EFFECTS OF MAGNETIC TRANSITIONS ON THE OPTICAL-ENERGY GAP VALUES OF SOME I2.MN.IV.VI4 PHASES

Optical absorption measurements were made to determine optical energy gap values as a function of temperature on samples Of Cu2MnGeS4, Cu2Mn SiSe4 (both antiferromagnetic), Ag2MnGeSe4 and Ag2MnSiTe4 (both ferrim agnetic) with the orthorhombic structure, and of Ag2MnSiTe4 with the c ubic structure (ferrimagnetic). These data were analysed to determine the contribution DELTAE to the energy gap due to magnetic effects in t he temperature region around the critical temperature T(N). It was sho wn that the magnetic contribution increased the energy gap for the ant iferromagnetic cases and decreased the energy gap for the ferrimagneti c ones. The variation of DELTAE with T was found to agree very well wi th the theoretical predictions of Alexander et al.