Charge and Orbital Ordering in Pr_{0.5} Ca_{0.5} MnO_3 Studied by ^{17}O NMR

TL;DR

This study uses ^{17}O NMR to investigate charge and orbital ordering in Pr_{0.5} Ca_{0.5} MnO_3, revealing temperature-dependent magnetic correlations and the gradual development of orbital order and CE-type magnetic correlations.

Contribution

First application of ^{17}O NMR to study charge and orbital ordering in Pr_{0.5} Ca_{0.5} MnO_3, providing detailed insights into magnetic correlations and phase transitions.

Findings

Charge and orbital order at T_{CO} = 225 K detected by NMR.

Gradual development of CE-type magnetic correlations below T_{CO}.

Presence of charge-disordered regions between T_{CO} and T_N.

Abstract

The charge and orbital ordering in Pr_{0.5} Ca_{0.5} MnO_3 is studied for the first time by ^{17}O NMR. This local probe is sensitive to spin, charge and orbital correlations. Two transitions exist in this system: the charge and orbital ordering at T_{CO} = 225 K and the antiferromagnetic (AF) transition at T_N = 170 K. Both are clearly seen in the NMR spectra measured in a magnetic field of 7T. Above T_{CO} there exists only one NMR line with a large isotropic shift, whose temperature dependence is in accordance with the presence of ferromagnetic (FM) correlations. This line splits into two parts below T_{CO}, which are attributed to different types of oxygen in the charge/orbital ordered state. The interplay of FM and AF spin correlations of Mn ions in the charge ordered state of Pr_{0.5} Ca_{0.5} MnO_3 is considered in terms of the hole hopping motion that is slowed down with decreasing temperature. The developing fine structure of the spectra evidences, that there still exist charge-disordered regions at T_{CO} > T > T_N and that the static (t > 10^{-6}s) orbital order is established only on approaching T_N. The CE-type magnetic correlations develop gradually below T_{CO}, so that at first the AF correlations between checkerboard ab-layers appear, and only at lower temperature - CE correlations within the ab-planes.