Emergence of half-metallic ferromagnetism and valley polarization in transition metal substituted WSTe monolayer

TL;DR

This work tackles the challenge of achieving coexisting spintronic and valleytronic functionality in a 2D Janus TMDC by substituting WSTe with transition metals Fe, Mn, and Co. Using SOC-enabled DFT on a 4x4x1 supercell, it demonstrates intrinsic Rashba spin splitting ($\alpha_R$ ≈ 422 meVÅ along Γ–K) and Zeeman splitting (~403 meV at K) in pristine WSTe, and reveals strain-tunable spin splittings. TM substitution induces ferromagnetism and, at specific dopant contents (e.g., Fe at 6.25% and 18.75%, Mn at 25%, Co at 18.75–25%), half-metallic behavior with 100% spin polarization; valley polarization reaches up to $Δ_{KK'}$ ≈ 112 meV under 3% biaxial tensile strain, with Fe-WSTe showing the strongest tunability. Overall, the results position TM-WSTe as a versatile platform for strain-engineered spintronic and valleytronic devices, leveraging both Rashba/Zeeman spin textures and dopant-induced magnetism.

Abstract

Two-dimensional (2D) Janus materials hold a great importance in spintronic and valleytronic applications due to their unique lattice structures and emergent properties. They intrinsically exhibit both an in-plane inversion and out-of-plane mirror symmetry breakings, which offer a new degree of freedom to electrons in the material. One of the main limitations in the multifunctional applications of these materials is, however, that, they are usually non-magnetic in nature. Here, using first-principles calculations, we propose to induce magnetic degree of freedom in non-magnetic WSTe via doping with transition metal (TM) elements -- Fe, Mn and Co. Further, we comprehensively probe the electronic, spintronic and valleytronic properties in these systems. Our simulations predict intrinsic Rashba and Zeeman-type spin splitting in pristine WSTe. The obtained Rashba parameter is $\sim$ 422 meVÅ\; along the $Γ- K$ direction. Our study shows a strong dependence on uniaxial and biaxial strains where we observe an enhancement of $\sim$ 2.1\% with 3\% biaxial compressive strain. The electronic structure of TM-substituted WSTe reveals half-metallic nature for 6.25 and 18.75\% of Fe, 25\% of Mn, and 18.75 and 25\% of Co structures, which leads to 100\% spin polarization. The obtained values of valley polarization 65, 54.4 and 46.3 meV for 6.25\% of Fe, Mn and Co, respectively, are consistent with the literature data for other Janus materials. Further, our calculations show a strain dependent tunability of valley polarization, where we find an increasing (decreasing) trend with uniaxial and biaxial tensile (compressive) strains. We observed a maximum enhancement of $\sim$ 1.72\% for 6.25\% of Fe on application of 3\% biaxial tensile strain.

Figures (13)

• Figure 1: Top view of (a) crystal structure of pristine WSTe. panels (b), (c), (d), and (e) show the crystal structure of Fe-WSTe for 6.25, 12.5, 18.75, and 25% concentrations. Blue, pink and purple spheres represent W, S, and Fe atoms, respectively. (f) 2D Brillouin zone of WSTe monolayer showing equivalent high-symmetry points.
• Figure 2: The electronic band structure of WSTe, (a) without and (b) with spin-orbit coupling. The inset in panel (b) shows the Rashba splitting at the $\Gamma$ point of VBM. Panel (c) and (d) show the atom and orbital-projected density of states, respectively. The Fermi level is set to zero.
• Figure 3: The electronic band structure of WSTe unit cell, (a) without and (b) with spin-orbit coupling. (c) A schematic digram showing Rashba spin splitting. (d) The zoomed view of Rashba splitting at the $\Gamma$ point of VBM. (e) The zoomed view of Zeeman splitting at $K$-point of VBM.
• Figure 4: The spin texture of VBM around $\Gamma$ point in $k_x - k_y$ plane at 0.29 eV. Red and blue colors represent the spin up and spin down states, respectively.
• Figure 5: (a) The strain evolution of band gap. (b), (c) Rashba parameter under uniaxial and biaxial strains, respectively. (d) Zeeman spin splitting in WSTe.