We investigated the spin-orbit torque (SOT) switching of the single CoTb layer with or without the heavy metals (Pt and W) in different thicknesses to identify the self-torque effects generated in the CoTb layer. The generation of self-torque heavily depends on the thickness of CoTb. The SOT switching was not observed for a single CoTb layer with 3nm in thickness, suggesting self-torque may be quite weak in a very thin film. A deterministic SOT switching can be obtained for a single 9 nm CoTb layer. The amplitude of the self-torque generated in CoTb is comparable to the Pt/Co case with the same switching polarity as the Pt/Co one. When 3 nm CoTb has deposited on Pt or W underlayer, the Jc is smaller in W/CoTb than that of Pt/CoTb, which can be attributable to a high spin hall angle of W and negligible self-torque of CoTb. On the other hand, when the CoTb is increased to 9 nm, the Jc of W/CoTb is significantly increased and becomes higher than that of Pt/CoTb. The enhanced Jc of W/CoTb may result from the opposite sign of spin-orbit torque generated in W and CoTb. Since the self-torque generated in 9 nm CoTb becomes substantial, the competition of spin-orbit torque between W and CoTb occurs, leading to a high Jc. We define the switching efficiency η=(2Hk2-Hx2)1/2*Mst/Jc based on the macrospin model 1, in which Hk and Hx are anisotropy field and applied magnetic field along the x-direction. The variations of η with the layer structure are shown in Table 1. The addition of SOT from Pt and self-torque from CoTb leads to enhanced switching efficiency when the thickness of CoTb is increased. On the other hand, the competition of SOT from W and self-torque in TbCo of W/CoTb devices was revealed by increasing the thickness of CoTb. We further confirm this interplay by reversing the stack order, that is, CoTb/W, in which the efficiency is substantially enhanced.

References
1 T. Taniguchi, S. Mitani, and M. Hayashi, Physical Review B., Vol.92, 024428 (2015).