M. Schmidt, Zhe Wang, Ch. Kant, F. Mayr, S. Toth, A. T. M. N. Islam, B. Lake, V. Tsurkan, A. Loidl, J. Deisenhofer
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. Via the exciton-magnon sidebands of the spin-forbidden crystal-field transitions of the Cr3+ ions (spin S = 3/2), it is possible to optically trace magnon excitations in the system and search the existence of antiferromagnetic spin correlations even far above the magnetic ordering temperature. For example, in CuCrO2 the temperature dependence of the line width of exciton-magnon transitions can be associated with the density of topologically protected Z2 vortices, which were proposed to occur in this class of systems. In alpha-CaCr2O4 we observe magnon sidebands at energies corresponding to low-lying roton-like spin waves. In CdCr2O4 magnon sidebands are found to persist across the magnetic ordering transition up to the corresponding Curie-Weiss temperature, indicating that short-range helical spin correlations are present also in the classical spin-liquid regime between the N\'eel and the Curie-Weiss temperatures. In ZnCr2O4 only a weak fine structure related to the magnetic ordering in the system has been observed.
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http://arxiv.org/abs/1210.3938
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