  Focused ion beam (FIB) technology has been used to fabricate miniature Nb DC
SQUIDs which incorporate resistively-shunted microbridge junctions and a
central loop with a hole diameter ranging from 1058 nm to 50 nm. The smallest
device, with a 50 nm hole diameter, has a white flux noise level of 2.6
microphy_{0}/Hz^{0.5} at 10^{4} Hz. The scaling of the flux noise properties
and focusing effect of the SQUID with the hole size were examined. The observed
low-frequency flux noise of different devices were compared with the
contribution due to the spin fluctuation of defects during FIB processing and
the thermally activated flux hopping in the SQUID washer.
