Increase of grain boundary resistance with time by impedance spectroscopy in La0.50Ca0.50MnO3+δ at 77 K

In order to investigate the effects of grain boundaries on the electrical properties of La0.50Ca0.50MnO3+delta impedance spectroscopic studies have been carried out at 77 K. With time there is irreversible formation of domains, which promotes the insulating behaviour of this charge ordered material. As time passes, the grain boundary resistance increases, showing the canted spin nature of the material. However, after 150 hours the grain boundary resistance becomes saturated, indicating that most of the metastable states have relaxed and transformed into stable charge ordered ones, most probably of antiferromagnetic nature. An equivalent circuit model, R-1(R2C2)(R3C3), i.e., a resistor-capacitor network, has been proposed to explain the impedance results. (C) 2007 Elsevier Ltd. All rights reserved.