The scanning SQUID "multi-tool" : advanced sensor development and novel applications
Keywords: Applied Physics.

The scanning Superconducting QUantum Interference Device (SQUID) is a versatile probe that can combine spatially-resolved magnetometry and magnetic susceptibility measurements in-situ. Expanding the scanning SQUID toolbox to develop new measurement modes and adapt them to new material systems can lead to potential scientific discoveries and technological applications. In this thesis, I will explore both topics of instrumentation and mesoscopic device measurement. In chapter 1, I will describe how we designed and built a scanning SQUID microscope in a Bluefors dry fridge. With increasing scarcity of liquid helium, low-temperature physics labs are quickly shifting to alternative (dry) cooling methods for sustainability. For this purpose, we built a new scanning SQUID microscope in a 2.8 Kelvin base temperature dry fridge, with a variable-temperature sample stage up to 100 Kelvin and sample-SQUID relative vibration down to tens of nanometers. Our low-vibration dry fridge will enable scanning SQUID studies of superconductors and magnetic devices over a wide temperature range, without the interruption of helium transfers. In chapter 2, I will describe the design, characterization and measurements of a novel time-resolved scanning SQUID sensor. The time-resolved scanning SQUID uses a sampling method and on-chip Josephson pulse generator to achieve a 40-picosecond time resolution. We demonstrated the sampling capability with measurements of Josephson relaxation oscillations in a resistively-shunted Josephson junction device. Combined with in-situ magnetometry and susceptibility measurement capabilities, the time-resolved scanning SQUID can be uniquely suited to characterize next-gen spintronics, superconducting/quantum computing devices. In chapter 3, I will present an in-depth study of the current-phase relation of an indium-arsenide (InAs) nanowire Josephson junction. Hybrid indium-arsenide/aluminum (InAs/Al) nanowires are a promising candidate for 1D topological superconductivity. In order to fully understand the electronic structure in the wire, we used the scanning SQUID to study the supercurrent in a Josephson junction made of InAs/Al nanowires as a function of superconducting phase across the junction. By fitting to theoretical models, we found resonant levels and evidence of Coulomb interaction in the junction. Schrodinger-Poisson numerical simulations showed that these emergent effects were most likely linked to band-bending at the InAs/Al interface. This work demonstrated the effect of electrostatics in this mesoscopic system, and therefore can serve as a prescription for device design or experimental analysis. In chapter 4, I will explore the magnetic properties of InAs nanowires covered by an epitaxial ferromagnetic insulator (FMI) europium-sulfide (EuS). FMI can induce magnetic exchange coupling into the InAs via proximity, thus satisfying the condition for topological superconductivity without a large external Zeeman field (usually 1-2 Tesla). Using the scanning SQUID, we were able to spatially resolve the orientation, uniformity, as well as sub-micron-scale domains of the magnetic moments in EuS. Our results showed that the epitaxial EuS layer was highly uniform and consistent with bulk magnetic properties of EuS. The next step involves doing a more comprehensive comparison between bulk EuS and InAs/EuS hybrid, in order to quantify the magnetic proximity coupling to the InAs nanowire.