Kondo effect in sputtered Nb films

We report a transport anomaly in sputtered niobium films after annealing in nitrogen or argon. It was found that the as-sputtered samples exhibit a single superconducting transition near 9K. However, after annealing in nitrogen and argon gases for an hour, the superconducting transition is suppressed down below 2 K. A resistance minimum appears around 20 K. This resistance minimum persists up to 8.5 T at which the superconductivity is completely suppressed. We attribute this anomaly to the Kondo effect and it is likely that iron impurities have been incorporated into the granular films during the annealing.