Optical phonons, spin correlations, and spin-phonon coupling in the frustrated pyrochlore magnets CdCr2O4 and ZnCr2O4

TL;DR

This study investigates the interplay of optical phonons, spin correlations, and spin-phonon coupling in frustrated pyrochlore magnets CdCr2O4 and ZnCr2O4 through various spectroscopic and thermodynamic measurements, revealing symmetry changes and coupling effects.

Contribution

It provides detailed experimental estimates of exchange constants, spin-phonon coupling, and symmetry changes in CdCr2O4 and ZnCr2O4, advancing understanding of spin-lattice interactions in frustrated magnets.

Findings

In CdCr2O4, phonon modes become both Raman and infrared active below TN, indicating inversion symmetry loss.

In ZnCr2O4, new modes appear below TN, but inversion symmetry is preserved.

Estimated spin-phonon coupling constants and exchange interactions from experimental data.

Abstract

We report on infrared, Raman, magnetic susceptibility, and specific heat measurements on CdCr2O4 and ZnCr2O4 single crystals. We estimate the nearest-neighbor and next-nearest neighbor exchange constants from the magnetic susceptibility and extract the spin-spin correlation functions obtained from the magnetic susceptibility and the magnetic contribution to the specific heat. By comparing with the frequency shift of the infrared optical phonons above TN , we derive estimates for the spin-phonon coupling constants in these systems. The observation of phonon modes which are both Raman and infrared active suggest the loss of inversion symmetry below the Neel temperature in CdCr2O4 in agreement with theoretical predictions by Chern and coworkers [Phys. Rev. B 74, 060405 (2006)]. In ZnCr2O4 several new modes appear below TN, but no phonon modes could be detected which are both Raman and infrared active indicating the conservation of inversion symmetry in the low temperature phase.