Low Temperature Annealing and the Generation of Superconductivity in Nickel-Bismuth Bilayers

Unconventional superconductivity has been suggested to be present at Ni/Bi interfaces in thin-film bilayers. We report a study of the structural, magnetic and superconducting properties of sputtered Bi/Ni bilayers. As-grown, our films do not display a superconducting transition. However, when stored at room temperature, after about 14 days our bilayers do develop a superconducting transition up to 3.8 K.
We perform structural characterization with X-ray diffraction and spin polarized neutron reflectometry, and measure magnetisation and low temperature electron transport for samples annealed at 70^○ C. We show that the onset of superconductivity in our samples is coincident with the formation of ordered NiBi₃ intermetallic alloy, known to be an s-wave superconductor. We find that the annealing process has a rather low activation energy of (0:86±0:06) eV. Gentle heating as typical for cleanroom processing will result in NiBi₃ formation which poses a challenge for studying distinct Bi/Ni interface behaviour.