Non-Hermitian control of helicity-selective antiferromagnetic resonance

Abstract

We study non-Hermitian antiferromagnetic resonance in an antiferromagnetic insulator/nonmagnetic metal junction with sublattice-dependent damping and spin-orbit torque. By formulating the linearized Landau-Lifshitz-Gilbert (LLG) equation as a 2x2 non-Hermitian eigenvalue problem, we derive the complex resonance frequencies, the exceptional-point condition, and the stability-threshold condition. We show that the largest absorption response is maximized by approaching the stability threshold from the stable side. Near the threshold on the stable side, the absorption spectrum exhibits strong enhancement and linewidth narrowing, together with pronounced helicity dependence in the sub-THz regime that can be switched by reversing the spin-orbit torque. We also confirm by solving the full LLG dynamics that crossing the threshold leads to gain and self-oscillation. These results identify the stability threshold as the key control principle for enhancing absorption and achieving polarization selectivity in the sub-THz/THz regime.