Comparative Study of Current-Induced Torque in Cr/CoFeB/MgO and W/CoFeB/MgO

Orbital torque (OT) in magnetic heterostructures has been actively discussed in terms of its actual existence and usefulness in comparison to the spin-orbit torque (SOT) that shows promise for next-generation magnetoresistive random access memories. The objectives of this study are 2-fold: (i) making an apples-to-apples comparison in two representative stacks where OT and SOT are expected to dominate and (ii) examining the potential emergence of OT in archetypal SOT stacks. Cr/CoFeB/MgO and W/CoFeB/MgO are chosen as the OT- and SOT-dominant systems, respectively. Systematic variations in each layer's thicknesses reveal that (i) Cr/CoFeB/MgO exhibits substantial torque comparable to or even exceeding that of the W/CoFeB/MgO stack when Cr and CoFeB layers are especially thick and (ii) the torque in W/CoFeB/MgO changes sign with increasing W and CoFeB thicknesses, suggesting a crossover of the dominant mechanism from SOT to OT. The findings clarify the opportunities and challenges of devices leveraging SOT and OT.