Proximity Effect in Nb/Cu/CoFe Trilayers

We have fabricated the Nb/Cu , Nb/CoFe bilayer and Nb/Cu/CoFe trilayer samples by varying the CoFe or Cu layer thickness using DC magnetron sputtering system and measured their superconducting transition temperature Tc. In Nb/Cu(d$_{Cu})$ and Nb/CoFe(d$_{CoFe})$ bilayers , we observed Tc behavior consistent with conventional SN and SF theory. In Nb/Cu/CoFe trilayer , as we increase d$_{Cu}$ with fixed values of d$_{Nb}$ and d$_{CoFe }$, $_{ }$Tc of Nb/Cu(d$_{Cu})$/CoFe trilayer increased rapidly for d$_{Cu} \quad <$ 5 nm and slowly saturated to a limiting value. We analyzed these data using the method based on Usadel formalism and obtained $\gamma _b ^{N}$=R$_{b}$A/$\rho _{n}\xi _{n}$[Nb/Cu]=0.41 , $\gamma _b ^{F}$= R$_{b}$A/$\rho _{f}\xi _{f}$ [Nb/CoFe] = 0.33, and $\gamma _b ^{F}$[Cu/CoFe]=0.35. In our Nb/Cu/CoFe trilayers, as we increased d$_{CoFe}$ with fixed values of d$_{Nb}$ and d$_{Cu }$, we observed the same dip structure as Nb/CoFe(d$_{CoFe})$. We will explain our data with interface resistance and its implications.