Charge-ordered state in half-doped Bi-based manganites studied byO17andBi209NMR

We present a $^{209}\mathrm{Bi}$ and $^{17}\mathrm{O}$ NMR study of the Mn electron spin correlations developed in the charge ordered state of ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$ and ${\mathrm{Bi}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3}$. The unusually large local magnetic field $^{209}H_{\mathit{loc}}$ indicates the dominant $6{s}^{2}$ character of the lone electron pair of ${\mathrm{Bi}}^{3+}$ ions in both compounds, probably, responsible for high charge ordering temperature ${T}_{\mathit{CO}}$. The observed difference in $^{209}H_{\mathit{loc}}$ for ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$ to ${\mathrm{Bi}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3}$ is suggested to be due to a decrease in the canting of the staggered magnetic moments of ${\mathrm{Mn}}^{3+}$ ions. The modification of the $^{17}\mathrm{O}$ spectra below ${T}_{\mathit{CO}}$ demonstrates that the line due to the apical oxygen is a unique local tool to study the development of the Mn spin correlations. In the AF state the analysis of the $^{17}\mathrm{O}$ spectrum of ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$ points toward a bond-centered model of the CO state in this material.