Coexistence of superconductivity and ferromagnetism in cluster-assembled Sn-Co nanocomposites

The coexistence of superconductivity and ferromagnetism is investigated in granular Sn-Co nanocomposites. The nanocomposites have been prepared by co-deposition of Sn atoms and Co clusters, the morphology and composition of which can be tuned by varying the deposition rate of Co clusters relative to Sn atoms. Flat isolated Sn islands are obtained at zero or low Co cluster flux, while granular nanocomposites are formed with increasing Co cluster flux, reaching Co concentrations up to 44 vol.%. Interfaces with a low electronic transparency between superconductor and ferromagnet are obtained by a combination of the granular nature of the nanocomposites and the formation of Sn-Co alloys at the Sn/Co interfaces. The structure and composition of the nanocomposites have been thoroughly characterized by atomic force microscopy, X-ray diffraction and conversion electron Mössbauer spectroscopy. Over the entire Co concentration range, the hybrids show a ferromagnetic response. The superconducting phase boundary and the Meissner response depend on the morphology and composition of the nanocomposites. In particular, the superconducting critical temperature decreases with increasing Co concentration, while the Meissner response varies from a reversible to a strongly hysteretic behaviour depending on the morphology of the samples with different Co content. The persistence of superconductivity at high Co concentrations is attributed to a suppression of the superconducting proximity effect in these nanocomposites, which is ascribed to the low interface transparency between the Sn and Co components that make up these hybrid systems.