Influence of magnetic fluctuations in the magnetocaloric effect on rare-earth intermetallic compounds

A theoretical model including both crystal-field and exchange interactions that considers the effect of magnetic fluctuations is developed to evaluate the temperature dependence of the isothermal magnetic entropy changes in ferromagnetic rare-earth-based intermetallic compounds. The Green's functions are derived from their equation of motion. The magnetic moment correlation functions are determined beyond the random phase approximation by incorporating a measure of magnetic spontaneous fluctuations in a way that ensures self-consistency with regard to the fluctuation-dissipation theorem. In particular, the exact magnitude of the entropy change without magnetic moment fluctuations depends on the ratio of both the crystal-field first- and the crystal-field third-order magnetic susceptibilities at the Curie temperature, TC. These theoretical predictions are compared with experimental data on cubic RM₂ (R=rare earth and M=Al and Ni) compounds, where the principal crystal-field and exchange parameters are well known.