Probing spin correlations in the frustrated magnets CdCr$_2$O$_4$ and ZnCr$_2$O$_4$

We performed optical, magnetic susceptibility, and specific heat measurements on CdCr$_2$O$_4$ and ZnCr$_2$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. In comparison with the frequency shift of the infrared optical phonons above $T_N$, we can obtain estimates for the spin-phonon coupling in both systems. Moreover, we will discuss optical signatures of the magnetic excitation spectrum which are found to persist up to room temperature in ZnCr$_2$O$_4$.