BICRYSTAL YBCO DC SQUIDS WITH LOW NOISE

We have fabricated 12 dc SQUIDs by laser depositing YBa2Cu3O7-x on a SrTiO3 bicrystal substrate with a misorientation angle of 24 degrees. At 77K all twelve devices had acceptable values of critical current, resistance and voltage modulation produced by an external magnetic field. The white noise energy of one device with an estimated inductance of 41 pH was 1.8 x 10(-30) JHz(-1). The noise power scaled as 1/f at frequencies below about 1kHz, however; by using a bias current reversal scheme we were able to reduce this noise by two orders of magnitude at 1 Hz, to a value of about 1.5 x 10(-29) JHz(-1). We made a magnetometer by coupling the SQUID to a flux transformer with a 5-turn input coil. The measured magnetic field gain was 60, and the white noise 36ff Hz(-1/2). However, the transformer produced relatively large levels of 1/f flux noise, not reduced by the bias reversal scheme, that limited the noise at 1 Hz to 1.7 pT Hz(-1/2). A single-layer magnetometer with a single-turn pick up loop is briefly described.