Average Lattice Symmetry and Nanoscale Structural Correlations in Magnetoresistive Manganites

TL;DR

This study uses x-ray scattering to explore nanoscale structural correlations in manganites, revealing their dependence on lattice symmetry and their role in insulating versus metallic phases.

Contribution

It demonstrates that nanoscale correlations are linked to orthorhombic symmetry and are absent in rhombohedral and tetragonal phases, highlighting their importance in phase behavior.

Findings

Correlations present in orthorhombic phase but absent in rhombohedral and tetragonal phases.

Correlations associated with insulating regions and increased resistivity.

Local structure correlates with average lattice symmetry and phase properties.

Abstract

We report x-ray scattering studies of nanoscale structural correlations in the paramagnetic phases of the perovskite manganites La$_{0.75}$(Ca$_{0.45}$Sr$_{0.55}$)$_{0.25}$MnO$_3$, La$_{0.625}$Sr$_{0.375}$MnO$_3$, and Nd$_{0.45}$Sr$_{0.55}$MnO$_3$. We find that these correlations are present in the orthorhombic $O$ phase in La$_{0.75}$(Ca$_{0.45}$Sr$_{0.55}$)$_{0.25}$MnO$_3$, but they disappear abruptly at the orthorhombic-to-rhombohedral transition in this compound. The orthorhombic phase exhibits increased electrical resistivity and reduced ferromagnetic coupling, in agreement with the association of the nanoscale correlations with insulating regions. In contrast, the correlations were not detected in the two other compounds, which exhibit rhombohedral and tetragonal phases. Based on these results, as well as on previously published work, we propose that the local structure of the paramagnetic phase correlates strongly with the average lattice symmetry, and that the nanoscale correlations are an important factor distinguishing the insulating and the metallic phases in these compounds.