Magnetometry based on sharpened high T/sub c/ GBJ Fraunhofer patterns

At EUCAS´95, we reported on the large enhancement of the sharpness of the diffraction pattern exhibited by YBCO grain boundary junctions on bicrystal substrates. This effect results from large focusing effects of the superconducting electrodes; the geometry used is a small square washer (630/spl times/630 /spl mu/m/sup 2/) with the junction in the center. For such devices, the Fraunhofer pattern is contracted around the value B/sub n//spl ap/0 where B/sub n/ is the perpendicular magnetic field applied to the washer. A FWHM of the Fraunhofer pattern of 155 mG has been observed leading to a field response of 176 A/T or 50 V/T corresponding to voltage and current biasing modes respectively. A white noise level of 3 pT//spl radic/Hz mas measured. Although a field modulation was applied to remove some of the "1/f" noise, an unusual telegraphic noise appearing as jumps of about 20 nT still remained. This is probably associated to vortex hopping between pinning sites in the vicinity of the junction. We propose a possible way for removing such large perturbations. These devices are to be used in active shielding systems and will be very useful in SQUID systems to be operated in an open environment.