A micro SQUID using ultra-small tunnel junctions

Basic characteristics of micro-SQUIDs (superconducting quantum interference devices) with very small tunnel junctions are presented. Through electron-beam lithography, angled deposition and lift-off, we have made aluminum SQUIDs, the loop size of which was from 2?m to 20?m. The Al-AlOx-Al tunnel junctions were 0.15 ?m? 0.15 ?m in size and the parallel resistance was of the order of 10 k? Transport measurement at low temperatures down to 30 mK reveals the following features of the device: (1) It does not show dissipationless supercurrent, i.e. the zero-bias resistance remains finite even at the lowest temperature. (2) The resistance varies periodically as a function of the magnetic fields. When the field is perpendicular to the SQUID loop plane, the period corresponds to one flux quantum through the loop. (3) In almost-parallel magnetic fields, the resistance oscillations continue up to about 0.9T, though the amplitude diminishes with the magnetic field. Thus, the device is applicable for the magnetic sensing in moderate magnetic fields. Noteworthy feature is that the power dissipation necessary for operation is extremely small, i.e., of the order of 1 fW.