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 ${\text{Cu}}_{43}{\text{Ni}}_{57}$. 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.