Unconventional magnetic flux response of Josephson junctions with a weak link made from alternating normal and ferromagnetic interlayers*

We present the first systematic study of critical current-vs-external magnetic flux dependences in Josephson junctions with a weak link formed by alternating normal (N=Al) and ferromagnetic (F=Ni or Py) nominally identical nm-thick films coupled to superconducting (S=Nb) electrodes. In contrast to the well-known Fraunhofer-type behavior for conventional Josephson trilayers, the S/(NF)_n/S hybrids with n > 5 (with or without an ultra-thin Al-oxide barrier) have revealed anomalous patterns. In the most samples, with increasing magnetic flux, the supercurrent underwent periodic, constant-amplitude modulation resembling that in SQUIDs, see preliminary data [1]. The oscillatory pattern was superposed on a background supercurrent having much larger oscillation period in an externally applied magnetic field. We explain the origin of the non-Fraunhofer critical current curves as being associated with a special type of diffusive charge trajectories confined to the edges of the periodic (NF)_n weak link. The Josephson devices in the surface-dominated transport regime can be exploited for nanoscale low-field detection.
[1] I.P. Nevirkovets and O.A. Mukhanov. Supercond. Sci. Technol., 31, 03LT01 (2018).