Observation of non-linear magnetic vortex gyration by X-ray microscopy   and micromagnetic simulations
  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. 
Arne Vansteenkiste, Ben Van de Wiele, Markus Weigand, Hermann Stoll,   Kang W. Chou, Tolek Tyliszczak, Georg Woltersdorf, Ch...
cond-mat.mtrl-sci cond-mat.other