Spin-orbit torque induced magnetization dynamics and switching in a CoFeB/Ta/CoFeB system with mixed magnetic anisotropy

Spin-orbit torque (SOT) induced magnetization switching in CoFeB/Ta/CoFeB trilayer with two ferromagnetic CoFeB layers exhibiting in-plane and perpendicular magnetic anisotropy (PMA), respectively, is investigated. Interlayer exchange coupling (IEC), measured using the ferromagnetic resonance technique is modified by varying thickness of the heavy metal Ta spacer (t(Ta)). The evolution of the IEC with t(Ta) leads to different orientations of the magnetic anisotropy axes of two CoFeB layers: for t(Ta) > 1.3 nm, magnetization prefers antiferromagnetic alignment, whereas for t(Ta) < 1.3 nm, ferromagnetic coupling is measured. The magnetization state of the CoFeB layer exhibiting PMA is controlled by the spin-polarized current originating from the Ta layer in micrometer sized Hall bars. The SOT induced critical current density with Ta spacer thickness is presented, showing an increase with decreasing t(Ta), which coincides with the CoFeB coercive field dependence. In a narrow range of t(Ta) corresponding to the ferromagnetic IEC, the field-free SOT induced magnetization switching is achieved.