Nb metallurgical transformations occurring in the microbridge area of an rf SQUID during its critical current adjustment

We describe the metallurgical transformations of Nb thin-film Dayem microbridges related to the adjustment of their critical current at operating temperature (4.2 K). The critical current is reduced to a few tens of microamperes when the microbridge is submitted to proper current pulses. Subsequent Transmission Electron Microscopy (TEM) observations show an increase of the mean Nb grain size in the microbridge area. Nb contamination by the silicon substrate is also observed, Both critical temperature and critical current decreases are attributed to these pnenomena.