Current‐Induced Reversible Split of Elliptically Distorted Skyrmions in Geometrically Confined Fe3Sn2 Nanotrack

Abstract Skyrmions are swirling spin textures with topological characters promising for future spintronic applications. Skyrmionic devices typically rely on the electrical manipulation of skyrmions with a circular shape. However, manipulating elliptically distorted skyrmions can lead to numerous exotic magneto‐electrical functions distinct from those of conventional circular skyrmions, significantly broadening the capability to design innovative spintronic devices. Despite the promising potential, its experimental realization so far remains elusive. In this study, the current‐driven dynamics of the elliptically distorted skyrmions in geometrically confined magnet Fe3Sn2 is experimentally explored. This study finds that the elliptical skyrmions can reversibly split into smaller‐sized circular skyrmions at a current density of 3.8 × 1010 A m−2 with the current injected along their minor axis. Combined experiments with micromagnetic simulations reveal that this dynamic behavior originates from a delicate interplay of the spin‐transfer torque, geometrical confinement, and pinning effect, and strongly depends on the ratio of the major axis to the minor axis of the elliptical skyrmions. The results indicate that the morphology is a new degree of freedom for manipulating the current‐driven dynamics of skyrmions, providing a compelling route for the future development of spintronic devices.

The left panels of a and b show the schematic view of two skyrmions with opposite helicity, respectively. The right panels of a and b are the corresponding simulated LTEM images based on their spin texture.

Supplementary Note 1
The Joule heating generated by the current is simulated based on the method given in [Appl.
Phys. Lett. 2018, 112, 212403] by using the MATLAB software. The model size (length l ×width w × thickness t) is 20 μm × 1.4 μm × 250 nm, which is the same as that of the sample in our experiments. The initial temperature is set to be 300 K and the relevant physical parameters for Fe 3 Sn 2 are summarized in Table S1. The values of mass density ρ m , heat capacity C p and resistivity ρ could be estimated by our experiments [see Fig. S3a and S3b] or previous references. However, the value of thermal conductivity k for Fe 3 Sn 2 single crystal is hard to estimate because the Fe 3 Sn 2 crystals are usually 1 mm × 0.5 mm hexagons while an 8 mm × 2 mm × 2 mm brick is required for exactly measuring k. Meanwhile, the heat transfer coefficient h c through the surfaces is also hard to be directly established in experiments. In our simulations, we have referenced the values of k and h c for the B 20 compound FeGe [Appl.
Phys. Lett. 2018, 112, 212403]. First, we have simulated the temperature distribution of the nanotrack for k = 3.39 W m -1 K -1 and h c = 5×10 6 W m -2 K -1 (the values of k and h c for FeGe are referenced) when a current pulse of density j = 3.4 × 10 10 A m -2 and pulse width τ = 100 ns is injected. It is found that the highest temperature (T h ) that the sample could be heated up to is about 480 K (see Fig. S3c). By varying the value of k or h c , the value of T h is only slightly affected in such a current density and pulse width (see Fig. S3c). Hence, our simulations are reliable though we use the values of k and h c for FeGe. We have further simulated the temperature distribution of the nanotrack for j ranging from 3.0 × 10 10 A/ m 2 to 4.4 × 10 10 A/m 2 . The corresponding value of T h ranges from 430 K to 690 K (see Fig. S3c). In experiments, we have established that the maximum current density for switching the helicity in the Fe 3 Sn 2 nanotrack is established to be approximately 4.2×10 10 A/m 2 , above which the 3 device could be easily heated beyond the Curie temperature (T c ) of Fe 3 Sn 2 (T c = 640 K). Our simulated results agree with the experiments qualitatively.  Simulated highest temperatures that the sample could be heated up to when parameters are adopted from Table R1. The pulse density ranges from 3.0 × 10 10 A/ m 2 to 4.4 × 10 10 A/ m 2 with a fixed pulse width τ of 100 ns. The value of k varies from 3.39 W m -1 K -1 to 10 W m -1 K -1 and the values of h c range from 5×10 5 W m -2 K -1 to 5×10 6 W m -2 K -1 . We found that the effect of h and k is insignificant.

Figure S4
The under-focused LTEM images of the w ~ 1.4 μm nanotrack. The external temperature ranges from 300 K to 550 K with a fixed magnetic field of 140 mT.