Flux pinning effect in a melt textured YBCO bulk evaluated by using tension measurements

Abstract

Research on the flux pinning effect in type-II superconductors has usually been focused on microor nanosize pinning centers, and mm-sized pinning centers have been relatively less studied. In order to investigate the flux pinning effect caused by mm-sized pinning centers, we introduce a tension measurement method in this research. A cm-sized melt-textured YBCO bulk, in which holes with a 2 mm diameter are made, is prepared. The YBCO bulk is field-cooled by using a strong magnet in liquid nitrogen, and the bulk and the magnet are separated from each other. Then, an attractive force (fa) between them is generated, and fa is detected by using a tension measuring device. As the distance (d) between them is increased, fa increases at short distance and decreases at long distance, showing a maximum value, fam, at a specific distance. The measurement of fa is stopped when d reaches a value defined as the breaking distance (dbk), as if a ‘string’ between the magnet and the YBCO bulk is broken. As the number of holes (n) made in the YBCO bulk increases from 1 to 6, fam and dbk increase, in spite of the superconducting volume loss. fam and dbk for n ≥ 7 converge to nearly constant values, which are smaller than the values for n = 6. This means that the critical current density can be calculated by using fam or dbk for a sufficient number of holes.