Topological spin textures in an antiferromagnetic monolayer

TL;DR

The study demonstrates that topological spin textures can be stabilized in an intrinsic antiferromagnetic Mn monolayer on Ta(110) by engineering a lateral boundary with a Mn double layer. Spin-polarized STM reveals a cycloidal AFM spin spiral in the Mn ML and a collinear AFM state in the Mn DL, with boundary frustration giving rise to AFM half-skyrmions of charge $|\frac{1}{2}|$ per sublattice; depending on the phase relation of adjacent DL regions, these half-loops either form trivial domain walls or combine into AFM skyrmions with charge $-1$. First-principles calculations show a cycloidal tendency in the Mn ML driven by DMI and MAE, while the DL remains largely collinear with negligible DMI, and micromagnetic simulations reproduce the boundary-induced half-skyrmions and their topological properties. This lateral-heterostructure engineering provides a pathway to design and control AFM topological textures in intrinsic AFMs, offering potential advantages for low-dissipation spintronic applications.

Abstract

Topological spin structures such as magnetic skyrmions are of fundamental interest and promising for various types of applications in spintronics. Skyrmions have been predicted to emerge also in antiferromagnetic materials where they exhibit superior transport properties. They were experimentally revealed in synthetic antiferromagnets, however, still remain elusive in intrinsic antiferromagnets. Here, we demonstrate the stabilization of topological spin structures in an antiferromagnetic monolayer. Using spin-polarized scanning tunneling microscopy, we observe an antiferromagnetic spin spiral in the Mn monolayer and a collinear antiferromagnetic state in the Mn double-layer on Ta(110). Near the boundary to the double-layer half-skyrmions form in the monolayer as revealed in combination with first-principles calculations and micromagnetic simulations. Our work shows how the topological state in antiferromagnetic material systems can be controlled by the configuration within a lateral heterostructure, resulting in trivial non-coplanar states or antiferromagnetic skyrmions.

Figures (7)

• Figure 1: AFM spin spiral ground state of the Mn ML on Ta(110).a, SP-STM current map of the AFM spin spiral in the Mn ML. $U=-15$ mV, $I=4$ nA. b, Sketch of an AFM cycloidal spin spiral. Each cone represents the position and magnetic moment of an atom, with the color encoding the magnetization in the [001]-[110]-plane. c, Line profile of the AFM spin spiral along the propagation direction as marked by the black line in a. 18 line-profiles from a were averaged for this plot.
• Figure 2: Magnetic state of the Mn ML and Mn DL on Ta(110).a, Constant-current STM topography image of 1.25 atomic layers of Mn on Ta(110). $U=+600$ mV, $I=500$ pA. b,c, Constant-current SP-STM current images of a Mn ML terrace with a region of Mn DL grown on top (bottom left), measured with two different tip magnetization directions. $U=-15$ mV, b: $I=2$ nA, c: $I=5$ nA.
• Figure 3: Half-loop formation at the DL/ML boundary.a, Constant-current STM image of a sample with 1.25 atomic layers of Mn on Ta(110). $U=+200$ mV, $I=300$ pA. b, SP-STM current image of a region of DL (top) growing on the ML (bottom). Scan area marked in a. Insets on the right show enlarged views of the regions indicated by white rectangles. $U=-15$ mV, $I=1$ nA. c, Sketch of Mn atoms (red and blue) on Ta (black). Red and blue color encode the spin direction of the Mn atoms. The white line follows a path similar to the line in b. Some Mn atoms in the DL were removed in the front. d, SP-STM current image of a ML region (left) bordering to DL across a buried Ta step edge (right). Scan region marked in a. $U=-15$ mV, $I=1$ nA. e, SP-STM current image of a region of ML bordering a DL island growing on top (left side). Scan region marked in a. $U=-15$ mV, $I=3$ nA. For b,d,e high frequency noise was removed. Raw data can be seen in the Supplementary Information.
• Figure 4: Effect of the phase relation between DL regions on the magnetic state in the ML.a, SP-STM current image of a sample with 1.25 atomic layers of Mn. The red lines indicate areas with mainly IP spin alignment. The green/red circle indicate favorable/unfavorable regions at the DL/ML interface. $U=-15$ mV, $I=1$ nA. The high frequency noise was removed (see Supplementary Information for raw data). b, SP-STM current image of a large ML area with regions of DL at top right and bottom left. The red lines indicate the regions with mainly IP spin alignment. $U=-10$ mV, $I=1$ nA. c,d, Enlarged views of the regions indicated by the black rectangles on the green/pink line in b.
• Figure 5: DFT energy dispersions of spin spirals in a Mn ML on Ta(110).a, Energy dispersion of spin spirals in the two-dimensional Brillouin zone in the Mn monolayer on Ta(110). Color-coded circles represent DFT results, while the coloring of the background represents an interpolation. b and c show zoomed-in regions along the $\overline{\text{N}\Gamma}$ and $\overline{\text{N}\text{P}}$ paths around the $\overline{\text{N}}$-point, respectively. The energy is displayed over the period of the spiral distortion $\lambda$ with respect to the $\overline{\text{N}}$-point. The DFT data, represented as circles, is fitted with parabolas representing the fit with a micromagnetic model. Data without SOC is blue, data with SOC in first order perturbation theory, which can be attributed to DMI, is shown in yellow.