Interlayer exchange coupling modulated spin-orbit torque and multi-state switching in GdCo/Ru/GdCo heterostructures

Abstract

The interlayer exchange coupling (IEC) in a synthetic antiferromagnet (SAF) contributes to the spin-orbit torque (SOT) effect. In this work, we study the SOT switching mechanism in a GdCo/Ru/GdCo SAF structure by varying the thickness of Ru (t Ru). It is found that the IEC changes from ferromagnetic to antiferromagnetic and then back to ferromagnetic coupling with increasing t Ru. By performing harmonic Hall measurements, it is confirmed that the antiferromagnetic IEC between two GdCo layers through Ru can significantly enhance the SOT, and a spin torque efficiency of up to 0.71 is obtained. Depending on the sign and strength of the IEC, the combination of SOT and IEC leads to various current-induced switching models. For samples with strong ferromagnetic and antiferromagnetic IEC, the magnetization switching between two parallel or antiparallel states is achieved. The current-induced multi-state switching processes, including four-steps-four-states and six-steps-four-states, are observed for samples with weakened IEC when t Ru is thicker than 0.57 nm. Our results enrich the SOT switching mechanism in SAF structures composed of rare-earth-transition-metal alloys.