Relaxation-oscillation-driven DC SQUIDs

Simple and reliable relaxation-oscillation (RO)-driven DC SQUIDs (superconducting quantum interference devices) have been fabricated with a flux-to-voltage factor H up to 1 mV/ Phi /sub 0/ and internal energy sensitivity E/sub v/ of (4+or-1) * 10/sup -31/ J/Hz at frequencies above 10 Hz and 1*10/sup -29/ (1 Hz/f) J/Hz at lower frequencies. Supplied with a 50- mu m-wire 20-turn spiral input coil and a 6-cm/sup 2/ pickup loop, such SQUIDs provided magnetic field sensitivity close to (6+or-3)*10/sup -15/ T/Hz/sup 1/2/ above 10 Hz. The dynamic range of the magnetometers was close to 1.5*10/sup 7/ Hz/sup 1/2/ above 10 Hz. The maximum slew rate was about 10 mu T/s at 0.1 kHz. Successful records of Earth-ionosphere magnetic resonances have been obtained with these magnetometers housed in a stainless steel transport dewar. A balanced version of the RO-driven DC SQUID is suggested with improved H and E/sub v/.<
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