Janus monolayers of magnetic transition metal dichalcogenides as an all-in-one platform for spin-orbit torque

We theoretically predict that vanadium-based Janus dichalcogenide monolayers constitute an ideal platform for spin-orbit torque memories. Using first-principles calculations, we demonstrate that magnetic exchange and magnetic anisotropy energies are higher for heavier chalcogen atoms, while the broken inversion symmetry in the Janus form leads to the emergence of Rashba-like spin-orbit coupling. The spin-orbit torque efficiency is evaluated using optimized quantum transport methodology and found to be comparable to heavy nonmagnetic metals. The coexistence of magnetism and spin-orbit coupling in such materials with tunable Fermi-level opens new possibilities for monitoring magnetization dynamics in the perspective of nonvolatile magnetic random access memories.

Figure 1

Relative changes of the magnetic anisotropy $E_A$, exchange coupling $J$, magnetic moment ${\mu }_s$, Bader charge $e_B$, and orbital angular momentum $L$ in $\mathrm{V}{\mathrm{Te}}_2$ on substitution of Te atoms by sulfur (S) and selenium (Se) and also on the transformation from the Janus form VSeTe into VSSe and VSeTe.

Figure 2

Phonon band diagrams of (a) VSSe, (b) VSTe, and (c) VSeTe monolayers.

Figure 3

Projected density of states of [(a) and (b)] $\mathrm{V}{\mathrm{Te}}_2$ and [(c) and (d)] VSeTe. The left panels display the projection of Se-$p$, Te-$p$, and V-$d$ orbitals, whereas the right panels display the orbital-resolved projection on the V-$d$ orbitals. Positive (negative) density of states stands for up (down) spins.

Figure 4

Left panels: Spin-resolved band structure of 1T-VSeTe projected on (a) $p$-Se orbitals, (b) $p$-Te orbitals, and (c) $d$-V orbitals. The three subpanels refer to $S_x$, $S_y$, and $S_z$ components of the spin density; red (blue) indicates a positive (negative) value. Right panels: $S_x$ component of the spin texture in momentum space projected on (d) $p$-Se orbitals, (e) $p$-Te orbitals, and (f) $d$-V orbitals. During these calculations, the magnetization is set perpendicular to the plane.

Figure 5

Orbital hybridization: Kohn-Sham orbitals of (a) 1T-$\mathrm{V}{\mathrm{Se}}_2$, (b) 1T-$\mathrm{V}{\mathrm{Te}}_2$, and (c) 1T-VSeTe.

Figure 6

(a) Schematics of the tight-binding model of a Janus monolayer with vanadium (red) surrounded by dissimilar chalcogens (yellow and blue). The phases acquired by the electron Bloch state when hopping from vanadium to the chalcogen element are given. (b) View of the model in the ($x,y$) plane and (c) in the ($y,z$) plane. In the latter the angles formed by the top and bottom bonds with $\mathbf{z}$ are given. (d) Representation of the spin-orbit coupling scheme between the three $d$ orbitals considered in the model.

Figure 7

Current-driven torque efficiency as a function of the transport energy, when the magnetization lies perpendicular to the plane (top) and in plane (bottom). The figure shows both the dampinglike (orange) and fieldlike (blue) components.