Polar phonons and spin-phonon coupling in HgCr2S4 and CdCr2S4

TL;DR

This study investigates the polar phonons and spin-phonon interactions in HgCr2S4 and CdCr2S4 using far-infrared spectroscopy, revealing magneto-dielectric effects and bond charge transfer without soft mode anomalies.

Contribution

It provides the first detailed analysis of phonon behavior and spin-phonon coupling in these sulfides, highlighting magnetic field effects on polar modes and charge transfer.

Findings

No phonon splitting at magnetic transitions.

Magneto-dielectric effects observed at infrared frequencies.

Charge transfer influences bond properties.

Abstract

Polar phonons of HgCr2S4 and CdCr2S4 are studied by far-infrared spectroscopy as a function of temperature and external magnetic field. Eigenfrequencies, damping constants, effective plasma frequencies and Lyddane-Sachs-Teller relations, and effective charges are determined. Ferromagnetic CdCr2S4 and antiferromagnetic HgCr2S4 behave rather similar. Both compounds are dominated by ferromagnetic exchange and although HgCr2S4 is an antiferromagnet, no phonon splitting can be observed at the magnetic phase transition. Temperature and magnetic field dependence 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 HgCr2S4 we provide experimental evidence that the magnetic field dependence of specific polar modes reveal shifts exactly correlated with the magnetization showing significant magneto-dielectric effects even at infrared frequencies.