Highly balanced long-baseline axial gradiometer based on high-Tc superconducting tape

The improving quality of high-T_c superconducting (HTS) tape with critical current densities larger than 1 MA/cm² creates the possibility to construct high quality flexible superconducting electronics operating at liquid nitrogen temperatures. We patterned a symmetric flux transformer into a 700 nm thick YBa₂Cu₃O_7-x film on a 70 μm thick, 85 mm long Hastelloy tape. The center loop of the transformer was coupled to the pickup loop of a SQUID magnetometer in a flip-chip configuration. The two outer pickup loops of the transformer were bent such that they were facing each other perpendicular to the magnetometer plane. The resulting axial gradiometer has a long baseline of 35 mm and a gradient sensitivity of 7.3 nT/(cmΦ₀). A superconducting shield was used to reduce uniform magnetic fields applied perpendicular to the magnetometer plane. Common-mode rejection ratios less than 10^-4 were achieved in the best case. The noise-limited gradient field resolution was approximately 330 fT/(cm√Hz) at a frequency of 10 Hz. This resolution was mainly limited by the flux noise level of our dc-SQUID magnetometer of approximately 45 μΦ₀/√Hz. In an unshielded laboratory environment, external noise contributions at 50 Hz were reduced by a factor of approximately 10³.