Low Temperature Annealing and the Origins of Superconductivity at Nickel Bismuth Interfaces

It has been suggest that Nui/Bi interfaces may host unconventional superconductivity in thin-film bilayers. We have studied the structural, magnetic, and superconducting properties of sputter deposited Bi/Ni bilayers. In the as-grown state, our films do not display a superconducting transition; however, when stored at room temperature, after about 14 days they develop a superconducting transition up to 3.8 K. To systematically study the effect of low temperature annealing on our bilayers, we performed structural characterization with x-ray diffraction and polarized neutron reflectometry, along with magnetometry and low-temperature electrical transport measurements on samples annealed at 70°C . We find that the onset of superconductivity in our samples is coincident with the formation of ordered NiBi₃ intermetallic alloy, a known s-wave superconductor. We calculate that the annealing process has an remarkably low activation energy of ( 0.86 ± 0.06 ) eV . As a consequence, gentle heating of the bilayers will cause formation of the superconducting NiBi 3 at the Ni/Bi interface, which poses a challenge to studying any distinct properties of Bi/Ni bilayers without degrading that interface.