Generation energetics of skyrmion bubbles concerning perpendicular magnetic anisotropy

Skyrmion bubbles with non-trivial topological number emerge from a complex energy landscape that involves multiple magnetic energy contributions. While they hold a unique position in applications such as racetracks or logic devices due to their stability and controllability, the fundamental properties of their dynamics also gained considerable attention as they inevitably affect the operation of skyrmion devices. Especially the generation process of the skyrmions is known to depend critically on magnetic parameters [1], and the role of entropic effects on activation energy can vary widely depending on the detailed structure of the magnetic textures [2,3].

In this work, we investigate the generation dynamics of skyrmion bubbles in W/CoFeB/Ta/MgO. We insert the Ta wedge layer between CoFeB and MgO to generate a spatial gradient of perpendicular magnetic anisotropy and quantify it using MOKE microscopy. We measure the temporal variation of the skyrmion number after applying an external magnetic field at various temperatures to obtain the activation energy and the attempt time by Arrhenius fitting. The appearance of the Meyer-Neldel-type rule indicates the presence of entropic effects in the nucleation of skyrmions in thin-film CoFeB. We also present a trend in the activation barrier and attempt time with respect to the anisotropy field, demonstrating their role in the skyrmion bubble nucleation process.