Domain-wall enhancement of superconductivity in superconductor/ferromagnet hybrids: Case of weak ferromagnets

We use magnetotransport measurements to study the influence of the domain structure on the superconducting transition temperature T(c) in ferromagnetic (F)/superconducting (S) bilayers and F/S/F trilayers based on Nb and the weak ferromagnet Cu(43)Ni(57). By comparing the anisotropic magnetoresistance above T(c) with the resistance behavior in the transition and with critical current measurements below T(c), we show that in bilayers enhanced superconductivity is found when the F layer is in a domain state. We also find that, below T(c), the magnetic fields where domains occur are significantly higher than above or around T(c), suggesting that the magnetization rotation in the F layers is influenced by the adjacent superconductor. In trilayers the effects are similar. A domain-state dominated mechanism even for reported spin-valve effects therefore cannot be ruled out.