🔬 materials science

Topological spin textures in an antiferromagnetic monolayer

This study demonstrates the stabilization of topological spin textures, specifically half-antiferromagnetic skyrmions, in a manganese monolayer on a Ta(110) substrate by utilizing lateral heterostructures to control magnetic configurations near double-layer boundaries.

Original authors: Felix Zahner, Tim Drevelow, Roberto Lo Conte, Roland Wiesendanger, Stefan Heinze, Kirsten von Bergmann

Published 2026-11-02

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CC BY 4.0

Original authors: Felix Zahner, Tim Drevelow, Roberto Lo Conte, Roland Wiesendanger, Stefan Heinze, Kirsten von Bergmann

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Magnetism is often thought of as a simple force that makes a compass needle point north or holds a note to a refrigerator door. But at the scale of individual atoms, magnetism becomes a complex landscape where tiny magnetic arrows, called spins, can arrange themselves into intricate patterns. For decades, scientists have been fascinated by a specific type of pattern known as a skyrmion. Imagine a knot tied in a rope; in a magnetic material, a skyrmion is a similar knot where the spins twist and turn to form a stable, particle-like structure. These knots are not just theoretical curiosities; they are promising candidates for the next generation of data storage because they are small, stable, and can be moved with very little energy. While these structures have been found in materials where the magnetic arrows generally point in the same direction, scientists have long wondered if they could exist in a different kind of material called an antiferromagnet. In these materials, neighboring magnetic arrows point in opposite directions, canceling each other out. This makes them invisible to external magnetic fields and potentially faster for computing, but it also makes creating and controlling these magnetic knots much harder.

A team of researchers from Germany and the Netherlands has now demonstrated how to stabilize these topological spin structures within a single atomic layer of an antiferromagnetic material. Working with a surface made of tantalum, they deposited manganese atoms to create a thin film that was either one atom thick or two atoms thick. Using a highly sensitive microscope that can detect the magnetic orientation of individual atoms, they observed that the single-layer film naturally formed a spiraling pattern of magnetic spins. In contrast, the two-layer film settled into a simple, straight-line arrangement where the spins pointed up and down in a rigid, alternating fashion. The breakthrough occurred at the boundary where these two different layers met. Here, the rigid order of the two-layer film clashed with the spiraling tendency of the single layer, creating a zone of magnetic frustration. Instead of breaking, the spins in the single layer bent and twisted to accommodate this conflict, forming a distinct, localized structure that the researchers identified as being equivalent to half of an antiferromagnetic skyrmion.

The researchers found that the behavior of these magnetic knots depended entirely on how the surrounding layers were arranged. When a strip of the single-layer film was sandwiched between two sections of the two-layer film, the outcome changed based on the alignment of the outer sections. If the outer sections were aligned in opposite phases, the magnetic twists in the middle formed a single, winding line that canceled itself out, resulting in a magnetically neutral state. However, if the outer sections were aligned in the same phase, the twists on both sides of the strip connected to form a closed, elliptical loop. This loop acted as a complete, stable skyrmion, carrying a specific topological charge that makes it robust against disturbances. The team confirmed these observations by combining their microscope images with detailed computer simulations and calculations of the atomic forces at play. These calculations revealed that the unique properties of the surface, specifically the interaction between the manganese atoms and the underlying tantalum, were responsible for forcing the spins into these specific shapes.

This work is significant because it shows a new way to create and control magnetic knots in materials that were previously thought to be too difficult to manipulate. Unlike their counterparts in ferromagnetic materials, these antiferromagnetic skyrmions do not suffer from a sideways drift when moved, a problem that often causes them to crash into the edges of a device and disappear. By engineering the boundaries between different magnetic layers, the researchers showed that they could switch these skyrmions on and off simply by changing the configuration of the surrounding material. This suggests a path forward for building information storage devices that are not only denser and faster but also more energy-efficient. The study proves that by carefully designing the atomic landscape, scientists can coax these elusive magnetic particles into existence, opening a door to a new class of spintronic technologies that operate without the interference of stray magnetic fields.

