Evolution of the Low-Frequency Spin Dynamics in Ferromagnetic Manganites

Abstract

Neutron scattering was used to study the ferromagnetic manganites ${\mathrm{Nd}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ ( $T_C=197.9\mathrm{K}$) and ${\mathrm{Pr}}_{0.63}{\mathrm{Sr}}_{0.37}{\mathrm{MnO}}_3$ ( $T_C=300.9\mathrm{K}$). The spin dynamical behavior of these two systems is similar at low temperatures but drastically different at temperatures around $T_C$. While the formation of spin clusters of size ( $\sim 20Å$) dominates the spin dynamics of the 197.9 K sample close to $T_C$, the paramagnetic to ferromagnetic transition for the 300.9 K sample is more conventional. These results, combined with seemingly inconsistent earlier reports, reveal clear systematics in the spin dynamics of the manganites.