Spin phonon coupling in frustrated magnet CdCr$_2$O$_4$

TL;DR

This study investigates how magnetic order influences phonon behavior in the frustrated magnet CdCr2O4, revealing spin-phonon coupling effects through temperature-dependent infrared spectroscopy and phonon splitting analysis.

Contribution

It provides the first detailed measurement of spin-phonon coupling in CdCr2O4 and compares it with similar phenomena in ZnCr2O4, highlighting the role of magnetic order in phonon dynamics.

Findings

Phonons soften in the paramagnetic phase and split in the antiferromagnetic phase.

Phonon splitting is linked to magnetic order, not just structural changes.

Spin-phonon coupling in CdCr2O4 is weaker than in ZnCr2O4.

Abstract

The infrared phonon spectrum of the spinel CdCr2O4 is measured as a function temperature from 6 K to 300K. The triply degenerate Cr phonons soften in the paramagnetic phase as temperature is lowered below 100 K and then split into a singlet and doublet in the low T antiferromagnetic phase which is tetragonally distorted to relieve the geometric frustration in the pyrochlore lattice of Cr$^{3+}$ ions. The phonon splitting is inconsistent with the simple increase (decrease) in the force constants due to deceasing (increasing) bond lengths in the tetragonal phase. Rather they correspond to changes in the force constants due to the magnetic order in the antiferromagnetic state. The phonon splitting in this system is opposite of that observed earlier in ZnCr2O4 as predicted by theory. The magnitude of the splitting gives a measure of the spin phonon coupling strength which is smaller than in the case of ZnCr2O4.