Néel-Vector-Orientation Induced Intrinsic Half-Metallicity in Two-Dimensional Altermagnets

Abstract

Whether a zero-moment antiferromagnet can host an intrinsic half-metallic ground state with a single-spin Fermi surface remains an open question in antiferromagnetic spintronics. Existing proposals in compensated magnets reach only transport analogues of this limit and do not realize a genuine AFM half-metallic ground state with a single-spin Fermi surface. Here we show that in two-dimensional altermagnets the Néel-vector orientation acts as an intrinsic knob for magnetic-space-group reduction that lifts the degeneracy between spin sectors. Using Janus monolayer Ta$_2$TeSeO as a realistic and clean platform and combining symmetry analysis with first-principles calculations, we demonstrate that rotating the Néel vector first breaks the relevant mirror symmetry, opening a gap in one spin sector of symmetry-related Weyl pairs, and then breaks the residual $C_{2z}$ symmetry, shifting the remaining Weyl cones in opposite energy directions so that a single spin sector retains a Fermi surface at the Fermi level. These two symmetry-lowering steps convert a compensated altermagnetic Weyl semimetal into an intrinsic AFM half-metal. The nearly degenerate in-plane magnetic anisotropy then enables reversible switching between the two spin channels using minute strain or weak anisotropic fields. Because this mechanism relies solely on Néel-vector-induced symmetry reduction, it provides a general low-power route to intrinsic half-metallicity in compensated altermagnetic Weyl semimetals.

Figures (4)

• Figure 1: (a) Mirror-symmetry operation $M_x$ with respect to a plane normal to the $x$ axis applied to magnetic moments for different Néel-vector orientations $\mathbf{n}(x, y, z$, and $d \equiv(110)$ ). Black arrows indicate the moment directions, while red circular arrows depict the direction of the equivalent loop current used to visualize the moment, blue lines/rectangles mark the mirror plane. Owing to the axial-vector character of the magnetic moment, $M_x$ leaves the $x$ component unchanged but reverses the components parallel to the mirror plane, i.e., $\left(m_x, m_y, m_z\right) \rightarrow\left(m_x,-m_y,-m_z\right)$. (b) Rotation-symmetry operation $C_{2z}$ applied to magnetic moments for different Néel-vector orientations $\mathbf{n}(x, y, z$, and $d \equiv(110)$ ).
• Figure 2: (a) Top and side views of monolayer Ta$_{2}$TeSeO. Yellow, red, blue, orange, and green spheres denote O atoms, spin-up-polarized Ta atoms, and spin-down-polarized Ta atoms, Se atoms, and Te atoms, respectively. (b) The first BZ of Ta$_{2}$TeSeO, with all Weyl points are shown. The green and blue areas are the two irreducible parts of the first BZ when SOC effect is not taken into account. (c) The first BZ of Ta$_{2}$TeSeO. The four areas in different color are the four irreducible parts of the first BZ when SOC effect is taken into account. (d) The band structure without SOC are shown. The up- and down-spin channels are depicted in red and blue, respectively.
• Figure 3: Spin-projected electronic band structures of Ta$_{2}$TeSeO for magnetic axis aligned along (a) the out-of-plane z direction, (b) the in-plane x direction, and (c) the in-plane diagonal (110) direction.
• Figure 4: Symmetry-selected spin-polarized transport under different Néel-vector orientations. Schematic distribution of spin-resolved Weyl cones on the Brillouin–zone boundary when the Néel vector is aligned (a) along $x$, (b) along $y$, (c) along $z$, and (d) along the in–plane diagonal $d\!\equiv\!(110)$. Red and orange colors denote the occupied and unoccupied states in spin-up channel, blue and cyan colors denote the occupied and unoccupied in spin-down channel.