Long range proximity effect in Ni nanowires*

We report measurements on the proximity effect of superconducting Nb electrodes on Ni nanowires (500nm to 1μm(length)× 400nm(width)× 40nm(thickness)). The fabrication of the devices was assisted by e-beam lithography. The spatial extent of the proximity effect is found to be on the order of 2 nm. Such a short range is expected due to the competing spin order of ferromagnetism and superconductivity. Interestingly, when a thin (3nm) Cu buffer layer is sandwiched between Nb and Ni, the spatial extent is dramatically increased to between 35 to 130 nm. Subsequent SQUID measurements of a sandwich film of Nb(40nm)/Cu(3nm) on SiO₂ substrate show evidence of weak ferromagnetism at 50K. The thermally evaporated Cu film on SiO₂, as in same the case for the fabrication of the nanowire devices, was exposed to air before the Nb film was sputtered. One way to understand the proximity effect result is that the buffer CuO layer with weak ferromagnetism facilities the conversion of s-wave superconductivity in Nb into triplet supercurrent along the Ni nanowires¹.