Effect of impurity doping on the superconducting properties of melt-processed bulk superconductors depending on the substituting site

The effect of doping impurities to melt-processed Gd-Ba-Cu-O bulk superconductors was studied. We doped Zn and Co in this work, the former substitutes for Cu of the CuO2 plane, while the latter to the Cu-O chain site. The trapped field increased for both Zn- and Co-doping, and the optimum content of ZnO and Co3O4 in this work was 0.1 wt.% and 0.225 wt.%, respectively. The trapped fields of these two optimally doped samples were very similar at liquid nitrogen temperatures. At lower temperatures, however, the trapped field of the Co-doped sample was larger than that of the Zn-doped one. We also observed that the creep of the trapped field of the former sample was faster than the latter one at 77 K, while it was slower at 50 K, which explains why the observed trapped field of the Co-doped sample was larger at lower temperatures although the two samples exhibited a very similar trapped field at 77 K.