Mechanism of vortex switching in magnetic nanodots under a circular magnetic field. II. Dynamics of a spin plaquette containing a vortex

For a theoretical explanation of the mechanism of switching of the polarization of magnetic vortices in an external circular magnetic field, a small spin plaquette in a vortex configuration is considered. An analytical investigation of the initial (linear) stage of the vortex switching process is carried out. The analytical results obtained confirm the data of a numerical calculation of the plaquette dynamics. Both the numerical simulation and an analytical treatment of the initial stage of activation show the importance of taking the azimuthal modes of the system into account. It is at the frequencies of these modes that the most rapid growth of the vortex energy and the total intraplane projection of the magnetization occur. Increasing the amplitude of these modes leads to parametric excitation of a low-frequency symmetric mode, and that causes vortex switching. The results provide a qualitative explanation of the data of a numerical simulation of vortex switching in large magnetic systems and can be used in experiments on the directed influencing of the polarization of vortices in magnetic nanodots.