Cryogen-free variable temperature scanning SQUID microscope
Abstract: Scanning Superconducting QUantum Interference Device (SQUID) microscopy is a powerful tool for imaging local magnetic properties, but it requires a low-vibration cryogenic environment, traditionally achieved by thermal contact with a bath of liquid helium or the mixing chamber of a “wet” dilution refrigerator. We mount a SQUID microscope on the 3 K plate of a Bluefors pulse tube cryocooler and characterize its vibrational spectrum by measuring SQUID noise in a region of sharp flux gradient. By implementing passive vibration isolation, we reduce relative sensor-sample vibrations to 20 nm in-plane and 15 nm out-of-plane. A variable-temperature sample stage that is thermally isolated from the SQUID sensor enables measurement at sample temperatures from 2.8 K to 110 K. We demonstrate these advances by imaging inhomogeneous susceptibility and vortex pinning in optimally doped YBCO above 90 K. Together with sub-micron spatial resolution and 350×350 μm2 scan range, these advances position us for further studies of magnetic and superconducting materials and devices over a temperature range not previously accessible to scanning SQUID microscopy.