Low noise YBa2Cu3O7-x SQUID magnetometers operated with additional positive feedback

Three magnetometers based on de superconducting quantum interference devices (SQUIDs) fabricated from YBa2Cu3O7-x, have been operated in a magnetically shielded room using a flux-locked loop involving additional 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 flux transformer with a 10X10 mm(2) pickup coil connected to a 16-turn input coil which was inductively coupled to a bicrystal SQUID. This device achieved a white noise of 16.2 fT/root Hz. High quality magnetocardiograms were obtained without signal averaging. (C) 1996 American Institute of Physics.