Neutron inelastic scattering techniques have been used to examine the dispersion relations for magnetic excitons propagating on the hexagonal sites of double-hcp Pr when uniaxial stress is applied along the $\left[1\mathrm{}\overline{2}10\right]$ direction. The mode of lowest energy (longitudinal optic) along $\Gamma M$ was found to exhibit a clear soft-mode behavior with increasing stress. Elastic satellite reflections corresponding to long-range magnetic ordering in a longitudinally modulated structure were observed: at 800 bars the Néel temperature is 7.5 K.

• Received 30 May 1978

DOI:https://doi.org/10.1103/PhysRevLett.41.343