Effect of interface roughness on superconducting transition temperatures of Nb/Co multilayers

Superconductor (SC)/ferromagnet (FM) Nb/Co multilayers have been produced by magnetron-sputtering with a 100 nm thickness of Nb and 5, 10, and 20 nm of Co. The superconducting properties have been investigated by electric transport measurements. It was found that the thicker Co layers decrease the superconducting transition temperature (Tc) less than the thinner ones. In order to understand this unexpected behavior, the microstructure of the layers has been investigated by means of Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) and by energy-dispersive X-ray spectroscopy line scan analyses in scanning TEM (STEM) mode. It was found that the decisive parameter which determines the effect of magnetic layers on Tc of the superconducting layers is not only the roughness (R), but the ratio of the roughness to thickness (dCo) of the magnetic Co layer, δ=R/dCo. For δ>1 the magnetic stray field of the magnetic layers is the main reason for the Tc reduction.