A new skyrmion topological transition driven by higher-order exchange interactions in Janus MnSeTe

Abstract

Two-dimensional (2D) van der Waals magnets offer a promising platform for pushing skyrmion technology to the single-layer limit with high tunability. While Dzyaloshinskii-Moriya interaction (DMI) is often recognized as central to skyrmion formation, their stability, collapse, and topological transition in 2D materials remain largely unexplored. In particular, the effect of higher-order exchange interactions (HOI) on these phenomena is unknown. Here, using first-principles calculations and atomistic spin simulations, we report a new topological transition generated by HOI, which we term 'ferric transition', in single-layer MnSeTe. Surprisingly, skyrmion stability and collapse remain largely unaffected by HOI due to the dominant role of DMI near the saddle point, whereas the Bloch point is strongly modified, giving rise to this novel transition. This mechanism is fundamentally distinct from the well-known radial and chimera transitions. Moreover, we predict that Janus MnSeTe exhibits remarkably high skyrmion energy barriers due to its strong DMI, among the highest reported for intrinsic 2D magnets. Our findings unveil an unexpected role of HOI in skyrmion topological transitions and establish Janus MnSeTe as a robust platform for 2D skyrmionics.

Figures (6)

• Figure 1: (a) Top (upper) and side (lower) views of the crystal structure of Janus MnSeTe. The black dashed lines indicate the 2D primitive cell. (b) Illustration of DMI vectors on the hexagonal lattice. For clarity, only the Mn atoms are shown. (c) Illustration of HOI constants on the hexagonal lattice. One of the six possible minimal hopping paths for the biquadratic interaction ($B_1$) is depicted by a black curve. One of the six possible minimal hopping triangular paths for the 4-spin 3-site interaction ($Y_1$) is depicted in yellow. Two of the twelve possible minimal hopping diamond-shaped paths for the 4-spin 4-site interaction ($K_1$) are depicted in gray.
• Figure 2: (a) Spin structures of multi-q states: two uudd states along $\overline{\Gamma \text{M}}$ and $\overline{\Gamma \text{K}}$, and the $3Q$ state at the $\overline{\text{M}}$ point along $\overline{\Gamma \text{M}}$ of the 2D BZ. (b) Energy dispersion of flat spin spirals ($E_{\text{ss}}$) for the MnSeTe monolayer along the high-symmetry path $\overline{\text{M} \Gamma \text{K} \text{M}}$ without SOC. The filled circles represent DFT total energies, while the solid lines are fits to the Heisenberg exchange interaction up to the tenth NN. The energies of the two uudd and the $3Q$ states are also denoted by red squares at the q values of the corresponding single-q states. (c) Energy contribution of flat spin spirals due to SOC ($\Delta E_{\text{SOC}}$), also referred to as the DMI contribution, is fitted up to the seventh NN. All energies are measured with respect to the FM state ($E_{\rm FM}$) at the $\overline{\Gamma}$ point.
• Figure 3: (a) Bogdanov (Boc) and experimental (Exp) skyrmion radii as functions of the applied magnetic field $B$ with (red) and without (black) HOI in Janus MnSeTe. The inset shows the radii difference with ($R_2$) and without ($R_1$) HOI, with the solid blue line for Boc and the dashed line for the Exp model. We observe that the Boc variation is smoother than the Exp one and hence numerically more stable. The inset further shows that the radii difference with and without HOI is very small, reaching maxima of about 0.15 nm at $B = 0.75$ T for the Boc model, and about 0.2 nm at $B = 0.4$ T and 0.22 nm at $B = 0.75$ T for the Exp model. (b) Total energy barriers of isolated skyrmions versus $B$ with and without HOI. The blue curve in the inset shows the difference between the two curves: black ($\Delta E_1$) and red ($\Delta E_2$).
• Figure 4: (a) MEP obtained for the transition from the skyrmion (sky) to the FM state through the SP for MnSeTe at $B = 0$ T with (red) and without (black) including HOI. The energies are shown w.r.t the skyrmion state. A zoom on the MEP near the SPs and the BPs is provided in the lower panel. SPs are marked as vertical dashed lines. BPs, identified via the magenta $y$-axis representing $Q$, are marked by vertical solid lines. (b) Corresponding spin textures (left panels) and topological charge densities (right panels) are also shown near SP and BP. Note that the topological phase transition occurs between BP-1 and BP. Interestingly, the skyrmion topological transition is radial without HOI, whereas it is ferric with HOI.
• Figure 5: (a) Decomposition of the MEP obtained for the skyrmion to FM transition in MnSeTe with and without including HOI at $B = 0$ T. The energy contributions of the different interactions are shown w.r.t the skyrmion state energy $E_\text{sky}$ (see legend). On the right axis the topological charge along the MEP is shown (magenta curves). SP and BP are marked by the vertical dashed lines. (b) Energy decomposition at the SP as shown in (a) with respect to the skyrmion state, both with and without HOI. Note that combined exchange and exchange denote the same quantity without HOI.