Temperature variation of exciton-magnon absorption bands in metamagnetic transition-metal dihalides

The temperature dependence of the oscillator strengths of exciton-magnon absorption bands is studied in the optical spectra of FeCl 2 , CoBr 2 and NiBr 2 , compounds in which strong intralayer ferromagnetic exchange interactions are combined with much weaker antiferromagnetic exchange between layers. FeCl 2 and CoBr 2 have large anisotropy, the former being an easy-axis system, the latter easy-plane, while in NiBr 2 the anisotropy is small and of easy-plane type. In Fe Cl 2 the intensity of the exciton-magnon band at 4270 AA varies as T 2 exp(-A/kT) over the temperature range 4.0-7.2K. In CoBr 2 , bands at 4940 AA have intensities varying approximately as T 3 , in agreement with theory, while in NiBr 2 the band at 6070 AA varies as T 2 .