Effect of lower critical fields on magnetization of a single-crystal Bi2Sr2CaCu2O8+d bulk superconductor

To test the utility of the modified Kim-Anderson critical-state model for study of the magnetic response of ceramic superconductors, we measured the initial magnetization, hysteresis loops and ac complex susceptibilities χn=χn'-iχn''(n=1,2,3,5) of a single-crystal Bi2Sr2CaCu2O8+d bulk sample. The results were simulated in the framework of the model. Analyses of χn have shown that generation of the higher harmonics can be qualitatively reproduced, where the material parameters k and B0, surface barriers ΔH and lower critical fields Hcl involved were used as fitting parameters. Comparison of the observed data with the calculated results has revealed that nonzero values of Hcl play an essential role in the magnetic characteristics of the sample. (author)