We report on optical transmission spectroscopy of the Cr-based frustrated triangular antiferromagnets CuCrO${}_2$ and $\alpha$-CaCr${}_2$O${}_4$, and the spinels CdCr${}_2$O${}_4$ and ZnCr${}_2$O${}_4$ in the near-infrared to visible-light frequency range. We explore the possibility to search for spin correlations far above the magnetic ordering temperature and for anomalies in the magnon lifetime in the magnetically ordered state by probing exciton-magnon sidebands of the spin-forbidden crystal-field transitions of the Cr${}^{3+}$ ions (spin $S=3/2$). In CuCrO${}_2$ and $\alpha$-CaCr${}_2$O${}_4$ the appearance of fine structures below $T_{\text{N}}$ is assigned to magnon sidebands by comparison with neutron-scattering results. The temperature dependence of the linewidth of the most intense sidebands in both compounds can be described by an Arrhenius law. For CuCrO${}_2$ the sideband associated with the ${}^4{A}_{\mathsf{2}}\to {}^2{T}_{\mathsf{2}}$ transition can be observed even above $T_{\text{N}}$. Its linewidth does not show a kink at the magnetic ordering temperature and can alternatively be described by a $Z_2$ vortex scenario proposed previously for similar materials. The exciton-magnon features in $\alpha$-CaCr${}_2$O${}_4$ are more complex due to the orthorhombic distortion. While for CdCr${}_2$O${}_4$ magnon sidebands are identified below $T_{\text{N}}$ and one sideband excitation is found to persist across the magnetic ordering transition, only a weak fine structure related to magnetic ordering has been observed in ZnCr${}_2$O${}_4$.

• Received 1 October 2012

DOI:https://doi.org/10.1103/PhysRevB.87.224424