Final Report Summary - NANOSQUID (Scanning Nano-SQUID on a Tip) The goal of the project was development of a novel scanning magnetic imaging tool for quantitative, high-sensitivity mapping of static and dynamic magnetic fields on nanometer scale, and its utilization to study of vortex dynamics in superconductors and of nano-magnetic phenomena. In the first stage of the project we have successfully developed Superconducting Quantum Interference Device (SQUID) that resides on the apex of a very sharp tip and acts as a highly sensitive detector of local magnetic fields. The main novelty of the approach is that the SQUID is fabricated on the end of a pulled quartz tube using “self-aligned” evaporation of superconducting material. The resulting SQUID-on-tip devices have significantly reduced dimensions and can be brought into very close proximity to the sample and scanned a few nm above the surface. With this approach we fabricate the smallest SQUIDs ever reported that have outstanding sensitivity for imaging magnetic moments. The first devices developed within the project were based on superconducting Al with typical diameter of 200 nm and reached flux sensitivity of 2 µΦ0/Hz1/2 and spin sensitivity of 65 μB/Hz1/2. Subsequently, nanoSQUID-on-tip devices made of Nb and Pb were developed using advanced deposition methods. The best performance was achieved with Pb nanoSQUIDs with diameter down to 46 nm, operation at fields of over 1 T at 4.2 K, flux noise of 50 nΦ0/Hz1/2, and a groundbreaking spin sensitivity of 0.38 μB/Hz1/2 that is about two orders of magnitude better than any previous SQUID. These new devices thus pave the way to scanning magnetic microscopy with single electron spin sensitivity.We designed and constructed two scanning SQUID-on-tip microscopes that allow sensitive imaging of local magnetic fields on the nanoscale at 300 mK and at 4 K over a wide range of magnetic fields. We developed a feedback mechanism in which the SQUID-on-tip is glued to a resonating quartz tuning fork. By monitoring the frequency and the amplitude of the resonance, a closed feedback loop is achieved that allows approach and scanning of the SOT within few nm above the surface of the sample. We have also developed sensitive readout electronics using a cryogenic amplifier based on series array of 100 SQUIDs. As a result we were able to obtain simultaneous sensitive magnetic and topographic imaging of superconducting samples. These microscopes were applied to study of topological insulators and vortices in superconductors. The outstanding sensitivity of our technique allows imaging of vortex displacements with subatomic precision down to 10 pm level. These studies of vortex dynamics with single vortex resolution reveal complex vortex trajectories within individual pinning centers, hysteretic hopping paths, flow patterns of moving lattices, dynamic instabilities, plastic flow, and ordering.
