Geometry and field dependence of the formation of magnetic antivortices in pound-key-like structures

Abstract

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. V C 2015 AIP Publishing LLC.

Observation of the dynamic modes of a magnetic antivortex using Brillouin light scattering

“…For details on the factors that influence the AV formation success rate including the role of the sample geometry and the angle of the applied field, refer to Ref. [31]. In Fig.…”

“…The differences in the shape anisotropy in the wide and narrow sections of structures with this general shape can be used to influence the reversal process and hence to promote the nucleation of magnetic AV's, as described in Refs. [5,31]. The AV's were prepared by applying first a saturating field and then a smaller field in the opposite direction along the structure diagonal that runs through the wider legs.…”