A low-noise integrated SQUID gradiometer for biomagnetic sensors

We developed a low-noise and compact SQUID gradiometer for biomagnetic multichannel applications. The SQUID is based on a double relaxation oscillation SQUID and provides very large flux-to-voltage transfer coefficients, allowing simple flux-locked loop electronics to be used for SQUID operation. The pickup coil is a first-order planar gradiometer integrated on the same wafer as the SQUID and has a baseline of 40 mm. The field gradient noise of the SQUID gradiometer in flux-locked loop mode is around 1 fT/cm/spl radic/Hz at 100 Hz, measured inside a magnetically shielded room.