Crystal structure and local ordering in epitaxial Fe${}_{100-x}$Ga${}_x$/MgO(001) films

Abstract

In this work we present a study of the structural properties of Fe${}_{100-x}$ Ga${}_x$ 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 $insitu$ Reflected High Energy Electron Diffraction. For substrate temperature $T_s$ below 400 ${}^o$C we obtain $bcc$ films while, for $x\approx$ 24 and $T_s\ge$ 400${}^o$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${}_3$-like ordered phase, leading to a minimization of the number of Ga-Ga pairs. On the other side, a true 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 magnetostrictive properties, and second, demonstrates the possibility of growing $fcc$ films at temperatures much smaller than those required to obtain bulk $fcc$ samples.

21 pages, 7 figures