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

TL;DR

This study uses $^{17}$O and $^{209}$Bi NMR to investigate charge ordering and Mn spin correlations in Bi-based manganites, revealing the role of Bi$^{3+}$ lone pairs and differences in magnetic structures.

Contribution

It provides new insights into the local magnetic fields and charge ordering mechanisms in Bi-based manganites using NMR spectroscopy.

Findings

Large local magnetic fields indicate dominant $6s^{2}$ character of Bi$^{3+}$ lone pairs.

Differences in $^{209}H_{loc}$ suggest variations in Mn magnetic moment canting.

$^{17}$O spectra reveal development of Mn spin correlations below $T_{CO}$.

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.