Effect of annealing on the structure and diamagnetic transition width of high-quality crystal Bi2Sr2CaCu2Oy studied by x-ray-diffraction and ac susceptibility measurements

Abstract

A large and highly oriented Bi2Sr2CaCu2Oy crystal with only a single domain was progressively air annealed at 500 and 750 °C for 4 and 8 h. Measurements of ac susceptibility showed that Tc (onset) and ΔTc (10%–90% of the complete transition) of the crystal were 74 and 3 K (as grown), 85 and 0.6 K (annealed at 500 °C for 4 h), and 89 and 2.5 K (annealed at 750 °C for 8 h), respectively. To explain this typical change of ΔTc with annealing temperature, a full structural characterization by x-ray diffraction in the b*c* plane and by Laue photographs of the crystal as grown and after each annealing treatment was carried out. Apart from variations of the lattice parameters, it is noted that the smaller the ΔTc, the more highly oriented and structurally homogeneous the crystal. We explain the annealing effect on ΔTc by variations in crystallinity, structural homogeneity, and surface phase decomposition, and the changes in oxygen stoichiometry caused by the annealing process.