Modulating Spin Current Induced Effective Damping in $β-W/Py$ Heterostructures by a Systematic Variation in Resistivity of the Sputtered Deposited $β-W$ films

Abstract: Utilizing the spin-induced pumping from a ferromagnet (FM) into a heavy metal (HM) under the ferromagnetic resonance (FMR) condition, we report an enhancement in effective damping in $\beta$- W/Py bilayers by systematically varying resistivity ($\rho_{W}$) of $\beta$-W films. Different resistivity ranging from 100 $\mu\Omega$-cm to 1400 $\mu\Omega$-cm with a thickness of 8 nm can be achieved by varying the argon pressure ($P_{Ar}$) during the growth by the method of sputtering. The coefficient of effective damping $\alpha_{eff}$ is observed to increase from 0.010 to 0.025 with $\rho_{W}$, which can be modulated by $P_{Ar}$. We observe a modest dependence of $\alpha_{eff}$ on the sputtering power ($p_{S}$) while keeping the $P_{Ar}$ constant. $\alpha_{eff}$ dependence on both $P_{Ar}$ and $p_{S}$ suggests that there exists a strong correlation between $\alpha_{eff}$ and $\rho_{W}$. It is thus possible to utilize $\rho_{W}$ as a tuning parameter to regulate the $\alpha_{eff}$, which can be advantageous for faster magnetization dynamics switching. The thickness dependence study of Py in the aforementioned bilayers manifests a higher spin mixing conductance ($g^{\uparrow\downarrow}_{eff}$) which suggests a strong spin pumping from Py into the $\beta$-W layer. The effective spin current ($J_{S(eff)}$) is also evaluated by considering the spin-back flow in this process. Intrinsic spin mixing conductance ($g^{\uparrow\downarrow}_{W}$) and spin diffusion length ($\lambda_{SD}$) of $\beta$-W are additionally investigated using thickness variations in $\beta$-W. Furthermore, the low-temperature study in $\beta$-W/Py reveals an intriguing temperature dependence in $\alpha_{eff}$ which is quite different from $\alpha_{b}$ of single Py layer and the enhancement in $\alpha_{eff}$ at low temperature can be attributed to the spin-induced pumping from Py layer into $\beta$-W.