Nd/Ba substitution and oxygen deficiency effects on the pinning properties of NdBa₂Cu₃O_7-δ single crystals

We studied the origin of peak effect in the magnetic field dependence of Jc in NdBa₂Cu₃O_7-δ single crystals. The single crystals were grown by the slow cooling method under various partial O₂ pressures(PO₂) (0.04∼1%) in Ar atmosphere. We found that the peak effect depends on PO₂ and it disappears for the crystals grown under low PO₂∼0.04%. This result demonstrates the important role of Nd/Ba substitution for the peak effect because it is known that the Nd/Ba substitution decreases with lowering PO₂. We also investigated the role of oxygen vacancies in the peak effect for the crystals with different amount of Nd/Ba substitution. We found that the main cause of peak effect in fully oxygenated crystals is due to local low Tc regions caused by the Nd/Ba substitution rather that those by the oxygen vacancies.