A terraced scanning superconducting quantum interference device susceptometer with submicron pickup loops

Superconducting quantum interference devices (SQUIDs) can have excellent spin sensitivity depending on their magnetic flux noise, pickup loop diameter, and distance from the sample. We report a family of scanning SQUID susceptometers with terraced tips that position the pickup loops 300 nm from the sample. The 600 nm-2 mu m pickup loops, defined by focused ion beam, are integrated into a 12-layer optical lithography process allowing flux-locked feedback, in situ background subtraction and optimized flux noise. These features enable a sensitivity of similar to 70 electron spins per root hertz at 4 K.