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

Abstract

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,uniformfield∕Aeff,gradientfield, where Aeff,uniformfield and Aeff,gradientfield 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 10×10mm2 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 ∼40fT∕Hz1∕2cm at 1kHz for two-SQUID gradiometers was demonstrated in a magnetically unshielded environment, which was comparable to the best gradient sensitivity of 30fT∕Hz1∕2cm, with gradiometric flip-chip flux transformer with a length of 24mm and a base line of 10mm in shielded environment.