Intergranular Tunnelling and Field-Induced Percolation Fluctuation of Granular Composites (La_1-zAg_zMnO₃)/(MnO₂/Mn₂O₃)

We investigate the giant magnetoresistance of composites (La_1-zAg_zMnO₃)_1-y(MnO₂/Mn₂O₃)_y. The magnetoresistive property of the composites shows the characteristics of intergranular tunnelling. A conductivity leap has been observed around y=0.7. The composite (La_0.926Ag_0.074MnO₃)_0.698(MnO₂/Mn₂O₃)_0.302 has been found to show the greatest magnetoresistance in the samples. Its room-temperature MR ratio reaches 24% under a field of 1.8T. These phenomena suggest a percolation transformation and a kind of field-induced fluctuation in percolation in the samples investigated.