Charge and orbital ordering in Pr0.5Ca0.5MnO3 studied by O-17 NMR

The charge and orbital ordering in Pr0.5Ca0.5MnO3 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 7 T. 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 Pr0.5Ca0.5MnO3 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-CO>T>T-N and that the static (t>10(-6) s) orbital order is established only on approaching T-N. The charge-exchange (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 do CE correlations within the ab planes appear.