Thickness dependence of magnetization reversal and magnetostriction in ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ 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 ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$. This study focuses on thin ${\mathrm{Fe}}_{0.81}{\mathrm{Ga}}_{0.19}$ films of thickness 5, 10, 20, and 60 nm deposited by sputtering onto glass substrates under a deposition field $H_{\mathrm{dep}}\sim 300$ 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 ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ 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.

Figure 1

$\theta -2\theta$ pattern for 60- and 20-nm-thick ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ films. Inset: Full angle scan.

Figure 2

High-resolution TEM image (left column: direct space; right column: reciprocal space) of a typical glass/${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$(7 nm)/$\mathrm{Ta}$ bilayer is shown in the top image. The bottom figure shows the image obtained for the glass/${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$(40 nm)/$\mathrm{Ta}$ thick film.

Figure 3

Saturation magnetic moment as a function of ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ film thickness. The solid line is a linear fit.

Figure 4

In-plane magnetization loops of thickness 5 and 60 nm. Longitudinal $M_L$ (red $▾$) and transverse $M_T$ (blue $\circ$) components are shown. $\phi$ is the angle between the deposition axis and the direction of the in-plane applied field. $\phi =0$ corresponds to $H$ applied parallel to the direction of the deposition field.

Figure 5

Azimuthal behavior of the normalized maximum transverse magnetization, $\mathrm{Max}(M_T)/M_s$, for different ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ film thicknesses.

Figure 6

Azimuthal behavior of the coercive field $H_c$ for different ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ film thicknesses.

Figure 7

Temperature evolution of the coercive field, $H_c$, for different ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ film thicknesses.

Figure 8

Azimuthal behavior of magnetostriction for $t=5,10,20,60$-nm-thick films. For each thickness, the solid line is a fit to experimental points using the function $A+B\cos (2\phi +\delta )$.

Figure 9

Magnetostrictive bending $H$ cycles of ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ cantilever films for thicknesses $t=5,10,20,60$ nm. Magnetic field is applied either parallel to the cantilever length ($\parallel$) or perpendicular to it ($\perp$).

Figure 10

Evolution of the magnetoelastic coefficient, $|b{|}_{\mathrm{eff}}$, and magnetostriction coefficient, ${\lambda }_{\mathrm{eff}}$, with ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ thickness. The solid line is a guide for the eye.