Coupling phenomena in superconducting Nb/Fe multilayers

Abstract

We measured the critical temperature $T_c$ and critical field $H_{c2\mathrm{}}$ 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 $t_{\mathrm{Fe}}=15\mathrm{}\AA .$ 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 $H_{c2\mathrm{}}\mathrm{}\left(T\right)\mathrm{}$ clearly show that the superconducting Nb layers are already decoupled for $t_{\mathrm{Fe}}=12\mathrm{}\AA ,$ magnetization measurements, however show that the Fe layers are ferromagnetic only for $t_{\mathrm{Fe}}>~16\AA .$ For constant $t_{\mathrm{Fe}}=25\mathrm{}\AA ,$ the observed strong decrease of $T_c$ with decreasing Nb thickness as well as the temperature dependence of $H_{c2\mathrm{}}$ are well described by the theoretical model for decoupled superconductor/ferromagnet multilayers.