Josephson tunnel junctions with ferromagnetic ${\text{Fe}}_{0.75}{\text{Co}}_{0.25}$ barriers

Josephson tunnel junctions with the strong ferromagnetic alloy ${\text{Fe}}_{0.75}{\text{Co}}_{0.25}$ as the barrier material were studied. The junctions were prepared with high quality down to a thickness range of a few monolayers of Fe-Co. An oscillation length of ${\xi }_{F2}\approx 0.79\mathrm{n}\mathrm{m}$ between 0 and $\pi$-Josephson phase coupling and a very short decay length ${\xi }_{F1}\approx 0.22\mathrm{n}\mathrm{m}$ for the amplitude of the superconducting pair wave function in the Fe-Co layer were determined. The rapid damping of the pair wave function inside the Fe-Co layer is caused by the strong ferromagnetic exchange field and additional magnetic pair breaking scattering. Josephson junctions with Fe-Co barriers show a significantly increased tendency towards magnetic remanence and flux trapping for larger thicknesses $d_F$.