Interplay of ferromagnetic and antiferromagnetic terahertz spin currents from Co/NiO/Pt multilayers

Abstract

Investigating spin current transport in ferromagnetic (FM)/antiferromagnetic (AFM) systems within the terahertz (THz) regime is crucial for advancing ultrafast spintronic technologies. In this work, we systematically explore the THz emission characteristics of Co/NiO/Pt trilayers with varying NiO interlayer thicknesses. A distinct coexistence of FM- and AFM-like THz amplitude symmetries emerges when the thickness of NiO interlayer (tNiO) is reduced to the ultrathin regime (1.6 nm < tNiO < 4 nm), exhibiting pronounced angular dependencies on both the applied magnetic field orientation and the sample azimuthal angle. This unique symmetry arises from the superposition of two spin currents, with one originating in the Co layer and propagating through the NiO, and the other intrinsically generated within the NiO, both of which contribute to THz emission upon reaching the heavy metal (HM) layer. Notably, subtle deviations from the ideal FM-AFM symmetry are observed, with calculations revealing a slight misalignment between the spin current polarization in Co and the external magnetic field direction, driven by exchange coupling between Co and NiO. These findings provide new insights into spin current dynamics in FM/AFM heterostructures and highlight the role of exchange coupling in shaping spintronic properties.