Direct flux noise measurement of YBCO grain boundary junctions

The flux generated by vortices in a field-cooled (10 muT) wide bicrystal gr ain boundary junction (GBJ) in a YBa2Cu3 O7-delta thin film was measured at 77 K, using a superconducting thin-film coil and Nb-based superconducting quantum interference device. The sample film was filled with slots, forming a mesh structure. When the field applied to the film was changed above a t hreshold value, either in positive or negative direction, random switching noise appeared. The switching heights implied long-distance movement of vor tices over 20-500 mum within the GBJ, driven by the shielding current. Almo st symmetric threshold fields showed the effectiveness of the mesh structur e in reducing demagnetization effect in thin-film geometry. The same slot s tructure made along the GBJ was also effective to suppress long-distance vo rtex motion in the GBJ to lengths of less than 1 mum. (C) 2001 Elsevier Sci ence BY. All rights reserved.