We have studied the spin dynamics in Pr0.63Sr0.37MnO3 above and
below the Curie temperature TC=301 K. Three distinct new features have been
observed: a softening of the magnon dispersion at the zone boundary for
T<TC, significant broadening of the zone boundary magnons as T→TC, and
no evidence for residual spin-wave like excitations just above TC. The
results are inconsistent with double exchange models that have been
successfully applied to higher TC samples, indicating an evolution of the
spin system with decreasing TC.Comment: 12 pages, Latex, 3 figure