Superconductorferromagnetsuperconductor

Superconductor–ferromagnet–superconductor (SFS) junctions are a class of Josephson devices in which a ferromagnetic metal layer is sandwiched between two superconducting electrodes. They rely on the superconducting proximity effect, where superconducting correlations penetrate the ferromagnet, decaying and simultaneously oscillating due to the exchange field in the ferromagnet. This leads to a Josephson coupling that can adopt either a 0-phase or a π-phase, depending on factors such as the ferromagnet thickness, material properties, and temperature. Consequently, the critical current can oscillate with the ferromagnet thickness and even change sign, signaling a 0–π transition.

Materials commonly used in SFS junctions include superconductors such as niobium (Nb) and ferromagnetic metals or alloys such as CuNi and PdNi, as well as pure Ni or Fe. Fabrication emphasizes high interface transparency and controlled magnetic properties. The decay length of superconducting correlations in a strong ferromagnet is typically a few nanometers, whereas weaker or diluted ferromagnets can extend this length. Temperature, magnetic domain structure, and interfacial scattering influence the observed 0–π behavior and the magnitude of the critical current.

In addition to conventional singlet proximity effects, SFS junctions with noncollinear magnetizations in multilayer structures can generate long-range spin-triplet superconducting correlations, potentially extending the proximity effect in ferromagnets and enabling new device concepts.

Applications include tunable π-junctions for superconducting qubits, rapid single flux quantum circuits, and superconducting spintronic devices. Experimental studies focus on Ic versus temperature and ferromagnet thickness to identify 0–π transitions and to extract characteristic coherence lengths and interface parameters.