Dense skyrmion crystal stabilized through interfacial exchange coupling:Role fo in-plane anisotropy

A Monte Carlo simulated-annealing algorithm was used to study the magnetic state in an in-plane helimagnet layer on triangular lattice that exchange couples to an underlayer with strong out-of-plane anisotropy.In the single helimagnet layer with in-plane anisotropy(K),the formation of labyrinth-like domains with local spin spirals,instead of parallel stripes,is favored,and these domains rapidly transform into dense skyrmion crystals with increasing interfacial exchange coupling(J'),equivalent to a virtual magnetic field,and finally evolve to an out-of-plane uniform state at large enough J'.Moreover,with increasing K,the skyrmion crystal state can vary from regular 6-nearest-neighboring circular skyrmion arrangement to irregular squeezed skyrmions with less than 6 nearest neighbors when the in-plane anisotropy energy is higher than the interfacial exchange energy as the skyrmion number is maximized.Finally,we demonstrated that the antiferromagnetic underlayer cannot induce skyrmions while the chirality inversion can be achieved on top of an out-of-plane magnetization underlayer with 180°domain walls,supporting the experimental findings in FeGe thin film.This compelling advantage offers a fertile playground for exploring emergent phenomena that arise from interfacing magnetic skyrmions with additional functionalities.

Skyrmion-(non)crystal structure stabilized by dipolar interaction

We report a numerical study on the role of longrange dipolar interaction played on the creation and stabilization of skyrmion-(non)crystal structure in chiral ferromagnetic thin films without any anisotropies,based on a Monte-Carlo simulation method.With the increase of external magnetic field,the microscopic spin configuration is transformed from a spin-spiral stripe or labyrinth structure,depending on the strength of dipolar interaction,into a skyrmion-(non)crystal structure,and then into a skyrmiongas structure,and finally into a ferromagnetic state.Interestingly,with the increase of dipolar interaction,the skyrmion-crystal structure evolves from a triangular arrangement into a square arrangement with the change of skyrmion shape from circle to square.For larger dipolar interactions,the skyrmion-crystal structure loses the regular arrangements and the skyrmions,remaining topological,exhibit different shapes and sizes and squeeze with each other,whose distributions are analogous to a non-crystal structure.Therefore,different skyrmion-(non)crystal structures are stabilized in different ranges of dipolar interactions,which further promotes the applications of skyrmions as non-volatile information carriers.