Hidden half-metallicity

Half-metals, featuring ideal 100\% spin polarization, are widely regarded as key materials for spintronic and quantum technologies; however, the half-metallic state is intrinsically fragile, as it relies on a delicate balance of exchange splitting and band filling and is therefore highly susceptible to disorder, external perturbations, and thermal effects. Here 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 $\mathrm{CrS_2}$, 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.