  We experimentally observe nonlinear spin torque in metallic bilayers of
platinum and permalloy by means of spin-torque ferromagnetic-resonance (ST-FMR)
under massive dc current injection. The observed nonlinear spin torque exerted
to permalloy magnetization is attributed primarily to nonlinear spin
polarization. Additional origin of the nonlinear spin torque is magnon
generation (annihilation) followed by shrinkage (expansion) of effective
magnetization, which is reveled by STFMR and unidirectional spin Hall
magnetoresistance measurements. The present study paves a way to spin-Hall
effect based nonlinear spintronic devices as well as time-varying nonlinear
magnetic metamaterials with tailor-made permeability.
