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

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.