Superconducting flux-flow type ultra-low-noise magnetic sensors - An alternative to dc-SQUIDs

A superconducting magnetometers based on the magnetic field dependence of the Eck step voltage in long Josephson tunnel junctions (LJTJs) is demonstrated. The field to be measured is applied perpendicular to a continuous superconducting pickup loop. Wherever the loop has a narrow constriction, the density of the flux-restoring circulating currents will become relatively high and will locally create a magnetic field large enough to bring a biased LJTJ in the flux-flow state, i.e., at a finite voltage proportional to the field strength. This method allows the realization of a novel family of robust and general-purpose superconducting devices which, despite their simplicity, function as ultra-low-noise, wide-band and high-dynamics magnetometers. The performances of low-T$_c$ sensor prototypes, among which a highly linear voltage responsivity and a magnetic spectral density $S_B^{1/2}< 3\,fT/Hz^{1/2}$, promise to be competitive with those of the best superconducting quantum interference devices.