HIGH SENSITIVE MAGNETOMETERS AND GRADIOMETERS BASED ON DC SQUIDS WITH FLUX FOCUSER

High sensitive dc SQUID magnetometers and gradiometers with flux focusers instead of traditional coupling coils have been designed and experimentally tested. All refractory material DC SQUIDs with shunted Nb-AlO(x)-Nb tunnel junctions were used 1. The outer dimension of the square washer electrode was 2.5 mm, the inner one was 70-mu-m. A wide superconducting washer concentrates the magnetic flux by twenty times and the flux sensitivity about 20 nT/PHI-o can be realized without any input coil. The application of the additional 14 mm diameter flux focuser has increased the flux concentration by four times more. As a result the magnetic field resolution 10 fT/square-root Hz has been achieved; this figure corresponds to the flux resolution about 2.10(-6) PHI-o/square-root Hz. Connecting the additional flux focuser to the placed apart compensation coils one can create gradiometers of both the first and higher orders. The flus concentration coefficients were calculated for the several possible mutual positions of the SQUID washer and focuser. The influence of the two focusers on each other was also calculated. The experimental test makes it possible to get the correct relation between the effective area of the SQUID with focuser and the outer sizes of the focuser. The first order gradiometer consisting of the rectangular focuser and compensation coil connected to it has been experimentally verified. Our results have demonstrated that high sensitivity magnetometers and gradiometers with flux focuser are very attractive for multichannel biomagnetic systems and for HT(c) SQUID magnetometers.