Thickness-Dependent Spin Pumping in YIG/W$_{90}$Ti$_{10}$ Bilayers

We investigate the spin pumping efficiency in YIG/YIG/W$_{90}$Ti$_{10}$ bilayers by measuring the thickness dependence of both the YIG and WTi layers using broadband ferromagnetic resonance (FMR) spectroscopy. The deposition of a 5-nm WTi layer leads to enhanced Gilbert damping in thinner YIG films, indicating efficient spin current injection. From the spin pumping contribution to the damping of the YIG/WTi bilayer, we determine an effective spin mixing conductance of $3.3 \times 10^{18}~\mathrm{m}^{-2}$ for the 5-nm WTi layer. Further measurements with varying WTi thickness reveal a non-monotonic dependence of spin mixing conductance, peaking at $4.2 \times 10^{18}~\mathrm{m}^{-2}$ for a 3-nm WTi layer. This behavior is attributed to a structural phase transition from the high-spin--orbit $\beta$-phase to the less efficient $\alpha$-phase in thicker WTi layers. Furthermore, comparative analysis with YIG/W bilayers shows that Ti doping significantly reduces $g^{\uparrow\downarrow}_{\mathrm{eff}}$. These findings highlight the critical role of alloy composition and structural phase in tuning spin transport for spintronic applications.