Low-noise magnetometers with asymmetric superconducting quantum interference devices

Fully integrated low-critical-temperature magnetometers based on asymmetric superconducting quantum interference devices have been designed, fabricated, and characterized. The large value of the flux to voltage transfer coefficient, typical of such configurations, allow a direct coupling to an external preamplifier without the need of an impedance matching flux transformer or additional positive feedback circuitry. This simplifies the read-out electronics, as required in multichannel systems for low-noise measurements. The values of the magnetic flux noise and field sensitivity, obtained so far, are 3–5 microF0 /Hz^(1/2) and 6–7 fT/Hz^(1/2) , respectively. These results are already suitable for biomagnetic applications and can be further improved by properly optimizing the device parameters.