O17NMR study of the doped electrons in lightly oxygen-deficient cubicSrMnO3−x

Abstract

The spin susceptibility of the localized $\mathrm{Mn}({t}_{2g})$ electrons, ${\ensuremath{\chi}}_{s}$, and the spatially distributed spin density of the doped electrons were investigated by $^{17}\mathrm{O}$ nuclear magnetic resonance (NMR) in the paramagnetic (PM) and antiferromagnetic (AF) phases of electron-doped ${\mathrm{SrMnO}}_{3\ensuremath{-}x}$ ceramics with the cubic structure. Three lightly doped samples $(2xl0.015)$ were studied with ${T}_{N}=220\phantom{\rule{4.pt}{0ex}}\text{K}--240$ K. In the PM state ${\ensuremath{\chi}}_{s}$ increases gradually from ${T}_{N}$ and reaches a broad maximum above $\ensuremath{\sim}1.5{T}_{N}$. The gapped behavior of ${\ensuremath{\chi}}_{s}$ indicates a low-dimensional short-range spin order persisting above ${T}_{N}$. These short-range one-dimensional correlations are consistent with $^{17}\mathrm{O}$ NMR results obtained at room temperature, which show that Mn magnetic moments are aligned along the edges of the cubic unit cell. Above 350 K all doped electrons are fast-moving ${e}_{g}$ electrons. They provide the uniform polarization of the localized spins which increases ${\ensuremath{\chi}}_{s}$ and the increasing doping shifts the oxygen-deficient ${\mathrm{SrMnO}}_{3\ensuremath{-}x}$ oxide towards a ferromagnetic (FM) metallic state. At lower $T$ the doped electrons are heterogeneously distributed in the oxide: The fraction of the fast-moving electrons diminishes and vanishes below 100 K, while the remaining doped electrons slow down their hopping and each of them creates a FM domain. These FM domains which are detected below 10 K by $^{55}\mathrm{Mn}$ NMR can be considered as small-size magnetic polarons. Their $T$-activated hopping in the G-type AF lattice was probed by $^{17}\mathrm{O}$ spin-echo experiments. The energy barrier of hopping shows a trend to grow with increasing doping, indicating that the de Gennes metallic ground state cannot be achieved in oxygen-deficient ${\mathrm{SrMnO}}_{3\ensuremath{-}x}$ oxides, probably due to detrimental oxygen vacancy defects.