Magnetic skyrmion shape manipulation by perpendicular magnetic anisotropy excitation within geometrically confined nanostructures

Abstract

Magnetic skyrmions are promising features of spintronic devices including race-track memory and nano-oscillators. Skyrmion shape can act as an additional degree of freedom along with other properties including topological charge, polarity and helicity etc. Skyrmion excitation associated with shape deformation is significant for microwave devices. By means of micromagnetic simulation study, we report skyrmion excitation within nanodiscs by dynamically modulated perpendicular magnetic anisotropy (PMA). At higher resonance frequencies, skyrmion shows shape deformation into regular polygons including elliptic, trigon, tetragon, up to decagon, characterized as n = 2, 3, 4 …, 10, respectively. We build a phase diagram showing that regular polygon shape deformation occurs within a PMA amplitude window with minimum and maximum limits depending on resonance frequency and disc radius. Further analysis reveals that with increase in disc radius, modes with higher n appear, however, skyrmion is distorted into irregular shape, with no specific polygon-like pattern, even for smaller PMA. With a semi-quantitative analysis, we relate n for different modes to resonance frequency, disc size and PMA magnitude. These findings can assist in getting fundamental physics underlying skyrmion deformation and can help in designing energy-efficient microwave generator/detector and nano-oscillator where skyrmion dynamics are very important.