2 1 O ct 2 00 1 Temperature - dependent electron paramagnetic resonance studies of charge ordered Nd 0

We report electron paramagnetic resonance measurements on single crystalline and powder samples of Nd0.5Ca0.5MnO3 across the charge-ordering transition at Tco = 240K down to the antiferromagnetic ordering transition at TN = 140K. The changes in the linewidth, g-factor and intensity as functions of temperature are studied to understand the nature of spin-dynamics in the system. We explain the observed large decrease in the linewidth from TN to Tco in terms of motional narrowing caused by the hopping of the JahnTeller polarons yielding an activation energy of Ea = 0.1 eV. Similar analysis of data on Pr0.6Ca0.4MnO3 published earlier gives Ea = 0.2 eV. Below Tco, the g-value increases continuously suggesting a gradual strengthening of the orbital ordering. We give a qualitative explanation of the maximum in the asymmetry ratio A/B observed at Tco and its temperature dependence in single crystal spectra which also supports the model of motional narrowing. PACS numbers: 76.30.-v, 75.70.Pa, 72.80.Ga, 71.30.+h