Current-induced spin-orbit torque switching of perpendicularly magnetized Hf|CoFeB|MgO and Hf|CoFeB|TaO_x structures

We study the effect of the oxide layer on current-induced perpendicular magnetization switching properties in Hf|CoFeB|MgO and Hf|CoFeB|TaO_x tri-layers. The studied structures exhibit broken in-plane inversion symmetry due to a wedged CoFeB layer, resulting in a field-like spin-orbit torque (SOT), which can be quantified by a perpendicular (out-of-plane) effective magnetic field. A clear difference in the magnitude of this effective magnetic field (H z F L) was observed between these two structures. In particular, while the current-driven deterministic perpendicular magnetic switching was observed at zero magnetic bias field in Hf|CoFeB|MgO, an external magnetic field is necessary to switch the CoFeB layer deterministically in Hf|CoFeB|TaO_x. Based on the experimental results, the SOT magnitude (H z F L per current density) in Hf|CoFeB|MgO (-14.12 Oe/10⁷ A cm^-2) was found to be almost 13× larger than that in Hf|CoFeB|TaO_x (-1.05 Oe/10⁷ A cm^-2). The CoFeB thickness dependence of the magnetic switching behavior, and the resulting H z F L generated by in-plane currents are also investigated in this work.