Quantitative analysis of nonmonotonic T$_{\mbox{c}} $ behavior in Nb/Co$_{60}$Fe$_{40}$, Nb/Ni, and Nb/Cu$_{40}$Ni$_{60}$ bilayers

We have studied the behavior of superconducting critical temperature T$_{\mbox{c}} $ in Nb/Co$_{60}$Fe$_{40}$, Nb/Ni, and Nb/Cu$_{40}$Ni$_{60}$ bilayers as a function of each ferromagnetic metal thickness d$_{F}$. The T$_{\mbox{c}} $'s of each bilayer show nonmonotonic behavior as a function of d$_{F}$ with a shallow dip feature. From the quantitative analysis based on Usadel formalism [1], we observed that the T$_{\mbox{c}} $ behavior of Nb/Co$_{60}$Fe$_{40}$ bilayers is in good agreement with the theoretical prediction with interface parameter $\gamma _{b }$= R$_{b}$A/$\rho _{f}\xi _{f}$ = 0.34 in the whole range of d$_{F}$. On the other hand, T$_{\mbox{c}} $ values of Nb/Ni and Nb/Cu$_{40}$Ni$_{60}$ bilayers are higher in the small d$_{F}$ regime than the theoretical calculation with $\gamma _{b }$= 0.7 for Nb/Ni bilayer and $\gamma _{b }$= 0.57 for Nb/Cu$_{40}$Ni$_{60}$ bilayer, respectively, although the theoretical calculations match the dip position and the saturation value of T$_{\mbox{c}} $ in the large d$_{F}$ regime. We propose that this discrepancy is due to the weakened magnetism resulting from the structural disorder in Nb/Ni bilayer and a change in relative composition between Ni and Cu in Cu$_{40}$Ni$_{60}$ ferromagnetic layer at its initial nucleation on Nb film. [1] H. Doh and H. Y. Choi, cond-mat/0407149 (2004).