  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
layers dominantly determines that 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 in terface but
also by the spin relaxation such as two-magnon scattering at the FMMgO
interface.The CoMgO interface leads to notably increased FMR linewidths, while
the Py|MgO interface has less effect. This different contribution of each
interface to the spin Hall angel and dissipation parameter suggests 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 promising method to optimize material parameters by engineering either
interfaces in contact with the heavy-metal or the oxide layer.
