LOW-FREQUENCY NOISE IN LOW-T-C MULTILOOP MAGNETOMETERS WITH ADDITIONAL POSITIVE FEEDBACK

The low-frequency noise in multiloop de superconducting quantum interference device (SQUID) magnetometers fabricated from low-T-c (transition temperature) materials was investigated. The magnetometers are operated in a flux-locked loop based on additional positive feedback (APF). A simple noise reduction scheme for de SQUIDs with APF is presented that eliminates the influence of critical current and resistance fluctuations without deteriorating the white noise performance. Applying this scheme to a magnetometer with a low-frequency two-level switching due to a single trap in the SixNy tunnel barrier reduced the switching amplitude of similar or equal to 10(-3) Phi(0) by at least two orders of magnitude down to a level below the intrinsic magnetometer flux noise. Using simple direct-coupled readout electronics with a 10 kHz bias reversal, a high bandwidth of similar or equal to 1 MHz, a low white flux noise of 1.5x10(-6) Phi(0)/root Hz, and a 1/f noise corner frequency of 4 Hz have been measured. (C) 1995 American Institute of Physics.