Electrical switchable room-temperature magnetic skyrmions in multiferroic MXene

Two-dimensional (2D) transition metal carbides and nitrides (MXene) host a plethora of interests, e.g., energy storage and conversions, and intriguing quantum phenomena ranging from electronic topological properties to superconductivity. However, topological magnetism is still a missing piece in MXenes. Here, via first-principles calculations, we validate the existence of significant Dzyaloshinskii–Moriya interaction (DMI) in 2D multiferroic MXenes, where the magnitude of DMI scales with the strength of electric dipole of the MXenes. Moreover, using atomistic spin model simulations, we demonstrate that room-temperature (RT) magnetic skyrmions can be spontaneously stabilized at monolayer ${\mathrm{Co}}_2\mathrm{C}{\mathrm{F}}_2$. The chirality of DMI can be tuned via ferroelectricity effect and, consequently, the skyrmions. We further show that a proper van der Waals substrate can effectively enhance the magnetic anisotropy of ${\mathrm{Co}}_2\mathrm{C}{\mathrm{F}}_2$ through interlayer charge transfer, while the tunable DMI and RT skyrmions can also be maintained. These results prove that MXenes and their heterostructure can be promising candidates in fabricating ultrathin skyrmionic devices and pave the way for the RT electrically controllable skyrmions toward spintronic applications.

Figure 1

(a), (b) Top and front view of multiferroic $M_{2}X{T}_2$ MXenes with different electric polarization orientations ($M=\mathrm{Co}$ and Ni, $X=\mathrm{C}$ and N, and $T=\mathrm{F}$ and O). (c), (d) spin charge density of ML ${\mathrm{Co}}_2\mathrm{C}{\mathrm{F}}_2$ at an isosurface value of $0.04e/{\AA }^3$. The gray vectors in (c) and (d) indicate the two spin configurations with opposite chirality used to extract the in-plane DMI parameters with up and down electric polarization, respectively.

Figure 2

(a) Calculated DMI strength $d^{\mathrm{tot}}$ of ${\mathrm{Co}}_2\mathrm{C}{\mathrm{F}}_2, {\mathrm{Co}}_2\mathrm{N}{\mathrm{F}}_2$, and ${\mathrm{Ni}}_2\mathrm{N}{\mathrm{O}}_2$ MXenes. The black solid squares, blue balls, and green arrows represent the dipole moment, magnetic atoms, and the DMI vectors, respectively. (b) Atom-resolved localization of the DMI associated SOC energy $\mathrm{\Delta }{E}_{\mathrm{SOC}}$ in ML MXenes. One should notice that all the values used here are positive for convenience. (c) Schematic diagram of potential difference (shallow red region) acts as effective SOC-active site for canting adjacent spins and the chirality reversal of DMI vectors (${\vec{D}}_{ij}$) when flipping the polarization.

Figure 3

Spin textures for ML ${\mathrm{Co}}_2\mathrm{C}{\mathrm{F}}_2$ with (a), (c) $P\uparrow$, and (b), (d) $P\downarrow$ state in real space without applying external field. The color map indicates the out-of-plane spin component of Co atoms. $M$ and $P$ denote the magnetization and electric polarization, respectively.

Figure 4

The charge density difference and spin textures of (a) ${\mathrm{Co}}_2\mathrm{C}{\mathrm{F}}_2/{\mathrm{Ti}}_2\mathrm{O}{\mathrm{F}}_2\text{−}P\uparrow$ and (b) ${\mathrm{Co}}_2\mathrm{C}{\mathrm{F}}_2/{\mathrm{Ti}}_2\mathrm{O}{\mathrm{F}}_2\text{−}P\downarrow$. Yellow (cyan) color isosurface (isovalue of $0.001e/{\AA }^3$) represents the charge accumulation (depletion). The relaxed magnetic skyrmion states in real space under zero external magnetic field are shown, wherein the zoom-in view of a local skyrmion with different helicity is highlighted. The color map indicates the out-of-plane spin component of Co atoms. (c) Comparison of skyrmion phase temperature in ${\mathrm{Co}}_2\mathrm{C}{\mathrm{F}}_2$ and other material systems. Except for ${\mathrm{Co}}_2\mathrm{C}{\mathrm{F}}_2$, the other data are taken from other reports.