  We investigated the spin dynamics of a vortex state in a core-shell magnetic
nanodisk driven by an oscillating field applied perpendicular to the disk plane
by means of micromagnetic simulations. The nanodisk comprises a Py (Fe0.2Ni0.8)
core of 100 nm in radius, surrounded by a 50 nm thick Fe shell. Fourier
transform analyses show that the Py core and the Fe shell dominate spin-wave
oscillation at the fundamental and higher order radial modes, respectively. For
oscillating driving field tuned to the fundamental eigenfrequency, the Py/Fe
interface effectively confines spin-wave excitation in the Py core region. This
effect leads to significantly more rapid vortex core (VC) reversal in
comparison to homogeneous disks. Our work demonstrates that the higher order
modes can drive much faster VC reversal than the fundamental mode, in sharp
contrast to the results obtained in homogeneous disks. With excitation levels
up to 30 mT, we find strong nonlinear spin-wave dynamics in the system, which
results in mode frequency redshifting, therefore the observation of the most
rapid VC reversals below eigenfrequencies and VC switching in wide ranges of
frequencies.
