Hidden half-metallicity.

Half-metals, featuring ideal 100% spin polarization without spin-orbit coupling, are widely regarded as key materials for spintronic and quantum technologies. Nevertheless, in symmetry-enforced net-zero-magnetization magnets, the mandatory degeneracy of spin-up and spin-down densities of states rigorously forbids the formation of a half-metallic state. To circumvent this fundamental restriction, we introduce the concept of hidden half-metallicity, whereby the global electronic structure of a symmetry-enforced net-zero-magnetization magnet is non-half-metallic, while each of its two symmetry-related sectors is individually half-metallic, enabling robust 100% spin polarization through a layer degree of freedom. Crucially, the vanishing net magnetization of the entire system suppresses stray fields and magnetic instabilities, rendering the half-metallic functionality inherently more robust than in conventional ferromagnetic half-metals. Using first-principles calculations, we demonstrate this mechanism in a PT-symmetric bilayer CrS2, and further show that an external electric field drives the system into a seemingly forbidden fully compensated ferrimagnetic metal in which hidden half-metallicity persists. Finally, we briefly confirm the realization of hidden half-metallicity in altermagnets, establishing a general paradigm for stabilizing half-metallic behavior by embedding it in symmetry-protected hidden sectors and opening a new route toward the design and discovery of unprecedented half-metallic phases.