-s >= 400 degrees C the nucleation of the fcc phase is observed. For both systems a Ga anticlustering or local range ordering phenomenon appears. The Ga/Fe composition in the first and second coordination shells of the bcc films is different from that expected for a random Ga distribution and is close to a D0(3) phase, leading to a minimization of the number Ga-Ga pairs. On the other side, a long-range D0(3) phase is not observed indicating that atomic ordering only occurs at a local scale. Overall, the epitaxial growth procedure presented in this work, first, avoids the formation of a long range ordered D0(3) phase, which is known to be detrimental for magnetostrictive properties, and second, demonstrates the possibility of growing fcc films at temperatures much lower than those required to obtain bulk fcc samples. (C) 2018 Elsevier B.V. All rights reserved.