Skyrmions in 2D chiral magnets with noncollinear ground states stabilized by higher-order interactions

Magnetic skyrmions are intriguing topological spin textures that have attracted great attention due to their potential for future spintronic devices. Skyrmions have so far been explored in different magnetic materials, such as ferromagnets, antiferromagnets, and ferrimagnets. Here, we propose a new type of unconventional skyrmions stabilized in noncollinear magnets. Using first-principles calculations and atomistic spin simulations, we demonstrate that a noncollinear ground state can be stabilized in Rh/Co and Pd/Co atomic bilayers on the Re(0001) surface by four spin exchange interactions, although Co – a material often used in applications – is a prototypical ferromagnet with strong pairwise exchange interaction. We further show that unconventional skyrmion lattices and isolated skyrmions can emerge on this noncollinear magnetic background. Transition-state theory calculations reveal that these metastable skyrmions are protected by large energy barriers, suggesting that they could be observed in experiments. These unconventional types of skyrmions in noncollinear magnets might open new possibilities for topological spin transport or magnet-superconductor hybrid systems.

💡 Research Summary

In this work the authors investigate the emergence of unconventional magnetic skyrmions on a non‑collinear magnetic background that is itself stabilized by higher‑order spin interactions. Using density‑functional theory (DFT) they calculate the full set of magnetic parameters for Rh/Co and Pd/Co atomic bilayers grown on the Re(0001) surface: Heisenberg exchange (Jij), Dzyaloshinskii‑Moriya interaction (Dij), magnetocrystalline anisotropy (K), and three types of higher‑order exchange (HOI) – biquadratic (B1), three‑site four‑spin (Y1) and four‑site four‑spin (K1). The key finding is that the four‑site four‑spin term is sizable and positive, which counteracts the strong ferromagnetic (FM) pairwise exchange of Co and drives the system into a non‑collinear ground state (nc‑GS) even at zero magnetic field.

When only pairwise interactions are considered the magnetic phase diagram follows the familiar sequence: a spin‑spiral (SS) at low field, a skyrmion lattice (SkX) at intermediate field, and a FM state at high field. Inclusion of the HOI dramatically reshapes the diagram. The SS relaxes into a superposition of two single‑Q spirals (SS‑2Q), and at about 1.7 T a three‑sublattice non‑collinear state becomes the lowest‑energy configuration. In this nc‑GS two sublattices have spins almost out‑of‑plane (θ≈17°) while the third is tilted toward the plane (θ≈39°). Fourier analysis shows a dominant Γ‑point peak together with weaker peaks at the ±K points, reflecting a uniform component plus a residual spiral modulation.

On top of this complex background the DMI and frustrated exchange still generate chiral textures. The authors construct isolated skyrmions (nc‑ISk) and a skyrmion lattice (nc‑SkX) on the nc‑GS using atomistic spin dynamics. Minimum‑energy‑path calculations based on the geodesic nudged elastic band (GNEB) method reveal large energy barriers (≈20–30 meV) for annihilation, comparable to or larger than those for skyrmions on a simple FM substrate. Importantly, the topological charge of each skyrmion is preserved despite the substantial reorientation of the surrounding spins, indicating that the skyrmion’s topology is robust against the underlying non‑collinearity.

The paper therefore establishes three major insights. First, higher‑order exchange interactions, especially the four‑site term, can destabilize the ferromagnetic state of a traditionally strong ferromagnet like Co and create a non‑collinear ground state. Second, a non‑collinear background does not preclude skyrmion formation; rather, DMI and exchange frustration cooperate to stabilize skyrmions that inherit the complex spin texture of the substrate. Third, the resulting skyrmions are metastable with sizable energy barriers, making them accessible to experimental detection, for example by spin‑polarized scanning tunneling microscopy (SP‑STM).

Beyond the immediate findings, the work opens new avenues for spintronic and quantum technologies. The three‑sublattice structure may enable novel spin‑wave modes, unconventional current‑driven dynamics, or coupling to superconductors that could host Majorana bound states. Moreover, the demonstration that higher‑order interactions can be harnessed to engineer exotic magnetic textures suggests that material design strategies should routinely include such terms, especially in ultrathin transition‑metal films where electronic confinement enhances multi‑spin processes. Overall, the study broadens the family of topological quasiparticles beyond ferromagnetic, antiferromagnetic, and ferrimagnetic hosts, pointing to a rich landscape of skyrmion physics in non‑collinear magnets.