Abstract
We present the measurement of dc magnetization curve of three samples of high-T c superconductors in relatively low magnetic field of ±50 mT and at temperaturesT=4.2 K andT=77 K by a SQUID susceptometer. The samples were polycrystalline YBa2Cu3O7−x, a single crystal of YBa2Cu3O7−x, and polycrystalline BiCaSrCu2O5.5. We have especially studied the influence of the Josephson medium inside our samples on their low-field magnetization. We have found closed hysteretic loops in the field ±4 mT with “virgin” and “granular” slopes in the sample of polycrystalline YBaCuO and clear flux jumps corresponding to the penetration of low-field (Josephson) vortices. The pinning of these vortices in our case is rather low. In polycrystalline sample of BiCaSrCuO, we have found only reversible magnetization curves corresponding to very low pinning and/or to relatively low number of weak links.
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