Temperature-dependent electron paramagnetic resonance studies of charge-ordered Nd_0.5Ca_0.5MnO₃

Abstract

We report electron paramagnetic resonance measurements on single crystalline and powder samples of Nd_0.5Ca_0.5MnO₃ across the charge-ordering transition at T_co=240K down to the antiferromagnetic ordering transition at T_N=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 T_co in terms of motional narrowing caused by the hopping of the Jahn-Teller polarons yielding an activation energy of E_a=0.1 eV. Similar analysis of data on Pr_0.6Ca_0.4MnO₃ published earlier gives E_a=0.2eV. Below T_co, 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.