Manipulating the Topology of Nanoscale Skyrmion Bubbles by Spatially Geometric Confinement [component]

The discovery of magnetic skyrmion bubbles in centrosymmetric magnets has been receiving increasing interest from the research community, due to the fascinating physics of topological spin textures and its possible applications to spintronics. However, key challenges remain, such as how to manipulate the nucleation of skyrmion bubbles to exclude the trivial bubbles or metastable skyrmion bubbles that usually coexist with skyrmion bubbles in the centrosymmetric magnets. Here, we report having
more » ... we report having successfully performed this task by applying spatially geometric confinement to a centrosymmetric frustrated Fe 3 Sn 2 magnet. We demonstrate that the spatially geometric confinement can indeed stabilize the skyrmion bubbles, by effectively suppressing the formation of trivial bubbles and metastable skyrmion bubbles. We also show that the critical magnetic field for the nucleation of the skyrmion bubbles in the confined Fe 3 Sn 2 nanostripes is drastically less, by an order of magnitude, than that what is required in the thin plate without geometrical confinement. By analyzing how the width and thickness of the nanostripes affect the spin textures of skyrmion bubbles, we infer that the topological transition of skyrmion bubbles is closely related to the dipole-dipole interaction, which we find is consistent with theoretical simulations. The results presented here represent an important step forward in manipulating the topological spin textures of skyrmion bubbles, making us closer to achieving the fabrication of skyrmion-based racetrack memory devices. Magnetic skyrmions are topologically protected vortex-like objects that were first discovered in the chiral, non-centrosymmetric magnets 1-6 where they are stabilized by the Dzyaloshinskii-Moriya interaction (DMI). Unlike the conventional, "rigid" magnetic domain walls, they are flexible in shape-deformation to avoid pinning centres, 7,8 and therefore only an ultra-low current density is needed to drive them to move. 7-12 This fascinating topological property, combined with their nanoscale size and stable particle-like features make skyrmions promising candidate for carrying magnetic information in further high-density and low-power consumption spintronic devices based on the racetrack memory concept. 8, 13, [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] Recent studies showed that some non-chiral centrosymmetric magnets could host skyrmion bubbles (SKBs), 25-32 stabilized by the interplay of the external magnetic field, ferromagnetic exchange interaction, uniaxial magnetic anisotropy, and dipole-dipole interaction (DDI). SKBs are topologically equivalent to magnetic skyrmions and exhibit similar topological properties, such as the topological Hall effect, 27 skyrmion Hall effect, 33 and ultra-low driving current density for current-induced motion. 25 More importantly, SKBs show a significantly high thermal stability over a wide temperature range crossing room temperature, 27,28,34 showing a high potential of SKBs for the construction of memory devices. Contrary to DMI-stabilized skyrmions with a fixed helicity, SKBs in centrosymmetric magnets possess two degrees of freedom, i.e., vorticity and helicity, 35 which makes them usually coexist with the topologically trivial bubbles (topological number is equal to 0), [28] [29] [30] [31] 37 or exhibit multiple topologies such as biskyrmions, 25, 27, 30, 36 and various metastable SKBs 28,30,36 (for example, pendulum-shaped SKBs 29 and bifurcation-shaped SKBs 28,30,36 ). When external stimuli, such as magnetic field H or spin-polarized current, are applied, the spin structures of the trivial and metastable SKBs may vary with the motion of Bloch lines (BLs), making them unsuitable for the application in magnetic racetrack memory devices. Therefore, in order to be suitable for such applications, it is essential to remove trivial bubbles and metastable SKBs. 21 Recent theoretical simulations, based on the nanostructured frustrated magnet, showed that the periodically modulated spin textures at geometrical boundaries had a significant influence on the magnetization dynamics of SKBs , 35 offering a new path that designing proper
doi:10.1021/acsnano.8b09689.s001 fatcat:6jxbwtbnjzfzrkqmyd32hbazpa