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

Three magnetometers based on dc 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 dc SQUIDs with outer diameters of 7 mm, achieved white noise levels of 10 fT/√Hz for bicrystal junctions and 30 fT/√Hz for step‐edge junctions. The third magnetometer involved a flux transformer with a 10×10 mm2 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/√Hz. High quality magnetocardiograms were obtained without signal averaging.