ELECTRICAL-MAGNETIC TRANSPORT AND MAGNETORESISTANCE IN La_0.7Ca_0.3MnO₃/Bi₂O₃ COMPOSITES

The structure and electrical-magnetic transport properties of ( La 0.7 Ca 0.3 MnO 3)1-x/( Bi 2 O 3)x has been investigated systematically by conventional solid-state reaction method. The composite samples were obtained by pure La 0.7 Ca 0.3 MnO 3 (LCMO) and Bi 2 O 3 powders and sintered at different temperature (Ts) 800°C and 1300°C for 2 h. For Ts=800°C samples, the results of X-ray diffraction (XRD) and scanning electronic microscopy (SEM) indicate that Bi 2 O 3 and LCMO coexist in the composites and Bi 2 O 3 mainly segregates at the grain boundaries or grain surface of LCMO, and double peaks related to the metal-to-insulator (M–I) transition have been observed. Corresponding to the two M–I transition peaks, the curves of magnetoresistance (MR) against temperature also showed two peaks for all composites. But for Ts = 1300°C samples, Bi 3+ ions mainly enter into the lattice of LCMO to form reaction products layer La 0.7-x Bi x Ca 0.3 MnO 3, only one peak occurs around the M–I transition temperature and the MR around T P has been enhanced. The results also indicate that the added Bi 2 O 3 has an important effect on the LFMR. These phenomena can be explained by the double exchange (DE) mechanism, the grain boundary effect and the intrinsic transport properties together. And the grain surface state of the particles is a key factor influencing the magnetoresistance effects.