Field-driven collapsing dynamics of skyrmions in magnetic multilayers

Magnetic skyrmions are fascinating topological particle-like textures which are promoted by a trade-off among interfacial properties (perpendicular anisotropy and Dzyaloshinskii-Moriya interaction) and dipolar interactions. Depending on the dominant interaction, complex spin textures, including pure Néel and hybrid skyrmions, have been observed in magnetic multilayers. A quantification of these different spin textures typically requires a depth-reoslved magnetic imaging or scattering techniques. In the present work, we will show qualitatively different collapsing dynamics for pure Néel and hybrid skyrmions induced by a perpendicular magnetic field in two representative systems, [Pt/Co/Ir]15 and [Ta/CoFeB/MgO]15 multilayers. Skyrmions in the former stack undergo a first morphological transition at zero field to labyrinth domains, and followed by a second morphological transition to skyrmions with opposite chirality and polarity, upon reversing the direction of perpendicular fields. On the other hand, skyrmions in [Ta/CoFeB/MgO]15 multilayers exhibit a continuous transition as a function of the field, which is mainly linked to a reversible change of the skyrmion size. A full micromagnetic phase diagram is presented to identify these two collapsing mechanisms as a function of material parameters. Since the two distinct collapsing dynamics rely on the detailed layer-dependent spin structures of skyrmions, we propose to use them as potential fingerprints for identifying the type of skyrmions in magnetic multilayers. Our work could be important for employing pure and hybrid skyrmions for specific applications, due to the strong correlation between the skyrmion dynamics and three-dimensional spin profiles.