Abstract:
The character of the evolution of a system of weak links in granular high-temperature superconductors under the action of an external magnetic field $H_{\mathrm{ext}}$ has been studied by measuring the current-voltage characteristics $E(j)_{H_{\mathrm{ext}}=\operatorname{const}}$ of YBa$_2$Cu$_3$O$_{7-\delta}$ ($\delta\approx$ 0.05) ceramic samples. The measurements have been performed at $T$ = 77.3 K in a range of very weak magnetic fields 0 $<H_{\mathrm{ext}}\lesssim 0.5H_{c2J}$, where $H_{c2J}$ is the upper critical field of the Josephson weak links. The results have been used to construct the field dependences of the magnetoresistance
$\Delta\rho(H_{\mathrm{ext}})$ of the superconducting ceramics. It has been established that the parameters of the power equation $E=A(j-j_{cJ})^\nu$ and the magnetoresistance$\Delta\rho$ are nonmonotonic functions of the external magnetic field. The presence of extrema in the curves $A(H_{\mathrm{ext}})$, $j_{cJ}(H_{\mathrm{ext}})$, $\nu(H_{\mathrm{ext}})$, and
$\Delta\rho(H_{\mathrm{ext}})$ indicates that different systems of weak links between grain boundaries, which are capable of forming extended Josephson contacts, undergo sequential transitions to a resistive state with an increase in $H_{\mathrm{ext}}$.
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