Technical Summary: Topological Spin Textures in an Antiferromagnetic Monolayer

Problem Statement
Magnetic skyrmions and related topological spin textures are of significant interest for spintronic applications due to their potential for information storage. While skyrmions in ferromagnetic (FM) backgrounds are well-studied and can be manipulated via external fields and currents, they suffer from the skyrmion Hall effect, which causes transversal motion and potential annihilation at boundaries. Antiferromagnetic (AFM) skyrmions offer theoretical advantages, including suppressed stray fields, faster dynamics, and the elimination of the skyrmion Hall effect. However, while AFM skyrmions have been observed in synthetic antiferromagnets (coupled FM layers), they have remained elusive in intrinsic atomic antiferromagnets. A primary challenge is that, unlike FM systems, applying external magnetic fields to intrinsic AFMs does not typically generate localized topological states. Furthermore, theoretical models often assume an out-of-plane magnetic background, whereas many intrinsic AFMs exhibit in-plane or complex spin structures.

Methodology
The authors employed a multi-faceted approach combining experimental characterization, first-principles calculations, and micromagnetic simulations:

Experimental: Spin-polarized scanning tunneling microscopy (SP-STM) was used to image Mn monolayers (ML) and double-layers (DL) grown on a Ta(110) substrate at 4.2 K. Measurements utilized magnetic Cr-bulk tips with varying magnetization directions to distinguish between in-plane and out-of-plane magnetic components. Both constant-current height and current channel maps were analyzed to resolve magnetic contrast.
First-Principles Calculations: Density Functional Theory (DFT) calculations were performed using VASP and FLEUR codes to determine the magnetic ground states, energy dispersions of spin spirals, Dzyaloshinskii-Moriya interaction (DMI) strengths, and magnetocrystalline anisotropy energy (MAE) for Mn ML and DL on Ta(110).
Micromagnetic Simulations: Using the Mumax3 code, the authors simulated the system by mapping the AFM sublattices to a ferromagnetic auxiliary system to model the energy landscape. These simulations incorporated DFT-derived parameters for exchange stiffness, DMI, and MAE to investigate the behavior of spin textures at the boundaries between ML and DL regions.

Key Results

Distinct Magnetic Ground States: The Mn double-layer on Ta(110) exhibits a collinear, out-of-plane AFM state with $c(2 \times 2)$ order. In contrast, the Mn monolayer hosts an AFM spin spiral with a propagation vector along the [001] direction. DFT reveals that while DMI induces a cycloidal rotation, the specific propagation direction is governed by a hard-axis MAE along [110], favoring modulation along [001].
Boundary-Induced Frustration: At the interface between the Mn ML and Mn DL, the collinear out-of-plane order of the DL suppresses the spin spiral in the adjacent ML. This creates magnetic frustration.
Formation of Half-Loops: To resolve this frustration, the system forms localized non-coplanar spin textures in the ML near the DL boundary. These structures appear as "half-loops" where in-plane spins connect regions of out-of-plane magnetization.
Topological Control via Lateral Heterostructures: The topology of these structures depends on the phase relationship between DL regions sandwiching an ML region:
- Anti-phase DLs: The half-loops on opposite sides connect to form a serpentine in-plane domain wall. The total topological charge is $Q=0$ (trivial).
- In-phase DLs: The half-loops pair up to form closed, elongated elliptical loops. Each loop carries a topological charge of $|Q|=1$ , equivalent to an AFM skyrmion.
Simulation Validation: Micromagnetic simulations, validated against SP-STM images, confirm that these half-loop structures are stable and resemble half of an AFM skyrmion. The simulated SP-STM contrast matches experimental observations, including the vanishing contrast for in-plane magnetization and the specific stripe patterns.

Significance and Claims
The paper demonstrates a mechanism to stabilize topological spin textures in intrinsic atomic antiferromagnets without the need for external magnetic fields. By utilizing lateral heterostructures (ML/DL boundaries), the authors show that the configuration of the surrounding magnetic layers can control the emergence of trivial non-coplanar states or topologically protected AFM skyrmions.

The authors claim that this approach addresses two central limitations of AFM skyrmions:

It provides a route to induce these states without external magnetic fields, which are ineffective for creating localized topological states in intrinsic AFMs.
It suggests that these skyrmions can be metastable or switchable. The simulations indicate a degeneracy between the presence and absence of an AFM skyrmion for specific widths of the central ML region, implying that these states could be written and deleted by manipulating the boundary conditions, similar to FM skyrmions under magnetic fields.

The work establishes that the interplay between competing magnetic states in lateral heterostructures can be exploited to engineer and control topological spin textures in antiferromagnetic materials.

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