Charge ordering and antiferromagnetic transitions in Nd_xCa _1-xMnO₃(x = 0.2,0.3) manganites

In this paper we present transport, magnetic and electron paramagnetic resonance (EPR) studies of electron doped manganites Nd_xCa _1-xMnO₃(x = 0.2,0.3) with a view to compare and contrast their properties with those of the hole doped Nd_0.5Ca _0.5MnO₃ and other manganites. The samples were prepared by the solid-state reaction method and the composition was verified using X-ray and EDAX measurements. Magnetization measurements on the x = 0.2 sample (NCMO 0.2) show a peak at 155 K accompanied by a sharp increase in the resistivity. In the case of x = 0.3 sample (NCMO 0.3) a peak in the magnetization along with an increase in the resistivity is observed at 220 K. From a comparison of the EPR results in the two compounds, we conclude that the transition in NCMO 0.2 is an antiferromagnetic transition which is likely to be a charge ordering transition as well and that in NCMO 0.3 is only a charge ordering transition at which long-range magnetic order is not established. We infer that in this compound the long-range antiferromagnetic order sets in at a much lower temperature below which the EPR signal disappears due to the opening of an antiferromagnetic gap.