Abstract:
We report electron paramagnetic resonance measurements on single crystalline and powder samples of Nd0.5Ca0.5MnO3 across the charge-ordering transition at Tco=240 K down to the antiferromagnetic ordering transition at TN=140 K. 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 Jahn-Teller 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.
