Magnetism and superconductivity of Fe/Nb/Fe trilayers

Sputtered Fe/Nb/Fe trilayers were prepared with Nh thicknesses d(Nb) in the range 300-800 Angstrom, and with Fe thicknesses d(Fe) in the range 0-30 Angstrom. X-ray reflectivity measurements revealed the high quality of the film structure with low surface and interface roughnesses. Magnetic properties of the samples were studied using the magneto-optical Kerr effect, ferromagnetic-resonance and magnetization measurements by a SQUID magnetometer. For constant d(Fe) a decrease of T-c with decreasing d(Nb) was observed up to a critical thickness d(Nb)(crit) below which superconductivity vanishes. We derived d(Nb)(crit) approximate to 320 Angstrom independent of d(Fe) for d(Fe) greater than or equal to 7 Angstrom. The temperature dependence of the upper critical field H-c2(T) showed two- and three-dimensional features for parallel and perpendicular orientations of the magnetic field, respectively. At the Nb/Fe interface in the trilayer systems an Fe-rich interlayer with nonmagnetic Fe is formed with a thickness depending sensitively on the preparation conditions. We observed a nonmonotonic dependence of the superconducting transition temperature T-c with increasing d(Fe) and a definite maximum in T-c(d(Fe)) at the onset of ferromagnetic order within the Fe layers. This maximum is attributed to a modification of the repulsive interaction between the electrons in the magnetically ''dead'' Fe-rich interlayer when an exchange field in the Fe layer is present.