Proximity effect in Nb/Cu/CoFe trilayers

We have fabricated Nb/Cu/CoFe trilayers, varying the thickness of the Cu or CoFe layer in order to control the coupling between the superconductivity and the ferromagnetism. The T-c behavior of the Nb/Cu bilayer exhibits the well-known proximity effect of an SN bilayer. In a Nb/Cu/CoFe trilayer with the thicknesses of the Nb and CoFe layers fixed, the T-c values increase on two different scales of length as the thickness of the Cu layer increases: one with a length scale of about 2 nm when the Cu layer is less than 5 nm in thickness and the other with a length scale of about 68 nm, the normal coherence length of Cu, when the Cu layer is larger than 5 nm in thickness. Furthermore, in a similar manner to the case of Nb/CoFe, the T-c behavior of a Nb/Cu/CoFe trilayer as a function of CoFe thickness shows a small dip structure. By employing Usadel formalism, we deduced all the relevant parameters that can explain our data in a consistent manner. In spite of the better interfaces in the Nb/Cu/CoFe system, no T-c oscillation as a function of the Cu thickness was observed, in contrast to the Nb/Au/CoFe system. We will discuss the difference and similarity between the Nb/Cu/CoFe and Nb/Au/CoFe systems.