Mechanism of vortex switching in magnetic nanodots under a circular magnetic field. I. Resonance action of the field on the nanodot eigenmodes

The resonance activation of the eigenmodes of a two-dimensional easy-plane ferromagnet of finite size by a circular magnetic field is considered as a basis for theoretical explanation of the mechanism of vortex switching in magnetic nanodots under the influence of such a field. It is shown analytically that in the case of weak easy-plane anisotropy, when the vortex has a nonzero polarization (a total magnetization along the hard axis), the influence of the field on the eigenmodes of the system is of a complicated nature. A circular field acts in a resonance manner on the azimuthal modes of the system, in which the magnetization depends on the azimuthal coordinate (in the form of a direct resonance at the eigenfrequencies of the azimuthal modes). The coupling of the azimuthal and symmetric (independent of the azimuthal coordinate) modes via the external field gives rise to complex parametric resonances at sum frequencies. The results obtained are compared with the data of previous numerical studies.