  Magnetic vortex gyration in a 500 nm wide, 50 nm thick Permalloy square
platelet was investigated using time-resolved scanning transmission x-ray
microscopy and micromagnetic simulations. The response of the vortex core on an
in-plane oscillating magnetic field was studied as a function of the excitation
frequency and amplitude. Non-linear behavior was observed in the form of a
redshift of the gyrotropic resonance. I.e., when the excitation amplitude was
increased, the resonance frequency was found to decrease. The nonlinearity is
in agreement with extensive micromagnetic simulations. This work complements
previous reports on thinner nanostructures where a frequency blueshift was
observed.
