Influence of intragranular Meissner currents and magnetic flux trapped in granules on the effective field in the intergranular medium and the magnetoresistance hysteresis of a granular HTSC

The concept of an effective field in an intergranular medium is used to describe the hysteresis magnetoresistance of granular HTSCs. This effective field is a superposition of the external magnetic field and the field induced by the magnetic moments of the superconducting granules. The magnetic moment of the granules is contributed by the shielding currents and the trapped magnetic flux. We studied and analyzed the effect of the trapped flux on the magnetoresistance. It has been experimentally shown that the dependence of the residual resistance $R_{\mathrm{Rem}}$ (after applying of the external field) on the trapped flux clearly correlates with the behavior of the remanent magnetization. In addition, special attention was paid to a detailed comparison of the magnetoresistance of the granular YBa$_2$Cu$_3$O$_{7-\delta}$ for two cases: (a) the magnetization of HTSC-granules is caused only by the trapped magnetic flux (in a zero external field) and (b) HTSC-granules are in the Meissner state (the external field is smaller than the first critical field of the granules). It has been found that Abrikosov vortices and intragranular Meissner currents differently affect the effective field in the intergranular medium (for the same values (of magnetization for cases (a) and (b)). A possible reason for this difference is discussed.