Development of a high-T/sub c/ first-order gradiometer system

The high-T/sub c/ superconducting quantum interference device (SQUID) gradiometer system was developed for operation in unshielded environment. The system consists of the first-order gradiometer and the flux-locked loop electronics. YBa/sub 2/Cu/sub 3/O/sub 7/ single-layer first-order SQUID gradiometers were fabricated on 15 mm/spl times/7.5 mm SrTiO/sub 3/ bicrystal substrates with a baseline of 7 mm. The gradient sensitivities at 77 K and 100 Hz were 0.1 pT/(cm/spl radic/Hz) in magnetic shielding and 1.4 pT/(cm/spl radic/Hz) when operated unshielded in our laboratory. The high-T/sub c/ SQUID gradiometer system demonstrated successful measurement of small magnetic fields in a laboratory environment without any shields.