  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 3 nm thick copper oxide
(CuO) buffer layer made by exposing an evaporated or a sputtered 3 nm Cu film
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 study
confirms the formation of a 3 nm thick CuO layer when an evaporated Cu film is
exposed to air. Magnetization measurements of such a 3 nm CuO film on a SiO2/Si
substrate and on Nb/SiO2/Si show clear evidence of ferromagnetism. One way to
understand the long-range proximity effect in the Ni nanowire is that the CuO
buffer layer with ferromagnetism facilitates the conversion of singlet
superconductivity in Nb into triplet supercurrent along the Ni nanowires.

Superconductivity;Mesoscale and Nanoscale Physics