Tunable spin–orbit torque switching in antiferromagnetically coupled CoFeB/Ta/CoFeB

We demonstrate a tunable spin–orbit torque (SOT) switching in an antiferromagnetically coupled CoFeB/Ta/CoFeB trilayer through careful design of magnetic anisotropies, where the thicker bottom CoFeB layer has a relatively weak perpendicular magnetic anisotropy (PMA) and PMA of the upper CoFeB layer is robust. The unique anisotropy feature causes a strong sensitivity to the assistant field during SOT switching, resulting in a slanted magnetic moment and a highly tunable switching window defined by the difference in Hall resistance between two SOT switching states at zero current. By further reducing the PMA of bottom CoFeB, only the upper layer can be switched by SOT. Until now, there are three types of SOT switching in antiferromagnetically coupled ferromagnetic metal/nonmagnetic metal/ferromagnetic metal trilayers: simultaneous switching between two antiparallel states with strong PMA for both ferromagnetic layers, field-free switching where one layer has in-plane magnetization and the other has canted out-of-plane magnetization, and tunable switching reported in this work. Our findings enrich the physical phenomena in trilayer SOT and may have potential application in multilevel storage and neural computing.