Spin-phonon coupling in antiferromagnetic chromium spinels

The temperature dependence of eigenfrequencies and intensities of the infrared (IR) active modes has been investigated for the antiferromagnetic chromium spinel compounds CdCr 2 O 4 , ZnCr 2 O 4 , ZnCr 2 S 4 , ZnCr 2 Se 4 , and HgCr 2 S 4 by IR spectroscopy for temperatures from 5 to 300 K. At the transition into the magnetically ordered phases, and driven by spin-phonon coupling, most compounds reveal significant splittings of the phonon modes. This is true for geometrically frustrated CdCr 2 O 4 , and ZnCr 2 O 4 , for bond frustrated ZnCr 2 S 4 and for ZnCr 2 Se 4 , which is also bond frustrated, but dominated by ferromagnetic (FM) exchange. The pattern of splitting is different for the different compounds and crucially depends on the nature of frustration and of the resulting spin order. HgCr 2 S 4 , which is almost FM, exhibits no splitting of the eigenfrequencies, but shows significant shifts due to FM spin fluctuations.