Critical current oscillations of elliptical Josephson junctions withsingle-domain ferromagnetic layers

Josephson junctions containing ferromagnetic layers are of considerable interest for thedevelopment of practical cryogenic memory and superconducting qubits. Such junctionsexhibit a ground-state phase shift of π for certain rangesof ferromagnetic layer thicknesses. We present studies of Nb based micron-scaleelliptically shaped Josephson junctions containing ferromagnetic barriers ofNi81Fe19 or Ni65Fe15Co20. Byapplying an external magnetic field, the critical current of the junctions is found tofollow characteristic Fraunhofer patterns and display sharp switching behavior suggestiveof single-domain magnets. The high quality of the Fraunhofer patterns enables us toextract the maximum value of the critical current even when the peak is shiftedsignificantly outside the range of the data due to the magnetic moment of theferromagnetic layer. The maximum value of the critical current oscillates as a function ofthe ferromagnetic barrier thickness, indicating transitions in the phase difference acrossthe junction between values of zero and π. We compare thedata to previous work and to models of the 0-π transitionsbased on existing theories.