We report on magnetic measurements carried out in a textured YBa2Cu3O 7−δ and YBa2(Cu1−xAgx)3O 7−δ (at x ≈ 0.02) crystals. The so-called fishtail-effect (FE) or second magnetization peak has been observed in a wide temperature range 0.4 < T /Tc < 0.8 for H c. The origin of the FE arises for the competition between surface barrier and bulk pinning. This is confirmed in a non-monotonically behavior of the relaxation rate R. The value Hmax for Ag-doped crystals is larger than for the pure one due to the presence of additional pinning centers, above all on silver atoms.
The superconducting properties of Y 1 Ba 2 (Cu 1−x Al x) 3 O 7 /Y 2 BaCuO 5 bulk superconductors with different Al contents were studied (x = 0.0025-0.05). A peak effect was observed at intermediate magnetic fields over a wide temperature range below T c. The influence of standard oxygenation (SO) at 400 • C and high pressure oxygenation (HPO) at 750 • C on the critical current density J c was investigated. We show that the critical current density is higher by a factor of 3 in samples oxygenated at high pressure. The magnetization relaxation rates were investigated for an undoped HPO sample as well as for the lowest Al concentration for HPO and SO in the temperature range from 15 to 77 K and at magnetic fields of 0.5 and 1 T. The normalized relaxation rates, S, and the pinning potentials, U 0 , have been calculated from the time-dependent magnetization curves.
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