Characterization of Nanoscale Superconducting Quantum Interference Devices in a Top-Loading Cryogen-Free 4 K Refrigerator

The imaging of magnetic structure at the nanoscale is a prominent need across many scientific disciplines. Superconducting quantum interference devices (SQUIDs) offer a minimally invasive and highly direct measurement of magnetic properties as compared with other methods of detection; however, conventional, lithographically-produced SQUIDs are not well suited for imaging magnetic structure at the nanoscale. The SQUID on tip (SOT) is a type of nanoSQUID that addresses this issue. The small size of SOTs and their close proximity to the sample yields high local magnetic sensitivity with nanoscale spatial resolution. Using custom-designed inserts for a top-loading cryogen-free 4 K refrigerator, this project characterizes SOTs for use in such a scanning SQUID microscope platform. The insert design features a two-stage configuration comprising an SOT and a SQUID series array amplifier with auxiliary instrumentation such as thermometry. Moreover, the insert uses a regulated amount of helium as a thermal exchange gas. Additionally, the insert incorporates a 0.5 Tesla magnet, with careful consideration given to the associated heat load. The project also examines strategies to minimize vibrational and electrical interference arising from the refrigerator pulse-tube cryocooler.