Flip-Chip High-Tc DC SQUID Magnetometer With a Ferromagnetic Flux Antenna

We have combined a flip-chip high-Tc dc SQUID magnetometer with a ferromagnetic flux antenna made from 250 insulated 25-μm-thick stripes of amorphous soft magnetic Vitrovac 6025 foil inserted through the pick-up loop of the superconducting flux transformer. The ferromagnetic flux antenna provides improved magnetic coupling of the high-Tc SQUID to magnetic fields of ~0.4 nT/Φ0 to ~0.18 nT/Φ0. The orientation of the stripes was chosen to be normal to the foil tape rolling direction in order to minimize contributions from Barkhausen noise. A magnetic field resolution of ~2 fT/√Hz at 1 kHz and 77 K was achieved. Details of the technology and noise spectra of the novel composite high-Tc SQUID sensor are presented and their prospective applications for biomagnetic research and nondestructive evaluation are discussed.