Direct detection of vortex motion in high-T/sub c/ grain boundary junctions

Using a superconducting thin-film coil and Nb-based SQUID, the flux generated by vortices in a field-cooled (10 /spl mu/T) wide bicrystal YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// grain boundary junction (GBJ) was measured directly at 77 K. At and above a small threshold field change, random switching noise was observed, with switching heights corresponding to long-distance movement of vortices over 20 to 500 /spl mu/m within the GBJ, driven by the shielding current. The vortex movement was suppressed to lengths of less than 1 /spl mu/m by making slots in the GBJ. From these results, we estimated the flux noise of a directly coupled dc SQUID magnetometer with a weak link in its pick-up coil.