Quantitative analysis of the proximity effect in Nb/Co60Fe40, Nb/Ni, and Nb/Cu40Ni60 bilayers -: art. no. 014518

We have studied the behavior of the superconducting critical temperature T-c in Nb/Co60Fe40, Nb/Ni, and Nb/Cu40Ni60 bilayers as a function of the thickness of each ferromagnetic metal layer. The T-c's of three sets of bilayers exhibit nonmonotonic behavior as a function of each ferromagnetic metal thickness. Employing the quantitative analysis based on Usadel formalism of the effect of the exchange energy, we observed that the T-c behavior of Nb/Co60Fe40 bilayers is in good agreement with the theoretical values over the entire range of the data. On the other hand, the T-c's of Nb/Ni and Nb/Cu40Ni60 bilayers show a higher value in the small thickness regime than the theoretical prediction obtained from the calculation, which matches the dip position and the saturation value of T-c in the large thickness limit. This discrepancy is probably due to the weakened magnetic properties of Ni and Cu40Ni60 when they are thin. We discuss the values of our fitting parameters and their implication for the validity of the current Usadel formalism of the effect of the exchange energy.