A simulation study of magnetic vortex structures and spin dynamic modes in permalloy materials

Transverse spin wave modes of a long thin-film permalloy stripe have been studied using micromagnetic simulations and a semi-analytical theory. Lower frequency modes were found localized near the edge of the stripe and higher frequency modes exhibited a crossover property. These features result from the inhomogeneous distribution of the equilibrium magnetization and internal field across the stripe, which is adjusted by an applied field across the width of the stripe. Mode frequencies and wave profiles show distinct characteristics under different applied fields. Mode softening happens at two critical fields corresponding to a symmetry transition of the magnetic system.;Spin dynamic modes of a magnetic antivortex in asteroid-shaped permalloy samples were systematically explored using micromagnetic simulations. A gyrotropic mode was found due to the shift of the antivortex from its equilibrium position. Azimuthal and radial spin wave modes have been excited by a pulsed magnetic field in different directions. Coupling between gyrotropic and azimuthal modes splits the degeneracy of paired azimuthal modes. Frequencies of the spin wave modes decrease with asteroid size and thickness while the frequency of the gyrotropic mode increases with asteroid thickness but decreases with asteroid size.;The vortex dynamics of a magnetic vortex-antivortex-vortex structure in elliptical permalloy samples was studied using a theoretical model and micromagnetic simulations. Three gyrotropic modes were found to exist and each of them can be excited by a special initial configuration of core displacements. The polarizations of three cores play an important role on the mode frequencies and relative core movements in each mode.;The annihilation process of a magnetic vortex pair confined in stadium-shaped permalloy samples was investigated using micromagnetic simulations. With the strength of an applied magnetic field across the width of stadia increasing, the equilibrium magnetization of the system exhibits three different states, which we attribute to the competition mainly between the Zeeman energy and the magnetostatic energy. The geometry dependence of the critical transition field and the linear response of the equilibrium vortex separation to the field were qualitatively explained through a dimensional analysis model.