Simultaneous observation of positive and negative giant magnetoresistances in composite (La_0.83Sr_0.17MnO₃)_1－x(ITO)_x

We have studied the transport property of the composites (La_0.83Sr_0.17 MnO₃)_1-x(ITO)_x ITO=(In₂O₃)_0.95 (SnO₂)_0.05, which were fabricated by mechanically mixing La_0.83Sr_0.17 MnO₃ and ITO grains. A giant positive magnetoresistance (PMR) has been observed above the Curie temperature T_c for samples with x around 0.40, in addition to the negative magnetoresistance related to spin-dependent interfacial tunnelling below T_c. For (La_0.83Sr_0.17MnO₃)_0.6(ITO)_0.4, the magnetoresistive ratio for the PMR can reach 39.3% under a magnetic field H=2.24\times10^5 A/m. Theoretical analysis suggests that the magnetic-field-induced broadening of the p--n barrier between both kinds of grains and the density of the p--n heterostructures should be responsible for the PMR behaviour.