Compensation techniques for HTS-rf-SQUID magnetometers operating in unshielded environments

We have developed two methods of compensating environmental disturbances applicable to high- and low-temperature superconducting quantum interference device (SQUID) systems, operating in a magnetically unshielded environment. For testing, we used first- and second-order axial electronic gradiometer setups with rf SQUID magnetometers operating at 77 K, with different baselines between 1.8 and 8 cm. The magnetometers were single-layer washer rf SQUIDs, with bulk or thin-film magnetic Bur concentrators and transformers in flip-chip geometry. The tested methods resulted in disturbance compensation levels comparable with those attained using electronically formed gradiometers. The white noise of the compensated magnetometers resulted in maximum gradiometric sensitivities of 13.5 fT/cm root Hz for first-order and 22 fT/cm(2) root Hz for second-order compensation, with baselines between 7 and 8 cm down to 10 Hz. Common mode rejection was balanced to, better than 10(4) for homogeneous fields, and better than 200 for gradient fields, with second-order compensation. Magnetically unshielded measurements of biomagnetic signals with different baselines were demonstrated. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.