Development of SQUID sensor for nondestructive evaluation: Optimization of the integrated concentric DC‐SQUID gradiometer

Abstract

SUMMARYWe have developed concentric multiloop DC-SQUID gradiometers for nondestructive evaluation, which were integrated on a 3 u 3 mm 2 Si chip using Nb thin-film fabrication technology. In order to obtain high spatial resolution and to cancel magnetic noise, a coplanar concentric second-order derivative coil was adopted for the pick-up coil. The gradiometer has obtained a perfectly symmetrical structure by directly using the pick-coil coil as the SQUID loop. Moreover, the self-inductance of the SQUID loop was reduced by forming the SQUID loop from the parallelconnected multiloop coil. We confirmed by experiment using a current line that the output of the gradiometer was independent of the scanning direction and that the gradiometer was effective in quantitative evaluations. In addition, the magnetic field sensitivity was raised by the multiloop coil, designed optimally. © 2001 Scripta Technica, Electron Comm Jpn Pt 2, 84(5): 3745, 2001