An active tunneling model for the magnetotransport in mixed-phase polycrystalline manganites

Based on phase separation between metallic and insulating domains, the evolution of magnetoresistance in mixed-phase polycrystalline manganites, as a function of temperature, has been modeled via a bond-disordered random resistor network. The tunneling bond is introduced in the resistor network to account for the contribution from grain boundaries, where the energy barrier is formed. The model is analytically worked out by means of the effective-medium approximation. Compared with the transport in single-phase ferromagnetic-ordered polycrystal, the transport in mixed-phase polycrystal is governed by not only the properties of grain boundaries but also the competing metallic and insulating phases. The fraction of tunneling bonds strongly depends on the temperature and external magnetic field. An active tunneling model is thus established in mixed-phase polycrystalline manganites. Within our model, the effect of crystallinity on the temperature dependence of magnetoresistance in mixed-phase polycrystalline manganites is well explained. (C) 2004 American Institute of Physics.