Optical phonons, spin correlations, and spin-phonon coupling in the frustrated pyrochlore magnets CdCr_2O_4 and ZnCr_2O_4

摘要

We report on infrared, Raman, magnetic susceptibility, and specific-heat measurements on CdCr_2O_4 and ZnCr_2O_4 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 T_N, 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 CdCr_2O_4 in agreement with theoretical predictions by Chern et al. [Phys. Rev. B 74, 060405(R) (2006)]. In ZnCr_2O_4 several new modes appear below T_N but no phonon modes could be detected which are both Raman and infrared active.