Multistep transition between topological spin textures in van der Waals magnets

Magnetic quasiparticles, such as skyrmions and merons in van der Waals (vdW) magnets, have attracted great attention due to their potential applications in low dimensional high-density memory storage. However, controlling the transition between different magnetic solitons is still a challenge. Here, we observed a multistep topological transition in monolayer $1H$-type magnets under magnetic field. The different magnetic states, including helical spiral spin textures, meron-antimeron pairs, and skyrmion-antiskyrmion lattices, are fully analyzed from the real-space spin textures, the propagation vectors in reciprocal-space, and the topological charges. Moreover, the formation of these special meron-antimeron pairs is demonstrated to originate from the competition between Dzyaloshinskii-Moriya (DM) interaction and Heisenberg-type frustration. A phase diagram shows that a weak frustration would lower the critical transition field between different topological states. Our results enable the realization of multistep transitions in vdW magnets and provide a promising material platform for studies on different magnetic solitons.