Proximity effect in MBE-grown superconducting/spin-glass multilayers

We have grown epitaxial superconductor/spin-glass multilayers, Nb/CuMn, as well as complimentary nonmagnetic Nb/Cu multilayers by molecular beam epitaxy. To probe the interaction of superconductivity and magnetism, we measured the resistivity and ac susceptibility as a function of temperature for multilayers of nominally constant Nb thickness and varying normal-metal thickness. The reduction of the transition temperature of the Nb/Cu multilayers with increasing Cu thickness is in excellent agreement with the de Gennes-Werthamer proximity effect theory. The inclusion of Mn in the Cu causes a significant additional suppression of the transition temperatures relative to the Nb/Cu multilayers. The extension of the de Gennes-Werthamer theory to include the effects of random magnetic impurities agrees well with the data from the Nb/CuMn multilayers for small CuMn layer thicknesses. However, deviations occur at the largest CuMn thicknesses studied. These deviations between the data and theory may be due to a decoupling of the Nb layers, as a result of the spin-glass ordering, causing a three-dimensional to two-dimensional crossover.