Ferromagnetic-superconducting thin-film layered device: critical current behaviour

We measured, at liquid-helium temperature and in st weak magnetic field, the electrical properties of a thin-film superconducting and ferromagnetic structure. The structure is composed of a narrow (5 mu m wide, 100 mu m long and 100 nm thick) Nb strip, crossed by a Permalloy (Ni80Fe20) (30 mu m wide, 1 mm long and 140 nm thick) bridge, produced by sputtering and photolithography. A hysteretic dependence of the critical current of the Nb strip versus the applied magnetic field was observed. The current hysteresis loop showed a close resemblance to the magnetization hysteresis loop of the ferromagnetic Permalloy bridge in the same applied magnetic field, as measured at room temperature with magneto-optical ellipsometric techniques. The phenomenon is qualitatively discussed in terms of a magnetostatic interaction.