Very large Dzyaloshinskii-Moriya interaction in two-dimensional Janus manganese dichalcogenides and its application to realize skyrmion states

The Dzyaloshinskii-Moriya interaction (DMI), which only exists in noncentrosymmetric systems, is responsible for the formation of exotic chiral magnetic states. The absence of DMI in most two-dimensional (2D) magnetic materials is due to their intrinsic inversion symmetry. Here, using first-principles calculations, we demonstrate that significant DMI can be obtained in a series of Janus monolayers of manganese dichalcogenides $\mathrm{Mn}\mathit{XY}$ ($X,Y$= S, Se, Te, $X\ne Y$) in which the difference between $X$ and $Y$ on the opposite sides of Mn breaks the inversion symmetry. In particular, the DMI amplitudes of MnSeTe and MnSTe are comparable to those of state-of-the-art ferromagnet/heavy metal heterostructures. In addition, by performing Monte Carlo simulations, we find that at low temperatures the ground states of the MnSeTe and MnSTe monolayers can transform from ferromagnetic states with wormlike magnetic domains into the skyrmion states by applying an external magnetic field. At increasing temperature, the skyrmion states start fluctuating above 50 K before an evolution to a completely disordered structure at higher temperature. The present results pave the way for new device concepts utilizing chiral magnetic structures in specially designed 2D ferromagnetic materials.

Figure 1

The top (a) and side (b) views of the crystal structure and the phonon spectra (c–e) for the $\mathrm{Mn}\mathit{XY}$ ($X,Y=\mathrm{S}$, Se, Te, $X\ne Y$) monolayers. The dashed lines in (a) show the primitive cell. The yellow vectors in (b) indicate the two spin configurations with opposite chirality used to extract the in-plane DMI parameters.

Figure 2

The calculated in-plane DMI parameters $d_{//}$ of the Janus $\mathrm{Mn}\mathit{XY}$ monolayers. Here $d_{//}<0\left(d_{//}>0\right)$ favors spin canting with clockwise (counterclockwise) chirality. The inset shows the DMI vectors ${\mathit{D}}_{\mathrm{ij}}$ (blue vectors) between the nearest neighbors of Mn atoms (red balls). For clarity, the chalcogen atoms are omitted.

Figure 3

Atomic-layer-resolved localization of the DMI associated SOC energy $\mathrm{\Delta }{E}_{\mathrm{soc}}$ for the Janus $\mathrm{Mn}\mathit{XY}$ monolayers. As can be seen $\mathrm{\Delta }{E}_{\mathrm{soc}}$ is dominated by heavy chalcogen atom $Y$.

Figure 4

Spin textures for (a–c) MnSeTe and (d–f) MnSTe monolayers in real-space. The corresponding temperatures and external fields for the simulations are labeled inside each panel. The color map indicates the out-of-plane spin component of Mn atoms.

Figure 5

The topological charge $Q$ per supercell of MnSTe monolayers as a function of temperature and external magnetic field, calculated from MC simulations.