Sputtered Fe/Nb/Fe trilayers were prepared with Nb thicknesses dNb in the range 300--800 {} and with Fe thicknesses dFe in the range 0--30 {}. 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 dFe a decrease of Tc with decreasing dNb was observed up to a critical thickness dNbᶜʳⁱᵗ below which superconductivity vanishes. We derived dNbᶜʳⁱᵗ{≈}320 {} independent of dFe for dFe{}7 {}. The temperature dependence of the upper critical field Hc2(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 Tc with increasing dFe and a definite maximum in Tc(dFe) 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.
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Mühge et al. (1997) studied this question.
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