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In this work, we assess the effects of field history and structure shape on the formation of magnetic antivortices. The magnetic reversal process was investigated for a series of patterned micron-sized permalloy pound-key structures with varying degrees of asymmetry using magneto-optical Kerr effect hysteresis measurements combined with magnetic force microscopy. The largest number of antivortices was observed in the structures with the highest level of structure asymmetry, which also show an intermediate state in the hysteresis loop. A significant enhancement of the antivortex formation rate--from 5% to almost 80%--was achieved by adjusting the structure dimensions. Images of the magnetic states obtained at various points in the hysteresis loop show that the highest rate of antivortex formation occurs near the coercive field, also the nucleation field, and that the antivortex formation is also sensitive to the angle of the applied field, where the highest antivortex formation rate is observed when the field is aligned along the structure diagonal. A comparison of the experimental results with micromagnetic simulations shows that the areas with lower shape anisotropy lead the reversal in the formation step and the upper field limit for the antivortex stability is related to the reversal of the regions with higher shape anisotropy, although the simulations suggest that the annihilation mechanism will change to one that involves domain wall propagation when the smallest structure dimensions are below ~60 nm. These results demonstrate how shape anisotropy can be used to promote the formation of isolated magnetic antivortices, which will facilitate future investigations of this topological magnetic state. [ABSTRACT FROM AUTHOR]

The dynamics of individual magnetic vortices and vortex pairs confined in lithographically defined ferromagnetic ellipses were measured using a microwave reflection technique. Resonance frequencies were detected in the subgigahertz range for Ni80Fe20 (Permalloy) ellipses ranging in size from 3 by 1.5 μm to 1 by 0.5 μm, 40 nm in thickness. Micromagnetic simulations indicate that the single mode observed for one vortex is a translational mode and that the two eigenmodes associated with a vortex pair correspond to coupled translational modes of vortices with parallel or antiparallel core polarizations. The single-vortex resonance scales inversely with the ellipse size, in agreement with the simulations and analytical theory. [ABSTRACT FROM AUTHOR]

Magnetic antivortices have potential for applications but they are considerably more difficult to create than their topological counterpart, the vortex state. Here, we describe a method to generate isolated magnetic antivortex (AV) states reliably in pound-key-like patterned structures. Magnetic force microscopy images confirm that AV states are obtained after a simple two-step magnetic field procedure that involves first a saturating field along the structure diagonal followed by a smaller field applied in the opposing direction. Micromagnetic simulations show that the second field reverses areas of the structure that have lower shape anisotropy first, which facilitates the subsequent antivortex formation. [ABSTRACT FROM AUTHOR]