The superconducting properties of directionally solidified YBa 2 Cu 3 O 7 -Y 2 BaCuO 5 composites, with critical currents above 10 5 A/cm 2 at 77 K and zero magnetic field, are reported in a wide variety of samples having very different contents of Y 2 BaCuO 5 ͑211 phase͒ precipitates with different particle size and magnetic fields up to 22 T. The field, temperature, and composition dependence of the critical currents allow us to identify interfacial pinning by 211 precipitates as a very effective pinning mechanism. Nevertheless, single vortex interfacial pinning has a dominant role only in a narrow region below about 1 T and temperatures 40 KрTр80 K. In this region the system shows a behavior very akin to that observed in ion irradiated single crystals and described as correlated disorder. On lowering the temperature the thermal wandering of the vortex from secondary weak pinning centers decreases leading to a new single vortex pinning regime extending up to very high magnetic fields where a mixture of strong and weak pinning centers are active. On the other hand, increasing the magnetic field, the characteristic footprints of the small bundle and large bundle regimes are identified at intermediate temperatures. The former is found below the H-T line given by the maximum of the macroscopic pinning force while the latter is observed above this line up to the irreversibility line. Finally, collecting all these results together a magnetic phase diagram of the mixed state of the YBa 2 Cu 3 O 7 -Y 2 BaCuO 5 textured composites is proposed.
We investigated with ac susceptibility the mixed state of twinned, deoxygenated single crystals and highly c-axis-oriented melt-textured samples of YBa 2 Cu 3 O 6ϩx with oxygen stoichometry corresponding to that of the orthorhombic II phase. In the single crystal we found, below H*Ϸ700 Oe and over almost four orders of magnitude in frequency, a frequency-independent ''irreversibility line'' ͑IL͒. Complementry dc magnetization measurements seem to disregard an IL arising from geometric effects when Hʈc, and they suggest a possible meltinglike transition of the flux-line lattice below H*. The reduced value of H* is associated with the increased anisotropy induced by the oxygen depletion, and an upper limit of the anisotropy parameter is estimated as ␥ max Ϸ147. However, in the twinned melt-textured sample having a major grade of disorder due to the presence of micrometric nonsuperconducting precipitates of Y 2 BaCuO 5 , the IL is frequency dependent for all applied magnetic fields, suggesting a cancellation of H* in the sufficiently disordered specimen.
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