Current-Induced Magnetization Switching of Exchange-Biased NiO Heterostructures Characterized by Spin-Orbit Torque

In this work, we study magnetization switching induced by spin-orbit torque in W (Pt)/Co/NiO heterostructures with variable thickness of W and Pt heavy-metal layers, a perpendicularly magnetized Co layer, and an antiferromagnetic NiO layer. Using current-driven switching and magnetoresistance and anomalous-Hall-effect measurements, we determine the perpendicular and in-plane exchange-bias field. Several Hall-bar devices possessing in-plane exchange bias from both systems are selected and analyzed in relation to our analytical switching model of the critical current density as a function of Pt and W thickness, resulting in estimation of the effective spin Hall angle and perpendicular effective magnetic anisotropy. We demonstrate in both the Pt/Co/NiO system and the W/Co/NiO system deterministic Co magnetization switching without an external magnetic field, which is replaced by an in-plane exchange-bias field. Moreover, we show that due to a higher effective spin Hall angle in the W-based system than in the Pt-based system, the relative difference between the resistance states in the magnetization current switching to the difference between the resistance states in magnetic field switching determined by the anomalous Hall effect (Delta R/Delta R-AHE) is about twice as high in W-based devices than in Pt-based devices, while the critical switching-current density in W-based devices is 1 order lower than in Pt-based devices. The current-switching stability and the training process are discussed in detail.