Separating the bulk and interface contribution of spin-orbit torque in ferromagnet-Heavy metal bilayers tuned by variation of resistivity of heavy metal

Abstract: Harmonic Hall measurements were conducted on a series of Ferromagnetic metal/Heavy metal (FM/HM) bilayers with beta-Tungsten (W) as the HM and in-plane magnetized permalloy (Py) as the FM and the efficiencies of the two orthogonal components of the spin orbit-torque were extracted. Two sets of Hall bar-shaped devices were considered where the HM resistivity systematically varied over a wide range (sim150-1000 muOmega-cm) while the FM layer remained the same and each set having a different aspect ratio of voltage pickup line width and Hall bar width. Using numerical simulations of current distribution at the region between voltage pickup lines we have normalised the SOT efficiencies and examined their dependence. The current-induced spin-orbit torque efficiency in ferromagnetic metal (FM)/heavy metal (HM) bilayers is quantitatively investigated in this study.beta-W, known for its high spin-orbit coupling, served as the HM layer, while Py, an FM with an in-plane magnetic anisotropy, comprised the other layer. We performed a thorough analysis of the second harmonic Hall resistance (R_{xy}^{2\omega}) obtained from Py/beta-W bilayer devices, systematically varying the resistivity (rho_W) of the beta-W layer within the range of 200 to 1000 \mu\Omega-cm by employing a fixed current density (J_W\sim0.8\times10^{11} A/m^2) through beta-W. Through this analysis, we derived the Slonczewski-like efficiency (xi_{SL}) and field-like efficiency (\xi_{FL}) as a function of rho_W. Notably, the device with a resistivity of 980 muOmega-cm exhibited the highest xi_{SL}, yielding a value of -0.42 0.09. These results highlight the promising potential of highly resistiv beta-W as a material of interest in spintronics research.