N\'eel-Vector-Orientation Induced Intrinsic Half-Metallicity in Two-Dimensional Altermagnets

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 N\'eel-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 N\'eel 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 N\'eel-vector-induced symmetry reduction, it provides a general low-power route to intrinsic half-metallicity in compensated altermagnetic Weyl semimetals.