Employing time-resolved x-ray microscopy, we investigate the dynamics of a pinned
magnetic vortex domain wall in a magnetic nanowire. The gyrotropic motion of the vortex
core is imaged in response to an exciting ac current. The elliptical vortex core trajectory at
resonance reveals asymmetries in the local potential well that are correlated with the
pinning geometry. Using the analytical model of a two-dimensional harmonic oscillator, we
determine the resonance frequency of the vortex core gyration and, from the eccentricity of …