Theory of transformations among skyrmion, antiskyrmion and non-topological bubble with in-plane magnetic field in noncentrosymmetric dipolar magnets

Topological magnetic structures are known to exhibit a variety of quantum transport phenomena and electromagnetic responses due to emergent electromagnetic fields that depend on skyrmion number. Control and manipulation of topological magnetic structures with different skyrmion numbers are expected to contribute to the development of multifunctional spintronic devices with high controllability. In this work, motivated by a recent experiment [Peng et al. Nat. Nanotech. 15, 181 (2020)], we have theoretically studied the field-induced transformations of topological magnetic structures in a disk-shape sample of noncentrosymmetric dipolar magnet. We have performed micromagnetic simulations based on the LLG equation with a classical Heisenberg model which includes both magnetic dipole interactions and Dzyaloshinskii-Moriya interactions and have successfully demonstrated the transformations among three magnetic structures with different skyrmion numbers Nsk, i.e., antiskyrmion (Nsk=+1), elliptical skyrmion (Nsk=-1), and non-topological bubble (Nsk=0) by application and/or reversal of an in-plane magnetic field. By calculating time and spatial profiles of energy contributions from respective interactions and magnetic anisotropy, we have revealed its physical mechanism and properties.