Stacking-Order Effect on Spin-Orbit Torque, Spin Hall Magnetoresistance, and Magnetic Anisotropy in Ni81Fe19–IrO2 Bilayers

The 5d transition-metal oxides are an intriguing platform to demonstrate efficient charge-to-spin-current conversion due to a unique electronic structure dominated by strong spin-orbit coupling. Here, we report on the stacking-order effect of spin-orbit torque (SOT), spin-Hall magnetoresistance, and magnetic anisotropy in bilayer ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$-5d iridium oxide, ${\mathrm{Ir}\mathrm{O}}_{2}$. While all ${\mathrm{Ir}\mathrm{O}}_{2}$ and $\mathrm{Pt}$ control samples exhibit large dampinglike SOT generation, stemming from the efficient charge-to-spin-current conversion, the magnitude of the SOT is larger in the ${\mathrm{Ir}\mathrm{O}}_{2}$($\mathrm{Pt})$ bottom sample than in the ${\mathrm{Ir}\mathrm{O}}_{2}$($\mathrm{Pt})$ top one. The fieldlike SOT has an even more significant stacking-order effect, resulting in an opposite sign in the ${\mathrm{Ir}\mathrm{O}}_{2}$ samples in contrast to the same sign in the $\mathrm{Pt}$ samples. Furthermore, we observe that the magnetic anisotropy energy density and the anomalous Hall effect are increased in the ${\mathrm{Ir}\mathrm{O}}_{2}$($\mathrm{Pt})$ bottom sample, suggesting enhanced interfacial perpendicular magnetic anisotropy. Our findings highlight the significant influence of the stacking order on spin transport and the magnetotransport properties of $\mathrm{Ir}$ oxide-ferromagnet systems, providing useful information for the design of SOT devices, including 5d transition-metal oxides.