The effect of Ca doping on the properties of melt-processed Gd-based bulk superconductors

The effect of Ca doping to melt-processed Gd-based bulk superconductors fabricated by the oxygen-controlled melt growth (OCMG) method was studied. First, we simply added CaCO3 to the starting materials to dope Ca and observed an increase in the trapped field and the critical current density (Jc) up to, at least, 0.31 wt.% CaCO3. However, an additional oxygen annealing resulted in a decrease of Jc in magnetic fields and in a monotonous exponential-like field dependence of Jc. This suggests that oxygen deficiency had caused the enhancement of Jc, although the same annealing process as the non-doped sample was applied. We also prepared samples by adding not only CaCO3 but also BaCO3 and CuO in a molar ratio of Ca:Ba:Cu = 1:2:3, and observed a large peak effect of Jc up to at least 0.31 wt.% CaCO3. It is likely that this large peak effect is also caused by oxygen defects, although the superconducting transition temperature was significantly large. These results indicate that Jc at 77 K can be significantly enhanced by introducing a proper amount of oxygen deficiency and compensating at the same time the decrease of carrier density by Ca doping.