Anomalous Supercurrent Transport Behavior in Ni-barrier Josephson Junctions

Critical current oscillation as a function of ferromagnetic barrier thickness in superconductor-ferromagnet-superconductor Josephson junctions (JJs) is a prominent manifestation of superconducting spin modulation due to magnetic order. We have developed fabrication and measurement methods that are both efficient and rigorous for magnetic JJs in order to investigate such superconducting spin modulation in detail. From Ni-barrier JJs, we obtain oscillatory characteristics at a deep subnanometer resolution without uncertainties that can arise from nonuniform magnetization in the ferromagnetic barriers. Although the oscillatory trend resembles one from an ideal theoretical model, we find anomalous features such as a finite oscillation phase shift with a nonmagnetic origin and nonsinusoidal current-phase relationship near the 0-π phase transition. These measurements demonstrate supercurrent transport physics beyond the common picture based on the simple Fulde-Ferrell-Larkin-Ovchinikov state modulation.