Regularities in the temperature evolution of magnetic flux trapped by the intergranular medium of a high-temperature superconductor

Granular high-temperature superconductors (HTSC) are characterized by the coexistence and interaction of two superconducting subsystems: HTSC granules and intergranular boundaries (Josephson medium). Depending on the thermomagnetic prehistory, the magnetic flux can be trapped either by both subsystems together or separately, or only in the weak subsystem of intergranular boundaries. In this paper, the conditions for the implementation of all these cases for the yttrium HTSC system are experimentally determined. The main attention is paid to the case when the Meissner state is realized in the HTSC granules, and the magnetic flux is trapped only by the Josephson medium. A previously undetected regularity in the temperature evolution of the remanent magnetization $M_r(T)$ is discovered in the case of flux capture only by the subsystem of intergranular boundaries. Namely, the temperature dependence of the normalized remanent magnetization $m_r(T)=M_r(T)/M_r$ ($T$ = 0) is identical for different values of the maximum applied field, despite the significant difference in the corresponding values of $M_r$ ($T$ = 0). At the same time, in a wide temperature range from 4.2 to $\sim$80 K (the transition temperature of the subsystem of intergranular boundaries $T_{\mathrm{CGB}}\approx$ 90 K), the functional dependence $m_r(T)$ follows the power law propto $\propto (1- T/T_{\mathrm{CGB}})^{0.5}$.