Measurement of the upper critical magnetic field of superconductors with magnetic impurities

Abstract

We have measured the upper critical magnetic field, $H_{c2\mathrm{}}\mathrm{}\left(T\right)\mathrm{}$, for In-Mn and Pb-Mn films, in which the Mn is a magnetic impurity. The results are compared with the additive-pair-breaking theory of Fulde and Maki, which predicts that the pair-breaking effect of the magnetic impurities is temperature-independent and that the pair-breaking effects of the magnetic impurities and of the applied magnetic field are additive. Furthermore, the theory predicts the explicit temperature dependence of $H_{c2\mathrm{}}\mathrm{}\left(T\right)\mathrm{}$ The results indicate that the temperature dependence of $H_{c2\mathrm{}}$ is well described by the Fulde-Maki theory for In-Mn, but not for Pb-Mn, probably because of the strong-electron-phonon coupling in Pb. For both In-Mn and Pb-Mn, the results indicate that the pair-breaking effect of the magnetic impurities is temperature independent, and that the pair-breaking effects of the magnetic impurities and the applied magnetic field are additive, so the concepts inherent in the Fulde-Maki theory are supported. We find no evidence in the data of the Kondo effect or of impurity spin alignment.