Atomistic Insights into Magnetostriction Enhancement in Boron-doped Fe80Ga20 Thin Film

Magnetoelastic materials have gained increasing interest due to their energy efficiency and versatility in applications such as actuators, sensors, and energy harvesters, by leveraging the strong magnon-phonon coupling and the long propagation distance of magnetoelastic waves [1]. Strong magnetoelastic coupling is often associated with materials with large magnetostriction. Fe80Ga20 alloy is one of the strong candidates for magnetoelastic devices by offering large magnetostriction coefficients and low Gilbert damping constants. For Fe80Ga20 thin films, it has been shown that B-doping can further enhance the magnetostriction, allowing device fabrication with higher efficiency and better performance [2]. However, it was also shown that magnetostriction constant of a (Fe80Ga20)100-xBx thin film also has a non-linear dependence on the Boron concentration, where 10% B-doping in 80 nm (Fe80Ga20)100-xBx thin films leads to an optimal magnetostriction constant of ∼50 to 70 ppm [2,3]. Therefore, clarification of the impact of B-doping on magnetostriction at atomistic scale in B-doped Fe80Ga20 thin films is crucial for the next generation magnetostrictive device design and fabrication. Here, we present a systematic structural and compositional analysis of (Fe80Ga20)100-xBx (0<x<16) thin films with 16 nm thickness using four-dimensional (4D-) scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Our results show that the sputtered (Fe80Ga20)100-xBx thin films consist of nanometer-sized grains where are Fe-rich, surrounded by Ga-rich network, indicating a non-uniform elemental distribution. Using 4D-STEM, we show that B-doping suppresses the long-range crystalline order of Fe-Ga alloy, leading to a mixed crystalline-amorphous phase for less than 6% B-doping and pure amorphous phase with more than 6% B-doping. More importantly, using exit-wave power cepstrum (EWPC) [4,5] based nearest-neighbor (NN) distance mapping, we show that the B-doping significantly decreases the Fe-Fe/Fe-Ga NN distances compared pure Fe80Ga20. Furthermore, Fe-Fe/Fe-Ga NN distances increase as B% increases. Combining with our macroscopic magnetostriction measurement of these thin films, we show that the NN distance and magnetostriction are negatively correlated, where a larger NN distance corresponds to a low magnetostriction in (Fe80Ga20)100-xBx, which can be tuned by B%. To conclude, using 4D-STEM/EELS we have established that the magnetostrictive properties of (Fe80Ga20)100-xBx is directly correlated to the Fe-Fe/Fe-Ga NN distance, which can be tuned by B-dopant concentrations. Our result has laid the groundwork for the design and fabrication of the magnetostrictive thin films with enhanced magnetoelastic coupling [6].