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Optical identification of hybrid magnetic and electric excitations in Dy3Fe5O12 garnet

Far-infrared spectra of magneto-dielectric Dy3Fe5O12 garnet were studied between 13 and 100 cm-1 and at low temperatures between 5 and 80 K. A combination of transmission, reflectivity, and rotating analyzer ellipsometry was used to unambiguously identify the type of the dipole activity of the infrared modes. In addition to purely dielectric and magnetic modes, we observed several hybrid modes with a mixed magnetic and electric dipole activity. These modes originate from the superexchange between magnetic moments of Fe and Dy ions. Using 4x4 matrix formalism for materials with Mu=/=1, we modeled the experimental optical spectra and determined the far-infrared dielectric and magnetic permeability functions. The matching condition Mu(Wh)*Se=Eps(Wh)*Sm for the oscillator strengths Se(m) explains the observed vanishing of certain hybrid modes at Wh in reflectivity.