Nuclear magnetic resonance study of insulator-metal transitions in(Pr,Ca)MnO3

Abstract

An NMR study of polycrystalline ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}{\mathrm{O}}_{3}$ ($x=0$ and 0.03) and ${\mathrm{Pr}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ at 3 K is presented. Zero-field spin-echo spectra of the Ga-doped compound consist of an overlapping $^{69,71}\mathrm{Ga}$ signal at 74 MHz (hyperfine field of 5.3 T) and a $^{55}\mathrm{Mn}$ double-exchange (DE) line at 375 MHz (35.5 T). Measurements in an applied field show a steplike increase in the DE line intensity, which corresponds to an increase of the amount of the ferromagnetic metallic phase. This coincides with a steplike feature in the bulk magnetization measurements. The effect is similar to that in the previous field-dependent $^{55}\mathrm{Mn}$ NMR measurements of ${\mathrm{Pr}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$. The insulator - metal transition for ${\mathrm{Pr}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$, at ambient pressure, occurs at fields greater than 5 T; however, at 1.1 GPa, the DE line corresponding to the ferromagnetic metallic phase is already present at zero field.