Measurement of the spatial sensitivity of miniature SQUIDs using magnetic-tipped STM

Large area superconducting quantum interference devices (SQUIDs) are known to be ultimate sensors of magnetic flux. By diminishing the size of a SQUID loop, the energy sensitivity of the device can be increased to near quantum-limited operation whilst also making it less sensitive to external magnetic field noise and improving its coupling to any nano-scale magnetic particle within the loop. This makes it an ideal nano-sensor for magnetic particles including single molecules. It is, however, important to optimize the coupling between any magnetic nano-particle placed within the SQUID and the SQUID itself in order to achieve maximum sensitivity for the spin detection. We have made some preliminary calculations of the expected SQUID response to a magnetic particle placed at different locations within the SQUID device, and using a magnetic tip in a scanning tunnelling microscope (STM) we are able to produce a spatial map of the SQUID sensitivity. Initial results on a 3 µm diameter loop SQUID are presented to demonstrate this method.