Creating and Deleting a Single Dipolar Skyrmion by Surface Spin Twists

We report deterministic operations on single dipolar skyrmions confined in nanostructured cuboids using in-plane currents. We achieve highly reversible writing and deleting of skyrmions in the simple cuboid without any artificial defects or pinning sites. The current-induced creation of skyrmions is well-understood through the spin-transfer torque acting on surface spin twists of the spontaneous 3D ferromagnetic state, caused by the magnetic dipole-dipole interaction of the uniaxial Fe3Sn2 magnet with a low-quality factor. Current-induced deletions of skyrmions result from the combined effects of magnetic hysteresis and Joule thermal heating. Our results are replicated consistently through 3D micromagnetic simulations. Our approach offers a viable method for achieving reliable single-bit operations in skyrmionic devices for applications such as random-access memories.

💡 Research Summary

This paper presents a deterministic method for electrically creating and deleting single dipolar skyrmions within a confined nanostructure, utilizing the unique magnetic properties of the uniaxial centrosymmetric magnet Fe₃Sn₂. The research demonstrates full control over individual skyrmion bits using in-plane current pulses, a crucial step towards skyrmion-based random-access memory (RAM).

The experimental platform consists of a nanostructured Fe₃Sn₂ cuboid. The system exhibits magnetic hysteresis, where skyrmion and ferromagnetic (FM) states coexist within a specific field range (approximately 470-558 mT). This hysteresis is essential for the operations. At a fixed magnetic field within this range (484 mT), the application of a current pulse can switch the state.

Skyrmion creation is achieved when the initial state is the FM phase. A current pulse with a density exceeding a first threshold (j_c1 ≈ 5.6×10¹⁰ A/m²) reliably writes a single skyrmion with a 100% success rate within an optimal current window. The mechanism is identified as the spin-transfer torque (STT) effect. Crucially, the STT acts not on a uniform FM state, but on a “3D ferromagnetic state” that spontaneously forms in Fe₃Sn₂ due to its low magnetic quality factor and strong dipole-dipole interaction. This 3D FM state features vortex-like, non-uniform spin twists at the surfaces while remaining uniform in the interior. These surface twists provide the necessary spatial magnetization gradient (∇M ≠ 0) for the STT to act upon, nucleating a seed domain that evolves into a stable skyrmion. The STT origin is confirmed by the independence of j_c1 on current pulse duration and the ability to create skyrmions with different current/field orientations, ruling out Joule heating and Oersted field effects as primary causes.

Skyrmion deletion is performed when the initial state is a skyrmion. A current pulse with a higher density, above a second threshold (j_c2 ≈ 12.0×10¹⁰ A/m²), annihilates the skyrmion and restores the FM state. This process is primarily driven by Joule heating combined with magnetic hysteresis. The heating transiently reduces the saturation magnetization (M_s), which alters the energy landscape. Simulations show that a reduction of M_s by about 21% destabilizes the skyrmion at the applied field, causing a transition to the FM state. Due to hysteresis, the system remains in the FM state even after cooling back to room temperature. This thermal mechanism is supported by the experimental observation that j_c2 decreases with increasing pulse duration for a fixed heating energy.

All experimental findings are consistently reproduced and elucidated through three-dimensional micromagnetic simulations. The simulations visually detail the dynamics of the surface spin twists during skyrmion creation and model the thermal deletion process.

In summary, this work establishes a robust scheme for deterministic single-skyrmion operations by leveraging intrinsic surface spin textures arising from dipolar interactions, eliminating the need for artificial defects. It provides a viable pathway for implementing reliable write/delete functions in future skyrmionic memory devices and suggests the potential applicability of this mechanism to other material systems hosting similar topological textures.