Magnetostriction and Temperature Dependent Gilbert Damping in Boron Doped Fe$_{80}$Ga$_{20}$ Thin Films

Magnetic thin films with strong magnetoelastic coupling and low Gilbert damping are key materials for many magnetoelectric devices. Here, we investigated the effects of boron doping concentration on magnetostriction and temperature dependent Gilbert damping in magnetron sputtered (Fe$_{80}$Ga$_{20}$)$_{1-x}$B$_{x}$ films. A crystalline to amorphous structural transition was observed for a boron content near 8% and coincided with a decrease in coercivity from 76 Oe to 3 Oe. A 10% doping concentration is optimal for achieving both large magnetostriction of 48.8 ppm and low Gilbert damping of $6 \times 10^{-3}$. The temperature dependence of the damping shows an increase at low temperatures with a peak around 40 K and we associate the relative increase $\Delta\alpha/\alpha_{RT}$ with magnetoelastic contributions to the damping, which has a maximum of 55.7% at 8% boron. An increase in the inhomogeneous linewidth broadening was observed in the structural transition regime at about 8% boron concentration. This study suggests that incorporation of glass forming elements, in this case boron, into Fe$_{80}$Ga$_{20}$ is a practical pathway for simultaneously achieving enhanced magnetoelastic coupling and reduced Gilbert damping.