Anomalous magnetotransport properties of the highly A-site mismatched manganite Nd0.7Ba0.3MnO3

The magnetoresistance of Nd0.7Ba0.3MnO3 has been studied for a single crystal and for ceramics. Besides the expected colossal magnetoresistance (CMR) effect, at about T-C = 140 K, two anomalies have been observed. It concerns the upturn of the resistivity below 50 K, never observed in manganites without barium. This electronic localisation which appears as well for the single crystal as for the ceramics is interpreted in terms of high mismatch between Ba2+ and Nd3+. The second anomaly is the appearance of a bump below T-C on the rho(T) curves. Thus the magnetoresistance ratio for the ceramic is no more maximum at T-C, contrary to the single crystal but below T-C. Moreover it remains larger at low temperature in the ceramics, in contrast with the single crystal. This property has been explained on the basis of grain boundary effects. (C) 1998 Elsevier Science Ltd. All rights reserved.