Spin-Orbit Torque in a Perpendicularly Magnetized Ferrimagnetic Tb-Co Single Layer

Spin-orbit torque (SOT) has been studied extensively in a heavy-metal (HM)/ferromagnet (FM) bilayer structure, where a HM is an essential ingredient because a spin current is generated via the spin Hall effect within the HM layer and/or the Rashba-Edelstein effect from the HM/FM interface. Here, we report the observation of SOT in a ferrimagnetic Tb-Co single layer with perpendicular magnetic anisotropy without a HM layer. Using harmonic Hall voltage measurements, we investigate the SOT-induced dampinglike effective field (B-DL) in a Tb-Co layer; the sign of B-DL is opposite from that of a Pt/Tb-Co bilayer, and the magnitude of B-DL increases as the Tb-Co composition approaches its magnetization compensation point. Moreover, we analyze the elemental composition of Tb-Co as a function of film thickness using scanning transmission electron microscopy and electron energy-loss spectroscopy, indicating that the sign and magnitude of the SOT are virtually insensitive to the vertical composition gradient within the Tb-Co layer. Our results demonstrate that the bulk spin-orbit interaction within the Tb-Co layer itself plays a major role in generating SOT in a Tb-Co single layer.