Ferro-Antiferromagnetic Coupling and Enhancement of Magnetoresistance in La<sub>0.67</sub>Ca<sub>0.33</sub>MnO<sub>3</sub>/LaMnO<sub>3</sub> Composites

Abstract

The samples with the nominal composition of (1-x)La0.67Ca0.33MnO3/xLaMnO3 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=0.3T, 3T. Experimental results show that with the increasing LaMnO3 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 La0.67Ca0.33MnO3 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.