Universal behavior and temperature evolution of the magnetoresistance hysteresis in granular high-temperature superconductors Y-Ba-Cu-O

The work is devoted to the establishment of regularities in the behavior of the magnetoresistance hysteresis $R(H)$ of granular high-temperature superconductors (HTS) of the yttrium system. We carried out a comparative study of the magnetotransport properties of granular HTSC samples, which have (i) approximately the same magnetic properties and temperatures of the onset of the superconducting transition (90.5–93.5 K, which characterizes HTS grains) and (ii) different values of the transport critical current $J_C$ (which characterizes the intergrain boundaries). Despite the significant scatter in the $J_C$ values (more than an order of magnitude) of the three samples studied, a universal behavior of the magnetoresistance hysteresis was found, apparently inherent in all granular Y-Ba-Cu-O. The $R(H)$ hysteresis is very wide, and in a sufficiently large interval of the external field, the dependence of the hysteresis width of the magnetoresistance $\Delta H$ on the field $H_{\mathrm{dec}}$ (external field $H=H_{\mathrm{dec}}$ for a decreasing hysteresis branch) is close to a linear function: $\Delta H\approx H_{\mathrm{dec}}$. This behavior is observed for the entire temperature range of realization of the superconducting state (the studies were carried out at temperatures of 77–88 K and 4.2 K). The explanation of the obtained result is based on the concept of consideration the effective field in the intergrain boundaries. This effective field is the superposition of the external field and the field induced by the magnetic moments of the grains. The field induced by the grains, in turn, is significantly enhanced in the region of intergrain boundaries due to the effect of the magnetic flux compression (the length of the intergrain boundaries is several orders of magnitude smaller than the size of the HTS grains). The above is confirmed from the analysis of the $R(H)$ hysteresis for a composite HTS sample based on Y-Ba-Cu-O and CuO, in which the length of the intergranular boundaries is purposefully increased, as a result, the effect of the flux compression is less pronounced, and the hysteresis $R(H)$ narrows.