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

摘要

We characterize the balance and noise properties of directly coupled high-T_c 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 =A_(eff,uniform field)/A_(eff,uniform field), where A_(eff,uniform field) and A_(eff,uniform 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 mm~2 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/Hz~(1/2) 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/Hz~(1/2) cm, with gradiometric flip-chip flux transformer with a length of 24 mm and a base line of 10 mm in shielded environment.