Magneto-optical effects and a Monte Carlo calculation for dysprosium aluminium garnet

The optical absorption spectrum of the antiferromagnet dysprosium aluminium garnet has been studied both above and below its Néel temperature (2·5 °K). Shifts and splittings of the absorption lines are interpreted in terms of magnetic interactions and, in particular, the finite shifts above T N are attributed to the presence of magnetic short-range order. A Monte Carlo calculation provides quantitative confirmation of these ideas. The value obtained for the nearest-neighbour exchange interaction is equivalent to an enhancement of the nearest-neighbour dipolar interaction by a factor 1·47, which is in good agreement with a value obtained by other authors. Further studies of the spectrum as a function of external field give a direct indication of the dominance of nearest-neighbour interactions in this substance, and also some insight into the form of the small non-dipolar interaction, which is clearly not of the simple Heisenberg J ij S i . S j form.