Deterministic Helicity Locking of Bloch Skyrmions in Centrosymmetric Systems

TL;DR

Problem: In centrosymmetric magnets, Bloch-type skyrmions exhibit helicity degeneracy ($η=±π/2$) in the absence of DMI, complicating deterministic control. Approach: interface a centrosymmetric ferromagnetic film with FM stripes or a notch to generate dipolar fields and interfacial exchange that lift helicity degeneracy without DMI. Findings: (i) FM-stripe geometry deterministically selects $η=+π/2$ or $-π/2$ depending on stripe configuration; (ii) a notch with current pulses enables deterministic nucleation of CW/CCW skyrmions; (iii) interfacial coupling strengthens helicity locking and enables guided motion. Significance: Provides a scalable, DMI-free platform for robust, helicity-locked skyrmions suitable for memory, logic, and neuromorphic spintronic concepts.

Abstract

Magnetic skyrmions in centrosymmetric materials exhibit Bloch-type spin textures with degenerate helicity states due to the absence of Dzyaloshinskii Moriya interaction (DMI), resulting in random nucleation and uncontrolled chirality. Here, we present a comprehensive micromagnetic study demonstrating a fully DMI free strategy for deterministic helicity control by interfacing ferromagnetic (FM) stripes or notch structures with centrosymmetric magnetic (CM) films. We first show that a geometrically constrained configuration comprising two FM stripes with opposite in-plane magnetizations stabilizes skyrmions with a selected helicity, either clockwise (CW) or counterclockwise (CCW). We further extend this concept to achieve deterministic nucleation of CW or CCW skyrmions using current pulses applied to an FM notch patterned on the CM film. The combined effects of the FM stripe/notch geometry and interfacial exchange coupling generate dipolar fields that lift the helicity degeneracy, enabling controlled formation of skyrmions with fixed chirality. These results establish FM/CM heterostructures as a robust, DMI-free platform for deterministic generation and guided motion of helicity-locked skyrmions, opening new pathways for advanced spintronic applications.

Figures (5)

• Figure 1: Design scheme for helicity locking in a centrosymmetric magnetic thin film system. (a) Schematic illustration showing ferromagnetic (FM)/spacer stripes with opposite in-plane (IP) magnetization on a centrosymmetric ferromagnetic thin film. Opposite skyrmion helicities ($\pm\pi/2$) are stabilized outside the stripe region, while only counterclockwise (CCW, $\eta$ = $+\pi/2$) skyrmions are stabilized between the stripes. (b) Deterministic skyrmion nucleation with locked helicity using triangular FM notch with IP and OP magnetic moments and under applied in-plane currents (Jx, Jy). The FM layer with OP ($+ z$) magnetization nucleates skyrmions with both helicities, whereas the in-plane ($\pm x$) FM layer nucleates only CCW ($\eta$ = $+\pi/2$) or CW ($\eta$ = $-\pi/2$) skyrmions.
• Figure 2: Micromagnetic simulations of dipolar-stabilized skyrmions in a heterostructures with different geometry. (a) Schematic of the simulated geometry along with their corresponding magnetization (my, mz) map for (a) a standalone CM film, (b) CM film with a single FM/spacer layer, (c) Bi-FM/spacer with parallel dipole configuration, (d) Bi-FM/spacer with antiparallel dipole configuration. For each simulated geometry, the schematic, mz magnetic profile, zoomed views of the mz magnetic profile shown in a boxed, and the with the my magnetization profile are shown under the column i, ii, iii, and iv, respectively. Color bars indicate the magnetization components (my, mz) in the equilibrium state.
• Figure 3: (a) Micromagnetic phase diagram of FM-coupled centrosymmetric film heterostructures as a function of FM width (WFM) and gap (GFM), Inset: schematic of the FM/Spacer/CM heterostructure with variable WFM and GFM. (b–d) Representative equilibrium spin textures corresponding to points (R1-R3) in (a): OP magnetization maps (mz, red = spin-up, blue = spin-down) and helicity-resolved in-plane magnetization maps. (color scale encodes (my, mz), arrows denote local magnetization). ‘The map "my" highlights individual cores, illustrating the transition from helicity-degenerate Bloch and anti-skyrmion states (R1), to coexisting helicity-locked Bloch skyrmion states ($\eta$ = $+\pi/2$)/ anti-skyrmion states (R2), and finally to fully helicity-locked Bloch skyrmion states ($\eta$ = $+\pi/2$) (R3). Bloch skyrmions with helicities $\eta$ = $+\pi/2$ and $-\pi/2$, and anti-skyrmions are highlighted by black, blue, and red squares, respectively.
• Figure 4: Deterministic and random skyrmion nucleation in FM/CM heterostructures under applied in-plane currents (Jx, Jy). (a) Schematic of helicity-locked skyrmion pair/random nucleation. Magnetization maps (my, mz) show nucleation of skyrmions with helicities (b–c) The OP magnetization of the FM layer lifts the helicity degeneracy, enabling deterministic nucleation of CW/CCW skyrmion pairs, (d–f) Nucleation of CCW ($\eta$ = $+\pi/2$) skyrmions with FM magnetization along $+x$, (g–i) Nucleation of CW ($\eta$ = $-\pi/2$) skyrmions with FM magnetization reversed to $−x$. Color maps show the IP (my) and OP (mz) magnetization components.
• Figure 5: Energy landscape and helicity selection mechanism in FM/CM heterostructures (a) Schematic double-well potential illustrating helicity degeneracy ($\eta$ = $\pm\pi/2$) in a centrosymmetric film, (b–c) The FM layer introduces an effective dipolar interaction , where $\pm$ corresponds to the magnetization direction of the FM layer along $+x$, and $-x$ axis, respectively, lifting the helicity degeneracy and stabilizing a preferred helicity. (d–f) Time evolution of total system energy ET during current-induced skyrmion nucleation for FM polarized along $+z$ (CW-CCW pair), $+x$ (CCW), and $-x$ (CW), respectively.