Magnetic fluctuations in singlet-ground-state systems

This paper discusses the possible effects of coupling between different excitation modes in singlet-ground-state ferromagnets above their Curie points. The picture we use is a Hartree-like one, with every magnetic exciton dressed (in a mean-square sense) by a cloud of other such excitons. This description finds a natural expression in a functional-integral language, where the result can be expressed as a renormalization of the single-ion susceptibility by averaging over a distribution of external fields. We find that the resulting excitation spectrum has a continuum part with a threshold at the crystal-field splitting, in addition to the usual spike at the exciton frequency. The transition temperature is also significantly depressed from its random-phase-approximation value. Some numerical results are presented which suggest that these effects may be important in real systems.