MAGNETISM AND SUPERCONDUCTIVITY OF FE NB/FE TRILAYERS/

Sputtered Fe/Nb/Fe trilayers were prepared with Nh thicknesses d(Nb) i n the range 300-800 Angstrom, and with Fe thicknesses d(Fe) in the ran ge 0-30 Angstrom. X-ray reflectivity measurements revealed the high qu ality of the film structure with low surface and interface roughnesses . Magnetic properties of the samples were studied using the magneto-op tical Kerr effect, ferromagnetic-resonance and magnetization measureme nts 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) approxi mate to 320 Angstrom independent of d(Fe) for d(Fe) greater than or eq ual to 7 Angstrom. The temperature dependence of the upper critical fi eld H-c2(T) showed two- and three-dimensional features for parallel an d perpendicular orientations of the magnetic field, respectively. At t he 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 wi thin the Fe layers. This maximum is attributed to a modification of th e repulsive interaction between the electrons in the magnetically ''de ad'' Fe-rich interlayer when an exchange field in the Fe layer is pres ent.