Effect of heat treatment on critical current density-magnetic field curve for YBa₂Cu₃O_7-δ single crystal and its microstructure

The dependences of critical current density-magnetic field (J_c-H) curves for YBa₂Cu₃O_7-δ (Y123) single crystals on heat treatment processes were studied. The single crystals were annealed by a two-step heat treatment process, during which the crystals were annealed at 873 K for 100 h, followed by annealing at 773 K for 200 h in a pure oxygen gas flow. Furthermore, the single crystals were annealed using three different untwinning processes with uniaxial stress such as a detwinning process and a twin-free process. The J_c-H curves and their peak effects with the H ∥ c-axis displayed a large dependence on the heat treatment processes. Furthermore, the microstructures in these single crystals were observed using transmission electron microscopy (TEM). The anti-phase boundaries, which depended on the heat treatment processes, were observed in the crystals. These results suggested that the anti-phase boundaries are responsible for the peak effects, which is attributed to the distribution of the pinning centers of oxygen deficient phases in the crystals.