Spin-Orbit Torque and Magnetic Damping in Tailored Ferromagnetic Bilayers

We study spin-orbit-torque-driven ferromagnetic resonance (FMR) in ferromagnetic (FM) bilayers, consisting of Co and permalloy (Py) sandwiched between Pt and MgO layers. We find that the FM layer in contact with the Pt layer dominantly determines the spin Hall angle, which is consistent with the spin-transparency model. By contrast, the FMR linewidths are considerably influenced not only by the spin-pumping effect across the Pt/FM interface but also by the spin relaxation such as two-magnon scattering at the FM/MgO interface. The Co/MgO interface leads to notably increased FMR linewidths, while the Py/MgO interface has less effect. The different contributions of each interface to the spin Hall angle and dissipation parameter suggest that the stack configuration of Pt/Co/Py/MgO requires less writing energy than Pt/Py/Co/MgO in spin-orbit-torque-driven magnetic switching. Our approach offers a practical method to optimize material parameters by engineering either interfaces in contact with the heavy metal or the oxide layer.