Controlled transformation of skyrmions and antiskyrmions in a non-centrosymmetric magnet

Control of topological spin textures in magnetic systems may enable future spintronic applications. Magnetic field pulses can switch the vortex polarity¹ or the winding number of magnetic bubbles². Thermal energy can reverse the helicity of skyrmions³ and induce the transformation between meron and skyrmion by modifying the in-plane anisotropy^4,5. Among the various topological spin textures, skyrmions^6,7 and antiskyrmions^8-10 are nanometric spin-whirling structures carrying integer topological charges (N) of -1 and +1 (refs. ^7,11,12), respectively, and can be observed in real space^8,13. They exhibit different dynamical properties under current flow^14-18, for example, opposite signs for the topological Hall effect. Here we observe, in real space, transformations among antiskyrmions, non-topological (NT) bubbles and skyrmions (with N of +1, 0 and -1, respectively) and their lattices in a non-centrosymmetric Heusler magnet, Mn_1.4Pt_0.9Pd_0.1Sn, with D_2d symmetry. Lorentz transmission electron microscopy images under out-of-plane magnetic fields show a square lattice of square-shaped antiskyrmions near the Curie temperature and a triangular lattice of elliptically deformed skyrmions with opposite helicities at lower temperatures. The clockwise and counter-clockwise helicities of the skyrmions originate from Dzyaloshinskii-Moriya interactions with opposite signs along the [100] and [010] directions, respectively. A variation of the in-plane magnetic field induces a topological transformation from antiskyrmions to NT-bubbles and to skyrmions, which is accompanied by a change of the lattice geometry. We also demonstrate control of the helicity of skyrmions by variations of the in-plane magnetic field. These results showcase the control of the topological nature of spin configurations in complex magnetic systems.