Ferro-antiferromagnetic coupling and enhancement of magnetoresistance in La_(0.67)Ca_(0.33)MnO_3/LaMnO_3 composites

摘要

The samples with the nominal composition of (1-x) La_(0.67)Ca_(0.33)MnO_3/xLaMnO_3 with x=0.00, 0.05, 0.15 and 0.25 were fabricated using a special experimental method. The electrical transport behaviour and magnetoresistance (MR) were studied for the composites in magnetic fields H=0.3 T, 3 T. Experimental results show that with the increasing LaMnO_3 doping level, the metal-insulator (M-I) transition temperature TP shifts to lower temperature and the resistivity increases sharply in zero magnetic field. Meanwhile, a significant enhancement in MR is observed for the composites especially in the low temperature range (below TP). Specially, the maximum MR at 3 T increased from 35% for the pure La_(0.67)Ca_(0.33)MnO_3 to 92% for the sample with x=0.25. We suggest that such enhancement in MR is attributed to the strong ferro-antiferromagnetic coupling effects in the composite system, which increase the magnetic disorder at the grain surface and boundary, will improve the spin-polarized tunneling process of the conducting electron between adjacent grains, and thus enhance the MR effects.