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

TL;DR

This study measures magnetoelastic coupling in thin FeGa films, revealing enhanced stress linked to the A2 crystal structure and electronic effects, differing from bulk behavior and emphasizing the role of epitaxial growth.

Contribution

It provides direct measurements of magnetoelastic coefficients in epitaxial FeGa thin films and links the enhanced stress to the A2 phase and electronic structure modifications.

Findings

$-B_1$ in thin films is about twice the bulk value for 22 at.% Ga.

No thickness dependence observed for $B_1$ and $B_2$ in the studied films.

Enhanced magnetoelastic stress is associated with the A2 crystal structure and electronic effects.

Abstract

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 $B_1$ and $B_2$ in epitaxial FeGa thin films below 50 nm deposited on the MgO(001) surface at 150 $^\circ$C by the cantilever method. Series of films with 22, 28, and 33 at. % Ga do not show thickness-dependent variations for $B_1$ and $B_2$, but $-B_1$ for the 22 at. % Ga composition is 10 MPa, roughly 2 times the bulk value and smaller than the bulk-like value of $-B_1$=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 D0$_3$ or other ordered nanometric precipitates proposed for bulk samples. Synchrotron diffraction excludes the formation of long-range L6$_0$, or D0$_3$ 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 D0$_3$ 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 $B_1$ in films with the A2 phase, stabilized using epitaxial growth.