Spin transfer torque in magnetic tunnel junctions with a perpendicularly magnetized polarizer

Spin-torque devices containing magnetic layers with perpendicular magnetic anisotropy are of interest for strategies to reduce the switching currents in memory applications. We report spin-torque-driven ferromagnetic resonance (ST-FMR) measurements of the bias-dependent torque in magnetic tunnel junctions containing [Co/Ni]$_{x}$ multilayers possessing perpendicular anisotropy, acting as the polarizer layer providing spin-polarized current. We observe unusual dependence of the bias-dependent torque as a function of the magnetic orientation of the [Co/Ni]$_{x}$ multilayer. We speculate that this sensitivity to the magnetic orientation may originate from changes in the occupation of spin-polarized states at the Co/Ni interfaces associated with the perpendicular magnetic anisotropy.