Thickness dependence of magnetization reversal and magnetostriction in Fe81Ga19 thin films

Among the magnetostrictive alloys, the one formed of iron and gallium (called "Galfenol" from its U.S. Office of Naval Research discoverers in the late 1990s) is attractive for its low hysteresis, good tensile stress, good machinability, and its rare-earth-free composition. One of its applications is its association with a piezoelectric material to form an extrinsic multiferroic composite as an alternative to the rare room-temperature intrinsic multiferroics such as BiFeO3. This study focuses on thin Fe0.81Ga0.19 films of thickness 5, 10, 20, and 60 nm deposited by sputtering onto glass substrates under a deposition field Hdep∼300 Oe. Magnetization reversal study reveals a well-defined symmetry with two principal directions independent of the thickness. The magnetic signature of this magnetic anisotropy decreases with increasing Fe81Ga19 thickness due to an increase of the nonpreferential polycrystalline arrangement, as revealed by transmission electron microscopy (TEM) observations. Thus, when magnetic field is applied along these specific nontrivial directions, magnetization reversal is mainly coherent for the thinnest sample, as seen from the transverse magnetization cycles. The magnetostriction coefficient reaches 20 ppm for the 5-nm film and decreases for thicker samples, where the polycrystalline part with nonpreferential orientation prevails.