Negative magnetoresistance in sputtered niobium thin films grown on silicon substrates

Nb-films (thicknesses of 20 nm, 50 nm and 100 nm), directly grown on orientated Si substrates by DC sputtering, have shown reduced superconducting critical temperatures TC as compared to the bulk value; an effect associated with their disordered granular character (the disorder phenomenon of the Nb-films depresses the density of states at the Fermi level, consequently shifts the TC value towards low temperatures). The disorder effect was also correlated to the semiconducting-like behavior observed in the R(T) measurements. At the normal state, while the 100nm Nb-film is dominated by a metallic-like behavior, the thinner Nb-films (20 nm and 50 nm thick) show significant negative magnetoresistances in a small temperature range about TC. This behavior was also explained assuming their granular characters, where the applied magnetic field first destroys the global superconducting character of the Nb-films, leaving Cooper pairs localized inside Nb-grains. A further increase of the applied field strength affects the superconductivity inside Nb-grains, enhancing the normal intergranular electric transport channel, which decreases the whole resistance of the Nb-films. The present study suggests that the microscopic disorder, at the grain surfaces/interfaces, seems to be an essential point to comprehend the negative magnetoresistance effect observed in some superconducting granular systems.