When a ferromagnet is placed in contact with a superconductor, owing to incompatible spin order, the Cooper pairs from the superconductor cannot survive more than one or two nanometers inside the ferromagnet. This is confirmed in the measurements of ferromagnetic nickel (Ni) nanowires contacted by superconducting niobium (Nb) leads. However, when a thin copper (Cu) buffer layer (3 nm, oxidized due to exposure to air) is inserted between the Nb electrodes and the Ni wire, the spatial extent of the superconducting proximity range is dramatically increased from 2 to a few tens of nanometers. Scanning transmission electron microscope images verify the existence of Cu oxides and the magnetization measurements of such a 3 nm oxidized Cu film on a SiO2/Si substrate and on Nb/SiO2/Si show evidence of ferromagnetism. One way to understand the long-range proximity effect in the Ni nanowire is that the oxidized Cu buffer layer with ferromagnetism facilitates the conversion of singlet superconductivity in Nb into triplet supercurrent along the Ni nanowires.
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