Magnetism and superconductivity of Fe/Nb/Fe trilayers

Sputtered Fe/Nb/Fe trilayers were prepared with Nb thicknesses ${\mathrm{d}}_{\mathrm{Nb}}$ in the range 300--800 \AA{} and with Fe thicknesses ${\mathrm{d}}_{\mathrm{Fe}}$ in the range 0--30 \AA{}. 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 ${\mathrm{d}}_{\mathrm{Fe}}$ a decrease of ${\mathrm{T}}_{\mathrm{c}}$ with decreasing ${\mathrm{d}}_{\mathrm{Nb}}$ was observed up to a critical thickness ${\mathrm{d}}_{\mathrm{Nb}}^{\mathrm{crit}}$ below which superconductivity vanishes. We derived ${\mathrm{d}}_{\mathrm{Nb}}^{\mathrm{crit}}$\ensuremath{\approx}320 \AA{} independent of ${\mathrm{d}}_{\mathrm{Fe}}$ for ${\mathrm{d}}_{\mathrm{Fe}}$\ensuremath{\geqslant}7 \AA{}. The temperature dependence of the upper critical field ${\mathrm{H}}_{\mathrm{c}2}$(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 ${\mathrm{T}}_{\mathrm{c}}$ with increasing ${\mathrm{d}}_{\mathrm{Fe}}$ and a definite maximum in ${\mathrm{T}}_{\mathrm{c}}$(${\mathrm{d}}_{\mathrm{Fe}}$) 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.