A PLANAR YBA2CU3O7 GRADIOMETER AT 77-K

We describe the fabrication and performance of a single-layer YBa2Cu3O7 (YBCO) d.c. SQUID in a first-order gradiometer configuration. The junctions are of the step-edge type. The edge formation in the MgO substrate and all patterning were performed by Ar-plasma etching utilizing standard photoresist masks, thus involving more common and simpler fabrication techniques than reported by other groups. The gradiometer consists of two 1.2 mm2 pick-up coils with a baseline of 1.7 mm coupled directly to a 90 x 90 mum2 SQUID loop. At 77 K a total white noise of 185 muPI0 Hz-1/2 and a 1/f corner of about 10 Hz have been measured. This corresponds to an intrinsic flux noise level of about 70 muPHI0 Hz1/2, an energy sensitivity of 6.3 x 10(-29) J Hz-1 and a gradient noise level of 2.2 nT m-1 Hz-1/2.