High-Tc SQUID magnetometers for use in moderate magnetically-shielded room

We have fabricated and characterized high-T_c planar SQUID magnetometers and first derivative gradiometers with directly-coupled pickup loops. The devices were made from single layer of YBa₂Cu₃O_7-δ thin film on LSAT bicrystal substrate with 30° or 24° misorientation angle. Magnetic properties were investigated by applying a magnetic field B₀ for the SQUID magnetometer patterned with holes to reduce the maximum structural width. We found an increasing low frequency noise with cooling fields B₀ larger than 1.5 μT. This value consists with the threshold field estimated from the maximum structural width. A magnetic field noise level of 500 fT/Hz^1/2 at 10 Hz was observed by using FLL electronics with a bias current reversal in a moderate magnetically-shielded room consisting of only two 1 mm thick layers of permalloy. Measurements of magnetocardiograms demonstrate the suitability of this magnetometer for biomagnetic applications. On the other hand, the gradiometer with two symmetric pickup loops was operated without any shielding. The performance obtained was a field gradient resolution of about 1 pT/cmHz^1/2 at 1 kHz and 10 pT/cmHz^1/2 at 1 Hz, with a baseline of 4 mm. The imbalance of this gradiometer was around 0.7%, limited by the sensitivity to homogeneous field of the SQUID itself