Exciton-magnon transitions in the frustrated chromium antiferromagnets CuCrO2, alpha-CaCr2O4, CdCr2O4, and ZnCr2O4

TL;DR

This study investigates exciton-magnon interactions in frustrated chromium antiferromagnets using optical spectroscopy, revealing temperature-dependent magnon sidebands and their persistence above magnetic transition temperatures, with implications for understanding spin correlations.

Contribution

It provides new insights into exciton-magnon sidebands in Cr-based frustrated antiferromagnets and their behavior across magnetic phase transitions, including the observation of sidebands above T_N.

Findings

Magnon sidebands appear below T_N in CuCrO2 and alpha-CaCr2O4.

Sideband line widths follow an Arrhenius law with temperature.

Some sidebands persist above the magnetic ordering temperature.

Abstract

We report on optical transmission spectroscopy of the Cr-based frustrated triangular antiferromagnets CuCrO2 and alpha-CaCr2O4, and the spinels CdCr2O4 and ZnCr2O4 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 Cr3+ ions (spin S = 3/2). In CuCrO2 and alpha-CaCr2O4 the appearance of fine structures below T_N is assigned to magnon sidebands by comparison with neutron scattering results. The temperature dependence of the line width of the most intense sidebands in both compounds can be described by an Arrhenius law. For CuCrO2 the sideband associated with the 4A2 -> 2T2 transition can be observed even above T_N. Its line width does not show a kink at the magnetic ordering temperature and can alternatively be described by a Z2 vortex scenario proposed previously for similar materials. The exciton-magnon features in alpha-CaCr2O4 are more complex due to the orthorhombic distortion. While for CdCr2O4 magnon sidebands are identified below T_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 ZnCr2O4.