High-T-c SQUID magnetometers for biomagnetic measurements

We have designed and fabricated three types of high-T-c SQUID (superconducting quantum interference device) magnetometers based on step-edge Josephson junctions using three different concepts of coupling magnetic flux into the SQUID: (i) a single pickup loop galvanically coupled to the SQUID, (ii) a flux transformer inductively coupled to the SQUID and (iii) a multiloop pickup loop used directly as the SQUID inductance. On a 10 x 10 mm(2) substrate we achieved an effective flux capture area of 1.70 mm(2) and 1.67 mm(2) for the inductively coupled and multiloop devices, respectively. Due to the low white noise levels of 44 fT HZ(-1/2) for the inductively coupled magnetometer and 30 fT Hz(-1/2) for the multiloop device high quality magnetocardiograms were recorded inside a magnetically shielded room without signal averaging.