Integrated DC SQUID magnetometer with simplified read-out

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Abstract
A process is described for fabricating all refractory, integrated DC SQUID magnetometers for biomagnetic measurement applications. The process consists of only four lithographic steps and three sputter depositions. Several magnetometers have been fabricated and successfully operated in a flux-locked-loop mode using a simplified read-out technique that does not require the use of an impedance matching circuit between the SQUID and the preamplifier in the room-temperature electronics. Noise measurements have been carried out with a superconducting shield installed, from which a typical value for the magnetic flux noise square root SPhi =2*10-5 Phi 0 Hz-1/2 in the white noise region was determined. The onset of 1/f noise was observed to occur at about 1 Hz. Although the magnetic flux noise square root SPhi is relatively high, the field noise square root SB= square root SsGF(B/ Phi ) has been minimised by choosing a large SQUID inductance LS=550 pH in order to achieve a high magnetic field sensitivity B/ Phi =0.27 nT/ Phi 0. In this way, a typical value for the magnetic field noise square root SB of 5 fT Hz-1/2 has been attained. This low value of square root SB makes these magnetometers well suited for biomagnetic measurement applications.

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