An NMR study of Pr$\text{}_{0.5}$Ca$\text{}_{0.5}$Mn$\text{}_{1-x}$Ga$\text{}_{x}$O$\text{}_{3}$ (x=0 and 0.03)

An NMR study of polycrystalline Pr$\text{}_{0.5}$Ca$\text{}_{0.5}$Mn$\text{}_{1-x}$Ga$\text{}_{x}$O$\text{}_{3}$ (x=0 and 0.03) at 3 K is presented. Zero field spin-echo spectra of the Ga doped compound consist of an overlapping $\text{}^{69,71}$Ga signal at 74~MHz (hyperfine field of 5.3 T), a $\text{}^{55}$Mn double exchange line at 375 MHz (35.5 T) and a weak Mn$\text{}^{3+}$ signal between 400 and 550 MHz. Measurements in an applied field show a step-like increase in the double exchange line intensity, which corresponds to an increase in the amount of the ferromagnetic metallic phase. This coincides with a step-like feature in the bulk magnetization measurements. The effect is similar to that in the previous field dependent $\text{}^{55}$Mn NMR measurements of Pr$\text{}_{0.67}$Ca $\text{}_{0.33}$MnO$\text{}_{3}$. At the demagnetized and remanent state, a variation of spin- spin relaxation time, T$\text{}_{2}$, across the $\text{}^{55}$Mn line, due to the Suhl-Nakamura interaction is observed, which suggests that the ferromagnetic metallic double exchange regions, at liquid helium temperatures, are at least 4 nm in size.