Janus Skyrmion: Dual Helicity in Magnetic Systems

• Janus skyrmions are two-faced topological spin textures with one half showing Nél-type and the other Bloch-type helicity, resulting from engineered DM interactions.
• They demonstrate distinctive static and dynamic properties, including 1D current-induced motion without the conventional skyrmion Hall effect.
• Their tunable interfacial design offers promising avenues for advanced spintronic devices and memory applications despite fabrication challenges.

A Janus skyrmion is a topological quasiparticle characterized by a spatially asymmetric helicity, realized at interfaces engineered between magnetic regions with different Dzyaloshinskii–Moriya (DM) interactions. Unlike conventional skyrmions—which maintain a uniform, centrosymmetric helicity determined by either Nél‐type or Bloch‐type spin textures—the Janus skyrmion incorporates both, presenting a “two-faced” helicity profile sharply divided along an engineered interface. This unique structure leads to distinctive static and dynamic properties, with implications for both fundamental topological magnetism and applied spintronics (Zhang et al., 9 Sep 2025).

1. Definition and Distinctive Characteristics

A Janus skyrmion is defined by its coexistence of two helicity structures within a single, topologically protected object of charge $|Q| = 1$:

• One half exhibits Nél‐type helicity (typically characterized by radial spin projections), while the other half exhibits Bloch‐type helicity (with spins winding tangentially in-plane).
• This bipartite nature is a direct result of interfacial engineering, where adjacent regions possess different signs and/or types of DM interaction.
• The topology is quantified in the standard way:

$$Q = \frac{1}{4\pi} \int \mathbf{m} \cdot \left( \frac{\partial \mathbf{m}}{\partial x} \times \frac{\partial \mathbf{m}}{\partial y} \right) dx\,dy$$

where $\mathbf{m}(\mathbf{r})$ is the local normalized magnetization.

• Visualizations reveal a heart-like in-plane magnetization pattern with a clear boundary between the two helicity domains.

2. Formation Mechanisms and Spin Texture Structure

Janus skyrmions are stabilized at the interface between two magnetic regions with engineered DM interactions, typically in ultrathin ferromagnetic heterostructures:

• The left domain ($x \in [0,20]$ nm) uses a Nél-type DM interaction ($D_N$), while the right domain ($x \in [20,40]$ nm) utilizes a Bloch-type DM interaction ($D_B$), both with $|D_N| = |D_B|$.
• The relevant micromagnetic energy density is:

$$\varepsilon = A|\nabla \mathbf{m}|^2 + D_N [m_z (\mathbf{m} \cdot \nabla) - (\nabla \cdot \mathbf{m}) m_z] + D_B [\mathbf{m} \cdot (\nabla \times \mathbf{m})] - K (\mathbf{n} \cdot \mathbf{m})^2 + \text{demagnetization/field terms}$$

with $A$ the exchange constant, $K$ perpendicular anisotropy, and $\mathbf{n}$ the film normal.

• The ground state features a continuous yet sharp transition in helicity across the engineered interface, producing a hybrid skyrmion whose left half exhibits Nél-type chirality and whose right half shows Bloch-type chirality, as confirmed by simulated spin textures (Zhang et al., 9 Sep 2025).

3. Static and Dynamic Properties

Janus skyrmions feature nontrivial static and dynamic behavior due to their helicity asymmetry:

• Field Response:
  • Under in-plane magnetic fields ($\pm x$ or $\pm y$), asymmetric forces on the two faces induce size and shape variations—distinct from symmetric skyrmions, which show minimal response under such fields.
  • Out-of-plane fields ($\pm z$) affect Janus skyrmion sizes in the conventional manner, shrinking or expanding the skyrmion core depending on field polarity.
• Current-Induced Motion:
  • When subjected to a vertical spin current (e.g., via spin Hall effect), Janus skyrmions exhibit strictly one-dimensional motion along the interface, described by:

  $$\mathbf{\tau}_d = u\, (\mathbf{m} \times \mathbf{p} \times \mathbf{m})$$

  where $u$ is proportional to current density, and $\mathbf{p}$ is spin polarization. - The velocity along the interface is given by

  $v_y = c \cos(\theta + \pi/4)$

  with $c$ a constant depending on current and damping, and $\theta$ the spin polarization orientation. - Crucially, there is no observable skyrmion Hall effect ($v_x \approx 0$), in contrast to conventional skyrmions which drift transversely to the current due to the Magnus force.

