Highly sensitive magnetometers based on $YBa2Cu3O7$ Josephson junction arrays

The dependence of the critical current of a Josephson junction on the magnetic flux in the junction area can be used for the sensitive detection of external magnetic fields. In contrast to superconducting quantum interference devices, also the measurement of absolute magnetic fields is possible. To increase the transfer function $∂V/∂B,$ we use serial arrays of up to 105 Josephson junctions between flux-concentrating areas on 24° $SrTiO3$ bicrystal substrates. We investigate the scaling properties of the critical current $Ic,$ the normal state resistance $Rn$ and the flux density noise $SB$ in dependence on the number of Josephson junctions in the serial array. By the use of a magnetic field modulation scheme, the $1/f$ noise from critical current fluctuations in the junctions can be suppressed. At 77 K, we achieve a white noise level of $SB=1.2$ $pT/Hz$ for a 105-junction array.