The Optical Absorption in Manganese Zinc Fluoride Single Crystals near the N el Temperature

Abstract

The visible and ultra-violet absorption spectra of single crystals of manganese fluoride, and of two manganese zinc fluoride mixtures, have been measured down to 62°K. The sharp lines arising from the level complexes {4Eg 4A1g} (4G) and 4Eg(4D), which are insensitive to the magnitude of the crystalline electrostatic field, show a non-linear shift with temperature, which varies with composition, and is correlated with the temperatures at which long-range antiferromagnetic order becomes appreciable. A separation of the exchange interaction effect from that due to the thermal expansion of the lattice can be made with some certainty in the material of medium dilution, except in the immediate neighbourhood of the ordering temperature where effects due to the lattice expansion anomaly may be appreciable. The magnitude of the antiferromagnetic interaction shift aids the identification of the transitions, and is interpreted as giving a measure of the variation of short-range spin order with temperature. Measurements of line width and oscillator strength have been made for several absorption lines, and the data for the magnetically dilute materials can be interpreted on simple vibrational models, but for pure manganese fluoride these models seem less satisfactory.