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

摘要

The charge and orbital ordering in Pr_{0.5} Ca_{0.5} MnO_3 is studied for thefirst time by ^{17}O NMR. This local probe is sensitive to spin, charge andorbital correlations. Two transitions exist in this system: the charge andorbital ordering at T_{CO} = 225 K and the antiferromagnetic (AF) transition atT_N = 170 K. Both are clearly seen in the NMR spectra measured in a magneticfield of 7T. Above T_{CO} there exists only one NMR line with a large isotropicshift, whose temperature dependence is in accordance with the presence offerromagnetic (FM) correlations. This line splits into two parts below T_{CO},which are attributed to different types of oxygen in the charge/orbital orderedstate. The interplay of FM and AF spin correlations of Mn ions in the chargeordered state of Pr_{0.5} Ca_{0.5} MnO_3 is considered in terms of the holehopping motion that is slowed down with decreasing temperature. The developingfine structure of the spectra evidences, that there still existcharge-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-typemagnetic correlations develop gradually below T_{CO}, so that at first the AFcorrelations between checkerboard ab-layers appear, and only at lowertemperature - CE correlations within the ab-planes.