Josephson tunnel junctions with ferromagnetic Fe0.75Co0.25 barriers

Josephson tunnel junctions with the strong ferromagnetic alloy Fe0.75Co0.25 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 ξF2 ∼ 0.79 nm between 0 and π-Josephson phase coupling and a very short decay length ξF1 ∼ 0.22 nm 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 dF.