We report measurements of the dispersion of singlet-triplet magnetic excitons as a function of temperature in the singlet-ground-state ferromagnets fcc Pr and ${\mathrm{Pr}}_3$T1. Well-defined excitons are observed in both the ferromagnetic and paramagnetic regions, but with energies which are nearly temperature independent, in disagreement with existing theory.

• Received 11 October 1971

DOI:https://doi.org/10.1103/PhysRevLett.27.1530