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

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 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 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.