Tailoring the Growth of $\beta$-Tungsten Using Substrate Bias and Its Effect on FMR-Driven Spin Pumping in $\beta$-W/Py Heterostructures
$\beta$-Tungsten ($\beta$-W), an A15 cubic phase of tungsten, exhibits a giant spin Hall angle compared to its bcc-phase $\alpha$-Tungsten ($\alpha$-W), making high-quality $\beta$-W films desirable for spintronic applications. We report the controlled growth of $\beta$-W films on SiO$_2$/Si substrates via DC sputtering, where substrate bias serves as a critical factor in stabilizing the $\beta$ phase by regulating the energy of deposited atoms. This approach enables the formation of $\beta$-W films over a wide thickness range. Additionally, we studied the spin pumping phenomena in different tungsten phases achieved through substrate bias. Ferromagnetic resonance measurements reveal an enhancement in the magnetic damping (\( \alpha_{\text{eff}} \)) for $\beta$-W/Py compared to $\alpha$-W/Py dominated film. The effective spin mixing conductance (\( g_{\text{eff}}^{\uparrow\downarrow} \)) is found to be higher in $\beta$-W/Py than in $\alpha$-W/Py, which is attributed to variations in the interface structures between these phases. Importantly, the use of substrate bias does not deteriorate the interface quality, underscoring its effectiveness. These findings highlight the potential of substrate bias in thin-film engineering, paving the way for its advanced utilization in spintronic applications.