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

TL;DR

This study investigates the structural properties of Fe-Ga films grown on MgO(001), revealing local ordering phenomena and phase transitions between bcc and fcc structures depending on substrate temperature, with implications for magnetic properties.

Contribution

It demonstrates the epitaxial growth of Fe-Ga films with local ordering without long-range D0$_3$ phase formation, and shows the possibility of fcc phase growth at lower temperatures than bulk methods.

Findings

Ga/Fe local ordering resembles D0$_3$-like phase

No long-range D0$_3$ phase observed

Epitaxial growth avoids detrimental long-range order

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 $in situ$ Reflected High Energy Electron Diffraction. For substrate temperature $T_s$ below 400 $^o$C we obtain $bcc$ films while, for $x \approx$ 24 and $T_s \geq$ 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.