Inhomogeneous superconductivity induced in a weak ferromagnet

Under certain conditions, the order parameter induced by a superconductor (S) in a ferromagnet (F) can be inhomogeneous and oscillating, which results e.g., in the so-called pi-coupling in S/F/S junctions, In principle, the inhomogeneous state can be induced at T-c as function of the F-layer thickness d(F) in S/F bilayers and multilayers, which should result in a dip-like characteristic of T-c(dF). We show the results of measurements on the S/F system Nb/Cu1-xNix, for Ni-concentrations in the range x = 0.5-0.7. where such effects might be expected. We find that the critical thickness for the occurrence of superconductivity is still relatively high, even for these weak ferromagnets. The resulting dip then is intrinsically shallow and difficult to observe, which explains the lack of a clear signature in the T-c(d(F)) data. (C) 2001 Published by Elsevier Science B.V.