Coupling phenomena in superconducting Nb/Fe multilayers

Abstract

We measured the critical temperature ${T}_{c}$ and critical field ${H}_{c2}$ of molecular-beam epitaxy-grown Nb/Fe multilayers in which the layer thicknesses ${t}_{\mathrm{Fe}}$ and ${t}_{\mathrm{Nb}}$ are systematically varied. For constant ${t}_{\mathrm{Nb}}$ we observe a continuous decrease of ${T}_{c}$ with increasing ${t}_{\mathrm{Fe}}$ up to ${t}_{\mathrm{Fe}}=15\AA{}.$ At higher ${t}_{\mathrm{Fe}}$ values a sudden drop of ${T}_{c}$ is measured related to the onset of ferromagnetism in the Fe layers. Measurements of the shape and the angular dependence of ${H}_{c2}(T)$ clearly show that the superconducting Nb layers are already decoupled for ${t}_{\mathrm{Fe}}=12\AA{},$ magnetization measurements, however show that the Fe layers are ferromagnetic only for ${t}_{\mathrm{Fe}}>~16\AA{}.$ For constant ${t}_{\mathrm{Fe}}=25\AA{},$ the observed strong decrease of ${T}_{c}$ with decreasing Nb thickness as well as the temperature dependence of ${H}_{c2}$ are well described by the theoretical model for decoupled superconductor/ferromagnet multilayers.