Enhanced flux pinning properties of RE123 melt-solidified bulks by dilute impurity doping

Abstract

In our previous study, dilute impurity doping to various cation sites of the Y123, particularly to the Cu site in the Cu-O chain, was found to dramatically enhance Jc of single crystals and its melt-solidified bulks. In order to clarify the essential role of dilute doping for improved pinning, TEM observation was performed for Co or Ga doped Y123 melt-solidified bulks. Density of twins did not change by the Co and Ga doping. The local compositional analysis by TEM-EDS indicated that the doped Co homogeneously dispersed in the Y123 matrix. These suggested generation of point-defect-like pinning centers by dilute doping for Cu site of the CuO chain. In addition, further enhancement of Jc was attempted for Dy123 melt-solidified bulks by the same method. The Co-doped Dy123 melt-solidified bulks also exhibited improved Jc, particularly for samples grown under higher cooling rate during crystal growth. A high Jc of ~60 kA cm-2 under 10~20 kOe at 77 K was achieved, when a cooling rate of 0.9°Ch-1 was applied. Systematic decrease in Jc with an increase of the cooling rate observed for undoped Dy123 bulks suggested that the high cooling rate suppressed partial Dy substitution for the Ba, resulting in a decrease of effective pinning sites. Therefore, the high Jc observed for the Co-doped Dy123 bulk grown under a high cooling rate can be explained by generation of effective pinning sites around Co ions in the strong superconducting matrix of Dy123 with low substitution level of Dy at the Ba site.