Impact of the honeycomb spin-lattice on topological magnons and edge states in ferromagnetic 2D skyrmion crystals

Abstract: We theoretically investigate the magnon band topology and associated topological edge states (TESs) in Neel-type ferromagnetic skyrmion crystals (SkXs) stabilized on a two-dimensional honeycomb lattice, using parameters relevant to monolayer CrI3. Employing stochastic Landau-Lifshitz-Gilbert simulations and discrete Holstein-Primakoff bosonization, we analyze the impact of the honeycomb spin-lattice structure on the magnonic spectrum, in contrast to the extensively studied triangular spin-lattice SkXs. Our analysis identifies topological features unique to the honeycomb lattice. In particular, certain characteristic magnon modes (e.g., elliptical distortion and triangular distortion modes) acquire nontrivial Chern numbers absent in triangular-based SkXs. Moreover, contrary to predictions based on triangular spin-lattice SkXs, we find that the counterclockwise (CCW)-breathing magnonic gap exhibits topological behavior only at large Dzyaloshinskii-Moriya interactions (DMI), losing its universality with decreasing DMI strength. Meanwhile, the second magnon gap consistently hosts robust TESs across the entire range of DMI and magnetic fields studied, closing at critical fields through field-induced topological phase transitions. The study further uncovers a remarkable richness in the magnon topology, identifying 65 distinct topological magnon phases generated by magnetic-field-driven skyrmion deformation. These findings underscore the profound role of lattice geometry in shaping magnon topology in non-collinear spin textures.