  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 90's)
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 a extrinsic multiferroic composite as an
alternative to the rare room temperature intrinsic multiferroics such as
BiFeO$_3$. This study focuses on thin Fe$_{0.81}$Ga$_{0.19}$ films of thickness
5, 10, 20 and 60 nm deposited by sputtering onto glass substrates.
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 FeGa thickness due to an increase
of the non-preferential polycrystalline arrangement, as revealed by
transmission electron microscopy (TEM) observations. Thus when magnetic field
is applied along these specific directions, magnetization reversal is mainly
coherent for the thinnest sample as seen from the transverse magnetization
cycles. Magnetostriction coefficient reaches 20 ppm for the 5 nm film and
decreases for thicker samples, where polycrystalline part with non-preferential
orientation prevails.
