Electronic high-temperature radio frequency superconducting quantum interference device gradiometers for unshielded environment

This article will discuss improved electronically formed gradiometers based on high-temperature radio frequency (rf) superconducting quantum interference device (SQUID) magnetometers. For gradiometer balancing, a system of adjustable superconducting plates was developed. This technique was used to build first- and second-order, axial gradiometers with adjustable baselines, which operate at 77 K. Each magnetometer combines a washer rf-SQUID with bulk or a thin-film flux concentrator in flip chip geometry. In an unshielded environment, the magnetic field sensitivity in the white noise region is about $\text{80\hspace{0.17em}}\mathrm{fT/\surd Hz}$ for first-order and $\text{150\hspace{0.17em}}\mathrm{fT/\surd Hz}$ for second-order gradiometer. Common mode rejection could be balanced to better than ${10}^4$ for uniform background fields and better than 200 for gradient fields.