Abstract
Optical absorption measurements were made to determine energy gap values as a function of temperature on samples of Cu2MnGeS4, Cu2MnSiSe4 (both antiferromagnetic), Ag2MnGeSe4 and Ag2MnSiTe4 (both ferrimagnetic) with the orthorhombic structure, and of Ag2MnSiTe4 with the cubic structure (ferrimagnetic). These data were analysed to determine the contribution triangle E to the energy gap due to magnetic effects in the temperature region around the critical temperature TN. It was shown that the magnetic contribution increased the energy gap for the antiferromagnetic cases and decreased the energy gap for the ferrimagnetic ones. The variation of triangle E with T was found to agree very well with the theoretical predictions of Alexander et al.
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