Closed-cycle refrigerator-cooled scanning SQUID microscope for room-temperature samples

We have designed, built, and operated a scanning superconducting quantum in terference device (SQUID) microscope that uses a closed-cycle refrigerator to cool a YBa2Cu3O7 (YBCO) dc SQUID to 77 K. The SQUID is mounted in custom vacuum housing that has a thin sapphire window that maintains thermal isol ation of the SQUID while allowing samples to be imaged in air at room tempe rature. Samples are mounted on an x-y scanning table and can be brought to within about 60 mum of the SQUID for magnetic field imaging. The SQUID has an effective pick-up area of 1.2x10(-9) m(2) and a level of flux noise of 1 0.5 mu Phi (0)/Hz(1/2) in the white noise region (above 500 Hz). We describ e the performance of the system and present images of a variety of samples. (C) 2001 American Institute of Physics.