Far-infrared spectra of the magnetic exchange resonances and optical phonons and their connection to magnetic and dielectric properties of Dy ₃Fe ₅O ₁₂ garnet

Far-infrared spectra of ferrimagnetic Dy ₃Fe ₅O ₁₂ single crystals have been studied in the spectral range between 12 and 700 cm ^-1 and in a wide temperature range between 5 K and 300 K using transmission spectroscopy and rotating analyzer ellipsometry. In the temperature range below T _C = 16 K for the magnetic ordering of Dy3 ⁺ spins, a number of ligand field and Kaplan-Kittel (KK) exchange resonance modes have been found. Temperature dependencies of their frequencies allowed us to estimate the ratio between the Fe-Dy and Dy-Dy exchange constants. We found that variation of the oscillator strength of the KK modes correlates with the magnetic-field-induced changes of magnetic permeability. The low-temperature peak in the static value of the dielectric function at T _C = 16 K may be related to the observed temperature-induced variation of the oscillator strength of the optical phonons.