LOW-NOISE YBA2CU3O7-X SQUID MAGNETOMETERS OPERATED WITH ADDITIONAL POSITIVE FEEDBACK

Three magnetometers based on de superconducting quantum interference d evices (SQUIDs) fabricated from YBa2Cu3O7-x, have been operated in a m agnetically shielded room using a flux-locked loop involving additiona l positive feedback with bias current reversal. Two of these devices, integrated multiloop de SQUIDs with outer diameters of 7 mm, achieved white noise levels of 10 fT/root Hz for bicrystal junctions and 30 fT/ root Hz for step-edge junctions. The third magnetometer involved a flu x transformer with a 10X10 mm(2) pickup coil connected to a 16-turn in put coil which was inductively coupled to a bicrystal SQUID. This devi ce achieved a white noise of 16.2 fT/root Hz. High quality magnetocard iograms were obtained without signal averaging. (C) 1996 American Inst itute of Physics.