Enhanced magnetoelastic stress in disordered iron-gallium alloy thin films revealed by direct measurement

The large magnetostriction in FeGa alloys is relevant for manifold applications, but for thin films, it can play a prominent role in controlling the strength of the magnetic anisotropy. Bulk samples show values depending on the extensive preparation procedure compendium, which is limited in its temperature range for high-quality thin-film synthesis. Here, we present a study of the magnetoelastic coupling coefficients B1 and B2 in epitaxial FeGa thin films below 50 nm deposited on the MgO(001) surface at 150 ºC by the cantilever method. Series of films with 22, 28, and 33 at. % Ga do not show thickness-dependent variations for B1 and B2, but -B1 for the 22 at. % Ga composition is 10 MPa, roughly 2 times the bulk value and smaller than the bulk-like value of -B1=12.1 MPa obtained for a film with 17 at. % Ga. This enhancement is correlated with the A2 crystal structure for the film rather than the coexistence with D03 or other ordered nanometric precipitates proposed for bulk samples. Synchrotron diffraction excludes the formation of long-range L60, or D03 precipitates in samples with (001)A2 peaks at concentrations around 25 at. % Ga, which implies partial chemical disorder. The analysis of extended x-ray absorption fine structure measurements points to a D03 local order with a residual number of Ga-Ga pairs. Considering that the substrate quenches the movable strain in the A2 phase described in dual-phase structures, our results point to the important role of the electronic structure of the iron atoms modified by the presence of Ga in the alloy. This effect enlarges B1 in films with the A2 phase, stabilized using epitaxial growth.