Oscillatory and negative magnetoresistance in granular nanowires of superconducting NiBi3

Local fluctuations in the superconducting order parameters in granular systems show interesting magnetotransport behavior, particularly relevant to superconducting nanowires. Kivelson and Spivak [Phys. Rev. B 45, 10490 (1992)] have shown that fluctuations in density and signs of supercurrent can lead to oscillating and negative magnetoresistance (MR). Here, we present magnetoresistance measurements of 100 nm wide NiBi3 superconducting nanowires, patterned by focused ion beam lithography, from granular NiBi3 films containing different concentrations of magnetic Ni impurities. The nanowire containing a higher concentration of Ni impurity showed oscillations in MR and simultaneously exhibited negative MR in certain temperature and field ranges. None of these effects were observed in the nanowire with nearly no Ni impurities. Unlike earlier experiments, we argue that excess magnetic Ni impurities in the nanowire realize a case of simultaneous magnitude and sign fluctuations in local order parameter, leading to MR oscillations and MR sign reversal in the same system, as discussed by Kivelson and Spivak. This result is important in the context of transport in superconducting quantum circuits.