• Thermal Fluctuations:

  • Thermal noise induces only a one-dimensional random walk along the interface; this is in stark contrast to the two-dimensional Brownian motion typical of standard skyrmions.

4. Implications for Device Engineering

Janus skyrmions offer several advantages and new functionalities for magnetic memory and logic devices:

• Suppression of the Skyrmion Hall Effect: The strictly 1D motion enables high-fidelity, drift-free transport along racetrack interfaces, suitable for reliable information propagation.
• In-Plane Field Modulation: The ability to modulate skyrmion size and shape via accessible in-plane fields provides a tuning knob for applications in magnetic logic, sensors, or multi-state memories.
• Design Versatility: The control over DM interaction sign and magnitude—demonstrated in recent synthetic heterostructures—enables the creation of networks or pathways supporting Janus skyrmion motion.
• Potential for Multi-Functional Computing: The unique spin textures and motion control could be harnessed for unconventional logic or neural-network-like architectures, leveraging the asymmetric responses to current and field stimuli.

5. Challenges and Future Directions

The realization and utilization of Janus skyrmions face several fundamental and technological challenges:

• Interface Engineering: Creating magnetic bilayers or multilayers with atomically sharp and stable boundaries between regions of different DM interactions is essential; imperfections might smear the helicity transition or destabilize the Janus structure.
• Robustness and Pinning: Control over interfacial potential wells is needed to prevent detachment or unwarranted deformations under current/thermal drive.
• Experimental Verification: Direct imaging, e.g., spin-polarized scanning tunneling microscopy or Lorentz TEM, is required to confirm the simulated heart-like spin profiles and dynamic responses.
• Parameter Optimization: Future studies will need to optimize material parameters—exchange stiffness, DM magnitude, uniaxial anisotropy, damping, and current density—to maximize stability and control over Janus skyrmion motion.
• Device Integration: Exploration of approaches to multiplex or route Janus skyrmions, and determine thresholds for depinning or topological transitions, will be vital for practical deployment in memory and logic circuitry.

6. Connections to Related Topological and Composite Skyrmion Concepts

The Janus skyrmion concept is distinct but conceptually linked to several broader themes in topological spin textures:

• Skyrmion Bags: Composite skyrmion textures are known to possess internal structure capable of hosting multiple sub-skyrmions (Foster et al., 2018). The ability to engineer asymmetric internal texture (e.g., different arrangement or helicity on each side) suggests routes toward Janus-like composite skyrmions.
• Multimoment Skyrmions and Hierarchical Phases: Results from twisted bilayer systems indicate that spatially modulated interactions can create skyrmions with spatially distinct dipole moments or charge distributions (Ray et al., 2021). A plausible implication is that such settings could host Janus skyrmions with higher-order multi-face structure.
• Degeneracy and Control: The Janus nature—dual helicity—introduces unique degeneracy and independent degrees of freedom with respect to conventional skyrmion parameter locking (chirality, polarity), analogous to the energetic degeneracy in n-skyrmion vortex states (Zhang et al., 20 Mar 2025).

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In conclusion, Janus skyrmions expand the taxonomy of topological quasiparticles in chiral magnets by introducing spatially asymmetric helicity structures confined to engineered interfaces. This two-faced helicity leads to unconventional static, dynamic, and transport phenomena, positioning Janus skyrmions as promising candidates for robust, highly controllable spintronic devices and as a fertile ground for further exploration of emergent topological matter (Zhang et al., 9 Sep 2025).