Softening and Broadening of the Zone Boundary Magnons inPr0.63Sr0.37MnO3

Abstract

We have studied the spin dynamics in ${\mathrm{Pr}}_{0.63}{\mathrm{Sr}}_{0.37}{\mathrm{MnO}}_3$ above and below the Curie temperature $T_C=301\mathrm{}$ K. Three distinct new features have been observed: a softening of the magnon dispersion at the zone boundary for $T<\mathrm{}{T}_C$, significant broadening of the zone boundary magnons as $T\to T_C$, 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 $T_C$.