Temperature-Dependent Spin–Orbit Torques in Perpendicular Magnetic [Co/Ni]_N/TbCo Composite Films

The temperature (T) dependence of spin–orbit torque (SOT) efficiencies and magnetic properties are investigated in antiferromagnetically coupled Ta/Cu/[Co/Ni]_N/TbCo perpendicular composite films. By changing the Co/Ni repetition number N, the magnetization compensation temperature T_M can be widely modulated, decreasing from 295 K for N = 0 to below 10 K for N = 2. From temperature-dependent harmonic Hall measurements, it is shown that damping–like SOT field per current density χ_DL = H_DL/J_ac varies in a similar way to the net saturation magnetization M_S and reaches a maximum at T = T_M, whereas field-like H_FL/J_ac is nearly independent with T. The observed slight deviation between χ_DL and 1/M_S versus T curves at reduced temperatures is attributed to the increased spin Hall angle and interfacial spin transparency. It is found that the SOT efficiency ξ varies from −0.04 to −0.06 and even to −0.13 for N = 0, 1 and 2, respectively, suggesting ξ is intensively dependent on the details of the Ta/ferromagnet (FM) interface and the FM layer material. These results provide more insight into the SOT effects for exchange-coupled perpendicular composite structures, which are valuable for developing high-performance SOT devices.