Spin-orbit torque generation in Ni Fe / Ir O 2 bilayers

The $5d$ transition-metal oxides have a unique electronic structure dominated by strong spin-orbit coupling and hence they can be an intriguing platform to explore spin-current physics. Here, we report on room-temperature generation of spin-orbit torque (SOT) from a conductive $5d$ iridium oxide, $\\mathrm{Ir}{\\mathrm{O}}_{2}$. By measuring second-harmonic Hall resistance of ${\\mathrm{Ni}}_{81}{\\mathrm{Fe}}_{19}/\\mathrm{Ir}{\\mathrm{O}}_{2}$ bilayers, we find both dampinglike and fieldlike SOTs. The former is larger than the latter, enabling easier control of magnetization. We also observe that the dampinglike SOT efficiency has a significant dependence on $\\mathrm{Ir}{\\mathrm{O}}_{2}$ thickness, which is well described by the drift-diffusion model based on the bulk spin Hall effect. We deduce the effective spin Hall angle of +0.093 \\ifmmode\\pm\\else\\textpm\\fi{} 0.003 and the spin-diffusion length of 1.7 \\ifmmode\\pm\\else\\textpm\\fi{} 0.2 nm. By comparison with control samples Pt and Ir, we show that the effective spin Hall angle of $\\mathrm{Ir}{\\mathrm{O}}_{2}$ is comparable to that of Pt and seven times higher than that of Ir. The fieldlike SOT efficiency has a negative sign without appreciable dependence on the thickness, in contrast to the dampinglike SOT. This suggests that the fieldlike SOT likely stems from the interface. These experimental findings suggest that the uniqueness of the electronic structure of $5d$ transition-metal oxides is crucial for highly efficient charge to spin-current conversion.