Observation of 0–π transition in SIsFS Josephson junctions

The 0–π transition in Superconductor-Insulator-superconductor-Ferromagnet-Superconductor (SIsFS) Josephson junctions (JJs) was investigated experimentally. As predicted by theory, an s-layer inserted into a ferromagnetic SIFS junction can enhance the critical current density up to the value of an SIS tunnel junction. We fabricated Nb′ | AlO_x | Nb | Ni₆₀Cu₄₀ | Nb JJs with wedge-like s (Nb) and F (Ni₆₀Cu₄₀) layers and studied the Josephson effect as a function of the s- and F-layer thickness, d_s and d_F, respectively. For d_s = 11 nm, π-JJs with SIFS-type j_c(d_F) and critical current densities up to j_c^π=60 A/cm² were obtained at 4.2 K. Thicker d_s led to a drastic increase of the critical current decay length, accompanied by the unexpected disappearance of the 0–π transition dip in the j_c(d_F) dependence. Our results are relevant for superconducting memories, rapid single flux quantum logic circuits, and solid state qubits