Site depending impurity effect on the properties of melt-processed bulk superconductors

The effect of doping impurities to Gd-based bulk superconductors prepared by melt processing was studied. The trapped field at liquid nitrogen temperature increased with ZnO content up to 0.1 wt%, with an enhancement of 37% compared to the non-doped sample. The trapped field decreased, however, when the content of ZnO was increased beyond this optimum level. When Co3O4 was added, on the other hand, the trapped field increased monotonically with the Co3O4 content up to 0.2 wt% and an enhancement of 32% was observed. We also measured the local superconducting properties on small samples that were taken from the bulk superconductors, which revealed that the difference of how the impurities affect the trapped field can be primarily attributed to the different influence on the critical temperature depending on the substituting site.