Effects of thermal annealing on the spin–orbit torque and unidirectional spin Hall magnetoresistance in Pt/CoFe/Ta trilayers

We investigated the spin–orbit torque (SOT) and unidirectional spin Hall magnetoresistance (USMR) in Pt/CoFe/Ta trilayer as well as Pt/CoFe and CoFe/Ta bilayers with in-plane magnetic anisotropy by performing transverse and longitudinal second harmonic resistance measurements. Compared to the two bilayers, we found that the trilayer exhibits enhanced SOT and USMR due to the opposite spin Hall angles of Pt and Ta, which work together to enhance the spin accumulation in the trilayer. Furthermore, we found that thermal annealing has a significant influence on the magnitude and sign of the SOT and USMR in the Pt/CoFe/Ta trilayers. Specifically, we observed that both the damping-like SOT and USMR of the trilayer decrease as the annealing temperature increases, and they even change signs at an annealing temperature between 235 and 265 °C. In contrast, the sign change of the SOT and USMR upon annealing is absent in the Pt/CoFe and CoFe/Ta bilayers. These findings suggest that the sign of the SOT and USMR in the Pt/CoFe/Ta trilayer can be easily manipulated by using an appropriate thermal annealing treatment, which has important implications for the development of novel spintronic devices.