Figure 1: Conversion between a skyrmion and a DW pair.

(a–c) A DW pair is converted into a skyrmion driven by STT from the narrow part to the wide part of the nanowire. (a) The snapshots of the magnetization configuration at four selected times corresponding to the four vertical lines in b and c. The in-plane current density along −x direction is 1.8 μA nm⁻² in the wide part. (b) The time evolution of the average spin components m_x, m_y, m_z. (c) The time evolution of the Pontryagin number Q. (d–f) Reverse process of a–c. The in-plane current density along +x direction is 1.8 μA nm⁻² in the wide part. (g–i) Conversion process of a DW pair into a skyrmion similar to a–c, when the width of the narrow part is larger than the skyrmion intrinsic size. The in-plane current density along −x direction is taken as 6.6 μA nm⁻² in the wide part so that the current density in the narrow part in g–i is the same as that of a–c. (j–l) Reverse process of g–i, where a skyrmion is not converted into a DW pair. The in-plane current density along +x direction is 1.8 μA nm⁻² in the wide part. For all simulations, the initial magnetization is assumed to align along +z except that the magnetization inside the DW pair (or skyrmion core) are aligned −z. During the first 0.2 ns, the system is relaxed to an energy minimum state without applying any current. The colour-scale shown between a and g has been used throughout this paper. A physical length scale bar is included in a.