Magnetic Vortex Core Dynamics in a Ferromagnetic Dot
  We report direct imaging by means of x-ray photoemission electron microscopy of the dynamics of magnetic vortices confined in micron-size circular Permalloy dots that are 30 nm thick. The vortex core positions oscillate on a 10-ns timescale in a self-induced magnetostatic potential well after the in-plane magnetic field is turned off. The measured oscillation frequencies as a function of the aspect ratio (thickness/radius) of the dots are in agreement with theoretical calculations presented for the same geometry. 
K. Yu. Guslienko, X.F. Han, D.J. Keavney, R. Divan, and S. D. Bader