High sensitive magnetometers and gradiometers based on DC SQUIDs with flux focuser

Highly sensitive DC SQUID magnetometers and gradiometers with flux focusers instead of traditional coupling coils were designed and experimentally tested. All refractory material DC SQUIDs with shunted Nb-AlO_x-Nb tunnel junctions were used. The outer dimension of the square washer electrode was 2.5 mm and the inner 70 μm. A wide superconducting washer concentrates the magnetic flux by twenty times, and a flux sensitivity of about 20 nT/Φ₀ 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. As a result, a magnetic field resolution of 10 fT/√Hz has been achieved; this figure corresponds to the flux resolution about 2×10^-6 Φ₀/√Hz. Connecting the additional flux focuser to the compensation coils, one can create gradiometers of both the first and higher orders. The flux concentration coefficients were calculated for the several possible mutual positions of the SQUID washer and the focuser. The influence of the two focusers on each other was calculated. The experimental test makes it possible to get the correct relation between the effective area of the SQUID with a 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. The results demonstrate that sensitivity of the magnetometers and gradiometers with flux focuses are very attractive for multichannel biomagnetic systems and for high-temperature SQUID magnetometers