The charge and orbital ordering in ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ is studied for the first time ${\mathrm{by}}^{17}\mathrm{O}$ NMR. This local probe is sensitive to spin, charge, and orbital correlations. Two transitions exist in this system: the charge and orbital ordering at $T_{\mathrm{CO}}=225\mathrm{}\mathrm{K}$ and the antiferromagnetic (AF) transition at $T_N=170\mathrm{}\mathrm{K}.$ Both are clearly seen in the NMR spectra measured in a magnetic field of 7 T. Above $T_{\mathrm{CO}}$ 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 $T_{\mathrm{CO}},$ 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 ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ is considered in terms of the hole hopping motion that slows down with decreasing temperature. The developing fine structure of the spectra evidences that there still exist charge-disordered regions at $T_{\mathrm{CO}}>T>\mathrm{}{T}_N$ and that the static $(t>\mathrm{}{10}^{-6}\mathrm{s})$ orbital order is established only on approaching $T_N.$ The charge-exchange (CE) type magnetic correlations develop gradually below $T_{\mathrm{CO}},$ so that at first the AF correlations between checkerboard ab layers appear, and only at lower temperature do CE correlations within the ab planes appear.

• Received 8 September 2002

DOI:https://doi.org/10.1103/PhysRevB.67.064414