Giant spin-Hall and tunneling magnetoresistance effects based on a two-dimensional nonrelativistic antiferromagnetic metal

The emergence of nonrelativistic band spin splitting in fully compensated collinear antiferromagnets provides an intriguing platform for investigating nontrivial spin-dependent phenomena. Here, we explore the relationship between magnetic symmetry and electronic states of two-dimensional (2D) antiferromagnets and demonstrate that nonrelativistic and giant band spin splitting and topological electronic features emerge in room-temperature antiferromagnetic metal ${\mathrm{V}}_2{\mathrm{Te}}_2\mathrm{O}$ that is sufficiently stable to be exfoliated from the layered compound. More interestingly, the anisotropic spin-momentum coupling of monolayer ${\mathrm{V}}_2{\mathrm{Te}}_2\mathrm{O}$ gives rise to the spin-Hall effect where the spin-charge conversion ratio is expected to be over 30%, and two fully spin-polarized and separated conduction channels with opposite spin polarization make it an ideal candidate for an electrode that is applied in the antiferromagnetic tunneling junction with large magnetoresistance. Our work elucidates the unexplored potential of 2D antiferromagnetic metals with nonrelativistic band spin splitting for their practical applications in spintronics.

Figure 1

(a) The schematic of a planar square lattice with $G$-type antiferromagnetism. The space group of this square lattice is $P4$/mmm. The Néel vector is set to be perpendicular to the material plane. Cartoons of band structure of AFM monolayer with (b) $PT{\tau }_{1/2}$ symmetry and (c), (d) without $PT{\tau }_{1/2}$ symmetry. In a 2D BZ, (c) $M_{\varphi }{\tau }_{1/2}$ symmetry makes the eigenstate at $k$ and $M_{\varphi }\mathit{k}$ exhibit the opposite spin, and (d) $M_z$ mirror symmetry could favor $M_z$-protected nodal loops.

Figure 2

The (a) side and (b) top view of crystal structure of monolayer VTO. Brown, dark green, and red balls represent V, Te, and O, respectively. Dark red and blue zones in (b) represent spin-up and spin-down magnetization densities. (c) Band structure and (d) Fermi surface. For electronic states around the $X$ or $Y$ point, the fully spin-polarized Fermi surface consists of two ellipses, where the flat one is dominated by the $p_y$ or $p_x$ orbital, and the fat one is dominated by the $d_{yz}$ or $d_{xz}$ orbital (Fig. S4 [58]). (e) The shape of nodal loops. The color map indicates the local gap between two crossing bands. In (c)–(e), red and blue correspond to spin-up and spin-down electronic states, respectively. DFT results elucidate that there is (b) anisotropic magnetization density in real-space and (c), (d) anisotropic spin-momentum coupling in the BZ.

Figure 3

(a) The electrical field $E$ induced the Fermi surface shift for monolayer VTO with $\sqrt{2}\times \sqrt{2}\times 1$ supercell. As $E$ is along the [100] direction, the combination of spin-up and spin-down current generates the longitudinal charge current and transverse spin current. (b) Illustration of $E$-induced spin current in real-space. (c) Charge and spin conductivities obtained from the Kubo formula.

Figure 4

The spin-charge conversion ratio ${\sigma }_{xy}^{z,\mathrm{odd}}/\sigma$ for monolayer VTO as functions of (a) $\mathrm{\Gamma }$ magnitude and (b) energy level.

Figure 5

The schematic of VTO/barrier/VTO AFMTJ with (a) parallel and (b) antiparallel spin configurations. Illustration of the tunneling process based on calculated density of states, for (c) parallel and (d) antiparallel states. The chosen energy level for density of states ranges from −0.38 to 0.54 eV where the spin-up and spin-down states are totally separated in the BZ. At the Fermi level, spin-down (-up) states are localized around the $X$ ($Y$) point [see labels in (c)], for monolayer VTO with the spin configuration shown in (a). Once the Néel vector is reversed, the positions of spin-down and spin-up states in the Brillouin zone are consequently switched [see labels in (d)].

Figure 6

The schematic structure of Ag/VTO/BiOCl/VTO/Ag AFMTJ with (a) parallel and (b) antiparallel alignment of Néel vectors of VTO. Brown, dark green, light green, red, purple, and silver balls represent V, Te, Cl, O, Bi, and Ag, respectively. The spin-resolved transition spectra in the BZ of AFMTJ with the (c) parallel and (d) antiparallel state.