Mechanism of formation of a negative magnetoresistance region in granular high-temperature superconductors

The field dependences of the magnetoresistance of Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x samples with different densities, which have a foam microstructure and exhibit different diamagnetic responses, were studied at 77.4 K to identify the mechanism responsible for the formation of a negative magnetoresistance region in granular high-temperature superconductors. A region with negative magnetoresistance was found to exist in samples with magnetizations highest in absolute magnitude. This behavior finds a reasonable interpretation as due to the effect exerted by dipole moments of high-temperature superconductor grains on the effective intergranular field. The strength of this effective field has been estimated.