Softening and broadening of the zone boundary magnons in Pr0.63Sr0.37MnO3

We have studied the spin dynamics in Pr0.63Sr0.37MnO3 above and below the Curie temperature T-C = 301 K. Three distinct new features have been observed: a softening of the magnon dispersion at the zone boundary for T < T-C, significant broadening of the zone boundary magnons as T --> T-C, and no evidence for residual spin-wave-like excitations just above T-C. The results are inconsistent with double exchange models that have been successfully applied to higher T-C samples, indicating an evolution of the spin system with decreasing T-C.