HTS scanning SQUID microscope cooled by a closed-cycle refrigerator

We have developed a scanning SQUID microscope which uses a commercially available closed-cycle refrigerator to cool a YBa/sub 2/Cu/sub 3/O/sub 7/ bi-crystal dc SQUID to about 77 K. The system allows magnetic imaging of samples which are at room temperature and pressure with spatial resolutions of 50 /spl mu/m or better. It is more compact and requires less maintenance than a more conventional liquid-nitrogen cooled system, while delivering equal sensitivity. In order to reduce the SQUID-sample separation while maintaining vacuum thermal isolation of the SQUID, the sensor is separated from the sample by a 25 /spl mu/m thick, optically transparent window. The noise spectrum of our SQUID shows a 1/f spectrum below 500 Hz with 72 pT//spl radic/Hz field sensitivity at 10 Hz, and a white noise level of 20 pT//spl radic/ Hz.