Formation of skyrmion and skyrmionium in confined nanodisk with perpendicular magnetic anisotropy

We theoretically investigate the micromagnetic dynamics and spin configurations in a CoFeB/Pt nanodisk with varying effective anisotropy constant (K) and radius (R). It is found that skyrmion and skyrmionium could be stabilized well in a certain region of the phase diagram of topological structures, and their formations physically originated from the competition between exchange interactions and anisotropy contributions. Furthermore, we achieve the transition between the skyrmion and the skyrmionium through fine tuning the magnetic anisotropy. Our results reveal the specific conditions needed to form the equilibrium states of skyrmions and skyrmionium and have significance for the development of skyrmion-based devices.