Giant Response and Harmonic Generation in Néel-Torque Antiferromagnetic Resonance.

We theoretically investigate the resonant and higher order magnetic responses of a collinear antiferromagnet induced by Néel spin-orbit torques (NSOTs). By deriving the dynamical susceptibilities up to the third harmonic, we find remarkable NSOT-induced amplifications of the linear and nonlinear magnetic dynamics by orders of magnitude compared to conventional spin-orbit torques, enabling highly efficient frequency conversion in the terahertz frequency range. From the effective dynamics, we uncover the strong coupling between the Néel vector and the driving field of NSOTs, providing a physical explanation of the gigantic responses at all orders. We then propose a multilayer antiferromagnetic nanodevice leveraging the gigantic harmonic generation to achieve unprecedented frequency amplifiers and converters. Our Letter uncovers a previously overlooked role of the NSOTs in nonlinear dynamics.