Spin-phonon coupling in frustrated magnet CdCr₂O₄

The infrared phonon spectrum of the spinel CdCr₂O₄ is measured as a function of temperature from 6 to 300 K. 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 temperature antiferromagnetic phase, which is tetragonally distorted to relieve the geometric frustration in the pyrochlore lattice of ions. The phonon splitting is inconsistent with the simple increase (decrease) in the force constants due to decreasing (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.