Polar phonons and spin-phonon coupling inHgCr2S4andCdCr2S4studied with far-infrared spectroscopy

Abstract

Polar phonons of $\mathrm{Hg}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ and $\mathrm{Cd}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ are studied by far-infrared spectroscopy as a function of temperature and external magnetic field. Eigenfrequencies, damping constants, effective plasma frequencies, Lyddane-Sachs-Teller relations, and effective charges are determined. Ferromagnetic $\mathrm{Cd}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ and antiferromagnetic $\mathrm{Hg}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ behave rather similar. Both compounds are dominated by ferromagnetic exchange, and although $\mathrm{Hg}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ is an antiferromagnet, no phonon splitting can be observed at the magnetic phase transition. Temperature and magnetic-field dependencies of the eigenfrequencies show no anomalies, indicating displacive polar soft-mode behavior. However, significant effects are detected in the temperature dependence of the plasma frequencies, indicating changes in the nature of the bonds and significant charge transfer. In $\mathrm{Hg}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$, we provide experimental evidence that the magnetic-field dependence of specific polar modes reveals shifts exactly correlated with the magnetization, showing significant magnetodielectric effects even at infrared frequencies.