Theory of magnon-plasmon polaritons in the layered structure metal–antiferromagnet–metal

The paper presents theoretical investigation of surface magnon-plasmon polaritons (MPPs) in a layered structure consisting of a thin uniaxial antiferromagnetic (AFM) film sandwiched between two non-magnetic metals. The study explores the dispersion relation of these hybrid electromagnetic waves, considering the impact of a non-diagonal magnetic permeability tensor inherent to AFMs. Unlike previous studies, our results demonstrate that in such AFM-based systems, the coupling between E-wave and H-wave modes leads to the formation of hybridized polaritonic states, which cannot be decoupled into independent wave modes. In particular, the dispersion relation transforms from quadratic to biquadratic, yielding two distinct frequency branches with anisotropy-driven frequency shifts. The frequency dependence of the AFM permeability tensor is considered, leading to a refined dispersion law. At high frequencies, two branches converge to a characteristic limit, while at lower frequencies, influence of the AFM's non-diagonal permeability tensor becomes pronounced. For lower frequencies, an exact (non-linearized) dispersion equation has to be used. This equation (fourth-order in squared frequency) is obtained; its solutions, four distinct branches, are analyzed. These solutions are categorized into two groups: one where the imaginary part of the frequency is significant, leading to substantial attenuation, and another where the solutions remain purely real, asymptotically approaching the characteristic frequency at large wavenumbers. These findings establish the foundation for the practical implementation of next-generation AFM-based magnonic and plasmonic devices in the THz spectrum. The tunability and low-loss nature of MPPs in AFM-based heterostructures offer promising applications in high-frequency waveguides, resonators, and sensors.