Integrated DC SQUID magnetometer with high dV/dB

A directly coupled DC SQUID magnetometer with very simple feedback electronics is presented. The magnetometer has been integrated on a 7.2*7.2 mm/sup 2/ chip and fabricated using a four-level Nb/Si/sub x/N/sub y//Nb process. Eight pickup loops are connected in parallel to directly form the SQUID inductance of about 0.4 nH which leads to a high sensitivity B/ Phi =0.47 nT/ Phi /sub 0/. An additional positive feedback (APF) circuit on the magnetometer chip has been used to increase the gradient of the V- Phi characteristic to dV/d Phi approximately=300 mu V/ Phi /sub 0/ at the SQUID operating point. The resulting gradient of the transfer function dV/dB approximately=640 mu V/nT makes it possible to directly readout the SQUID without helium-temperature impedance matching circuits or flux modulation techniques. Using a highly simplified feedback electronics consisting of an ultra-low-noise bipolar preamplifier and an integrator on a 10-cm/sup 2/ PC board, a white noise level below 3 fT/ square root Hz with a 1/f corner frequency around 3 Hz, a 3-dB bandwidth of about 40 kHz, and a maximum feedback range of +or-32 nT for frequencies up to 300 Hz have been measured.