The balance and noise properties of directly coupled high-Tc superconducting quantum interference device gradiometers with a serial array

We characterize the balance and noise properties of directly coupled high- Tc superconducting quantum interference device (SQUID) gradiometers with a series array. The gradiometer consisted of symmetrical pickup loops, in which a series of SQUID was symmetrically configured and directly coupled to pickup loops. We characterize the balance resolution B from the following formula: B= Aeff,uniform field Aeff,gradient field, where Aeff,uniform field and Aeff,gradient field are the parasitic effective area and the effective area of the gradiometer, respectively. We obtained B=1.25× 10-2 which was comparable to the typical balance resolution of ∼ 10-2 for a gradiometer fabricated onto the 10×10 mm2 substrate. This discrepancy was probably due to the many superconducting pads inside the pickup loops which deteriorated the uniformity of applied uniform magnetic fields. The gradient field sensitivity of ∼40 fT Hz12 cm at 1 kHz for two-SQUID gradiometers was demonstrated in a magnetically unshielded environment, which was comparable to the best gradient sensitivity of 30 fT Hz12 cm, with gradiometric flip-chip flux transformer with a length of 24 mm and a base line of 10 mm in shielded environment.