Proximity effect in Nb/Cu/CoFe trilayers

We have investigated the superconducting critical temperatures of $\\mathrm{Nb}∕\\mathrm{Au}∕\\mathrm{Co}\\mathrm{Fe}$ trilayers as a function of Au and CoFe thicknesses. Without the CoFe layer the superconducting critical temperatures of $\\mathrm{Nb}∕\\mathrm{Au}$ bilayers as a function of Au thickness follow the well-known proximity effect between a superconductor and a normal metal. The superconducting critical temperatures of $\\mathrm{Nb}∕\\mathrm{Au}∕\\mathrm{Co}\\mathrm{Fe}$ trilayers as a function of Au thickness exhibit a rapid initial increase in the small Au thickness region and increase slowly to a limiting value above this region, accompanied by a small oscillation of ${T}_{c}$. On the other hand, the superconducting critical temperatures of $\\mathrm{Nb}∕\\mathrm{Au}∕\\mathrm{Co}\\mathrm{Fe}$ trilayers as a function of CoFe thickness show nonmonotonic behavior with a shallow dip feature. We analyzed the ${T}_{c}$ behavior in terms of Usadel formalism and found that most features are consistent with the theory, although the small oscillation of ${T}_{c}$ as a function of the Au thickness cannot be accounted for. We have also found quantitative values for the two interfaces: $\\mathrm{Nb}∕\\mathrm{Au}$ and $\\mathrm{Au}∕\\mathrm{Co}\\mathrm{Fe}$.