Current-driven perpendicular magnetization switching in Ta/CoFeB/[TaO_xor MgO/TaO_x] films with lateral structural asymmetry

We study the current-driven perpendicular magnetization switching in Ta/CoFeB(wedge)/[TaO_xor MgO/TaO_x] devices with a lateral structural asymmetry introduced by a varying CoFeB thickness. In these devices, an in-plane current can generate a field-like torque and its corresponding effective magnetic field (H z F L) is out-of-plane, which can deterministically switch perpendicular magnetization at zero magnetic field. Experimental results indicate that the method used for breaking lateral structural symmetry greatly affects the resulting field-like torque, and that the gradient of perpendicular anisotropy, resulting from the CoFeB thickness variation, is not by itself sufficient to give rise to the current-induced H z F L. Analysis of the oxidation gradient at the CoFeB/TaO_xinterface indicates that the oxidation gradient may play a more important role than the gradient of magnetic anisotropy for the generation of H z F L. For practical applications, the demonstration of perpendicular magnetization switching in Ta/CoFeB(wedge)/MgO/TaO_xdevices potentially allows for using MgO-based magnetic tunnel junctions for readout in three-terminal memory devices without the need for external magnetic fields.