The charge ordered state in half-doped Bi-based manganites studied by ${}^{17}$O and ${}^{209}$Bi NMR

Abstract

We present a ${}^{209}$Bi and ${}^{17}$O NMR study of the Mn electron spin correlations developed in the charge ordered state of Bi${}_{0.5}$Sr${}_{0.5}$MnO${}_3$ and Bi${}_{0.5}$Ca${}_{0.5}$MnO${}_3$. The unusually large local magnetic field ${}^{209}{H}_{loc}$ indicates the dominant $6s^2$ character of the lone electron pair of Bi${}^{3+}$-ions in both compounds. The mechanism connecting the $s$ character of the lone pairs to the high temperature of charge ordering $T_{CO}$ is still not clarified. The observed difference in ${}^{209}{H}_{loc}$ for Bi${}_{0.5}$Sr${}_{0.5}$MnO${}_3$ to Bi${}_{0.5}$Ca${}_{0.5}$MnO${}_3$ is probably due to a decrease in the canting of the staggered magnetic moments of Mn${}^{3+}$-ions from. The modification of the ${}^{17}$O spectra below $T_{CO}$ demonstrates that the line due to the apical oxygens is a unique local tool to study the development of the Mn spin correlations. In the AF state the analysis of the ${}^{17}$O spectrum of Pr${}_{0.5}$Ca${}_{0.5}$MnO${}_3$ and Bi${}_{0.5}$Sr${}_{0.5}$MnO${}_3$ prompts us to try two different theoretical descriptions of the charge-ordered state, a site-centered model for the first manganite and a bond-centered model for the second one.

10 pages, 7 figures