Peak effect in critical current density induced by oxygen non-stoichiometry in cation-stoichiometric SmBa2Cu3O7-δ superconductor

Cation-stoichiometric SmBa2Cu3O7-delta superconductor (Sm-123) samples were synthesized by a conventional (without any liquid phases involved) solid-state-reaction technique, followed by annealing in flowing O-2, gas at 350 degrees C for 40-200 h. A ''peak effect'' was observed in the critical current density (J(c)) vs. applied magnetic field (H) characteristics measured at 77 K. Pronounced peak effect was observed in a sample annealed at 350 degrees C for 40 h, while the peak was suppressed in a sample annealed at 350 degrees C for 200 h. Since the average oxygen content determined by a coulometric titration method increased with decreasing J(c), value at the peak, certain oxygen-deficient regions induced by oxygen non-stoichiometry might have formed. They could work as the magnetic fluxon pinning centers, since transmission electron microscopy (TEM) observations revealed the presence of both the tetragonal phase and a twin-free orthorhombic phase with width of ca. 200 nm in the sample annealed in O-2 gas at 350 degrees C for 40 h which would have contained a significant amount of oxygen deficiency. The presence of the oxygen-deficient Sm-123 phase especially close to the boundary between either or both of the orthorhombic and tetragonal phases and an ordinary orthorhombic phase containing twins could be working for magnetic flux pinning to yield the peak effect in the J(c), vs. H characteristics. (C) 1994 Elsevier Science B.V. All rights reserved.