Relaxation-oscillation-driven dc SQUIDS

Simple and reliable relaxation-oscillation-driven dc SQUIDs have been fabricated with flux-to-voltage transfer factor up to 1 mV/PHI/sub o/ and internal energy sensitivity E/sub v/ = (4 +- 1)x10/sup -31/ J/Hz at frequencies above about 10 Hz, and 1x10/sup -29/(1 Hz/f) J/Hz at lower frequencies. Supplied with the 50-μm-wire 20-turn spiral input coil and 6 cm/sup 2/ pick-up loop, such SQUIDs provided magnetic field sensitivity close to (6 +- 3)x10/sup 15/ T/Hz/sup 1/2/ above 10 Hz. The dynamic range of the magnetometers was close to 1.5x10/sup 7/ Hz/sup 1/2/, and the maximum slew rate, about 10 μT/s at 0.1 kHz. Successful records of the Earth-ionosphere magnetic resonances have been obtained with these magnetometers housed in a stainless steel transport dewar. A new balanced version of the RO-driven dc SQUID has been suggested with the aim to improve both H and E/sub v/. An analysis and the first measurements have shown a noticeable increase of H with respect to the basic version of the device