We present measurements of spin-orbit torques (SOTs) generated by Ir as a function of film thickness in sputtered Ir/CoFeB and Ir/Co samples1 via spin-torque ferromagnetic resonance (ST-FMR)2 and second harmonic Hall3 techniques. We find that Ir provides a damping-like component of spin-orbit torque with a maximum spin-torque conductivity( and a maximum spin-torque efficiency of ξDL = 0.042 ± 0.005, which is sufficient to drive switching in an 0.8 nm film of CoFeB with perpendicular magnetic anisotropy. We also observe a surprisingly large field-like spin-orbit torque (FLT). Measurements as a function of Ir thickness indicate a substantial contribution to the FLT from an interface mechanism so that in the ultrathin limit there is a non-zero FLT with a maximum torque conductivity(. When the Ir film thickness becomes comparable to or greater than it’s spin diffusion length, 1.6 ± 0.3 nm, there is also a smaller bulk contribution to the field-like torque.

References
1. Dutta, S., Bose, A., Tulapurkar, A. A., Buhrman, R. A. & Ralph, D. C. Interfacial and bulk spin Hall contributions to fieldlike spin-orbit torque generated by iridium. Phys. Rev. B 103, 184416 (2021). 2. Liu, L., Moriyama, T., Ralph, D. C. & Buhrman, R. A. Spin-Torque Ferromagnetic Resonance Induced by the Spin Hall Effect. Phys. Rev. Lett. 106, 036601 (2011). 3. Hayashi, M., Kim, J., Yamanouchi, M. & Ohno, H. Quantitative characterization of the spin-orbit torque using harmonic Hall voltage measurements. Phys. Rev. B 89, 144425 (2014).