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

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Abstract
The temperature dependence of the oscillator strengths of exciton-magnon absorption bands is studied in the optical spectra of FeCl2, CoBr2 and NiBr2, compounds in which strong intralayer ferromagnetic exchange interactions are combined with much weaker antiferromagnetic exchange between layers. FeCl2 and CoBr2 have large anisotropy, the former being an easy-axis system, the latter easy-plane, while in NiBr2 the anisotropy is small and of easy-plane type. In Fe Cl2 the intensity of the exciton-magnon band at 4270 AA varies as T2exp(-A/kT) over the temperature range 4.0-7.2K. In CoBr2, bands at 4940 AA have intensities varying approximately as T3, in agreement with theory, while in NiBr2 the band at 6070 AA varies as T2.