Magnetic polaritons, magnetostatic waves and effective-medium approximation for antiferromagnetic superlattice with impurity in parallel magnetic field
We derive the general effective-medium expression for the surface-guided magnetic polaritons and magnetostatic waves, which propagate in the antiferromagnetic superlattice with antiferromagnetic impurity film, and investigate the influence of the external magnetic field on the energy of localized magnetic polaritons. Similarly as in the free-standing antiferromagnetic film, the spectrum of magnetic polaritons in the presence of an external magnetic field is reciprocal in the sense that the frequency is independent of the direction of propagation. In the system under consideration one finds both the surface polaritons which are strongly localized in the antiferromagnetic film which acts as a waveguide, and waves which are weakly localized within film. The first waves are the pure surface modes or guided modes where excitations have a standing-wavelike character. This important feature of the localized magnetic polaritons enables us to use these antiferromagnetic systems in the technologies for devices (for example, in resonators) that work at wavelengths in the infrared region. Second waves have the very small value of the decay parameter and appear in the regions where the surface mode penetrates into the bulk band, i.e. the magnetic polaritons are weakly localized in the impurity film region. Now we obtain themixed type mode having both bulk and surface characteristics. Also, the general way in which the dispersion curves vary with the volume fraction of the superlattice components and with impurity film is illustrated in this study.