  The interplay between fluctuating Cooper pairs and magnetic impurities in
conventional BCS low-Tc superconductors has been studied through measurements
of the magnetic field dependence of the fluctuation diamagnetism (FD) above Tc
in lanthanum with praseodymium impurities. These measurements provide a crucial
confirmation of our previous observation [Europhys. Lett. 73, 587 (2006)] that
in the dilute impurity regime the FD increases almost linearly with the
concentration of magnetic impurities. This striking effect is attributed to a
variation due to the presence of the fluctuating Cooper pairs of the coupling
between magnetic impurities. To describe these results at a phenomenological
level, we propose a Gaussian Ginzburg-Landau model for the FD which includes an
indirect contribution proportional to both the impurities concentration and the
Cooper pairs density. Our approach is able to explain simultaneously the FD
increase due to magnetic impurities and its decrease with the application of
large magnetic fields.
