We report on infrared, Raman, magnetic susceptibility, and specific-heat measurements on ${\text{CdCr}}_2{\text{O}}_4$ and ${\text{ZnCr}}_2{\text{O}}_4$ 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 Néel temperature in ${\text{CdCr}}_2{\text{O}}_4$ in agreement with theoretical predictions by Chern et al. [Phys. Rev. B 74, 060405(R) (2006)]. In ${\text{ZnCr}}_2{\text{O}}_4$ several new modes appear below $T_N$ but no phonon modes could be detected which are both Raman and infrared active.

• Received 9 June 2009

DOI:https://doi.org/10.1103/PhysRevB.80.214417