Magnetic Force Microscopy for Diagnosis of Complex Superconducting Circuits

The possibility of controlling Josephson vortices and fluxon states has always been an attractive area of research that can influence the development of superconducting devices and quantum computing. Here we demonstrate the capability of magnetic force microscopy to detect and manipulate Josephson vortices (JVs) and fluxons below the critical current in complex superconducting systems on the example of a dc superconducting quantum interference device. The simultaneous magnetic force microscopy and transport investigation method make it possible to generate JVs by direct current in Josephson junctions and simultaneously analyze their dynamics under various external conditions. Our results show a dynamic process of transition of $2\ensuremath{\pi}$-phase singularities of JVs and fluxons from one to another. Finally, we demonstrated that the magnetic force microscopy method could distinguish the difference in critical currents between several Josephson junctions in complex superconducting circuits without the need of direct electron transport measurements.