Abstract
Based on intrinsic phase separation between ferromagnetic metallic domains and paramagnetic insulating domains, and extrinsic band-bending effects at the grain boundaries, a tunneling percolation model is established for the magnetotransport in phase-separated polycrystalline manganites. Within this model, the competition between intrinsic colossal magnetoresistance and extrinsic tunneling magnetoresistance in these materials is mapped onto a multicomponent bond-disordered random resistor network, where the fraction of each kind of bond is determined self-consistently. For the existence of a distribution of intergranular distance and the exponential dependence of tunneling resistance on the intergranular distance, the network exhibits strong disorder dependence of low-field magnetoresistance and is expected to yield a nonuniversal behavior of electrical transport in some particular cases, which are also magnetic field dependent. Further numerical simulations on the temperature dependence of resistance and magnetoresistance give a reasonable explanation of the dependence of magnetotransport on the intergranular connectivity and the grain size. All of these results agree will with experimental observation.
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