Vortex dynamics induced by scanning SQUID susceptometry

We measured the local magnetic response of a niobium thin film by applying millitesla-scale AC field using micron-scale coil and detecting with pickup loop in scanning superconducting quantum interference device (SQUID) susceptometry measurement. Near film's critical temperature, we observed step-like nonlinear dissipative due to dynamics small number vortex-antivortex pairs induced applied field. modeled measurement combined two-dimensional London-Maxwell time-dependent Ginzburg-Landau approach, allowing us construct detailed real-space picture vortex motion causing response. This work pushes SQUID superconductors beyond regime linear lays foundation for microscopic studies pinning devices more exotic materials systems.