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

We have designed, built, and operated a scanning superconducting quantum interference 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 isolation of the SQUID while allowing samples to be imaged in air at room temperature. Samples are mounted on an x–y scanning table and can be brought to within about 60 μm of the SQUID for magnetic field imaging. The SQUID has an effective pick-up area of 1.2×10−9 m2 and a level of flux noise of 10.5 μΦ0/Hz1/2 in the white noise region (above 500 Hz). We describe the performance of the system and present images of a variety of samples.