Polarity Manipulation of Anomalous Hall Effect and Enhanced Spin-Orbit Torques in Perpendicular Synthetic Antiferromagnets

Asymmetric synthetic antiferromagnets (SAFs) with perpendicular magnetic anisotropy (PMA) have attracted considerable attention due to the advantages of both high operation speed and easy detection. In this paper, anomalous Hall effect (AHE) and spin-orbit-torque (SOT) effect are investigated for the SAF samples of [Ni/Co]4/Ru/[Co/Ni]3, which exhibit oscillatory interlayer exchange coupling when varying the Ru layer thickness tRu. It is found that the AHE loop amplitude decreases and even changes its polarity as tRu exceeds 1.27 nm, indicating the existence of a negative Hall coefficient at the Ru/Co interface. The calculated spin Hall angle θSH varies nonmotonically with the increase of tRu, following a similar variation trend to the antiferromagnetic (AF) coupling field HAF. Specifically, when HAF reaches its maximum at tRu = 0.95 nm, θSH is 7 times higher compared to the case of HAF = 0, indicating that the greatly enhanced SOT effect is strongly linked to the AF-coupling interaction. Nevertheless, although HAF increases significantly as the measurement temperature drops from 300 to 10 K, θSH keeps nearly unchanged. Our study demonstrates that the enhanced θSH with AF coupling is primarily determined by the modulation of the spacer-related band structure, rather than a magnetic mechanism of reduced magnon excitations in the magnetic layers. These results of strong AHE and SOT manipulation are valuable for the practical design of SAF-based spintronic devices.