Improved high-<i>T</i>_csuperconducting quantum interference filters for sensitive magnetometry

Abstract

Superconducting quantum interference filters (SQIFs) are arrays of superconducting loops, with Josephson junctions, of diverse loop sizes. The dependence of the SQIF voltage on external magnetic fields is non-periodic and shows only one unique peak at zero field. We present several kinds of SQIFs—serial arrays, parallel arrays and various combinations of both—which are all realized with high-Tc superconductors. Compared to a single SQUID all SQIF types show improved magnetic field sensitivity and noise-limited field resolution. In order to realize really sensitive magnetometers the SQIFs were coupled to pickup coils. Consequently, the sensitivity and noise of the SQIF-based magnetometers are comparable with that of their SQUID-based counterparts. For a flip-chip configuration using a serial–parallel SQIF and a superconducting flux transformer, a noise-limited field resolution of 70 fT Hz−1/2 was achieved. This is only twice the value of that using a normal SQUID. Further reduction should be possible by a better adaptation of the complete design to the SQIF options.