[Paper Review] Hidden half-metallicity

San‐Dong Guo, Pan Zhou|arXiv (Cornell University)|Jan 12, 2026

2D Materials and Applications0 citations

TL;DR

The paper introduces the concept of hidden half-metallicity in net-zero-magnetization magnets, where each symmetry-related layer is half-metallic but their contributions cancel globally; it demonstrates this in PT-symmetric CrS2 bilayers and shows electric-field tuning to a fully compensated ferrimagnetic metal with persistent hidden half-metallicity.

ABSTRACT

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.

Motivation & Objective

• Motivate and define the concept of hidden half-metallicity in symmetry-protected, net-zero-magnetization magnets.
• Demonstrate, via first-principles calculations, a PT-symmetric CrS2 bilayer as a prototypical example.
• Show how interlayer coupling and external electric fields can realize and control hidden half-metallicity.
• Suggest generalization to altermagnets and outline design principles for hidden half-metallic phases.

Proposed method

• Perform first-principles density functional theory calculations with PAW and PBE GGA, including a Hubbard U correction for Cr and V.
• Construct PT-symmetric bilayer CrS2 by stacking CrS2 monolayers into AA, AB, and AC configurations and analyze interlayer magnetic couplings (FM vs AFM).
• Evaluate spin-resolved band structures and layer-resolved projections to identify local half-metallicity in each sector while enforcing global spin degeneracy.
• Apply out-of-plane electric fields to break PT symmetry and observe lifting of spin degeneracy and realization of hidden half-metallicity in a fully compensated ferrimagnetic metal.
• Extend discussion to hidden half-metallicity in bilayer altermagnets (e.g., VI3) via layer-resolved spin splitting.
• Use stacking engineering and interlayer spacing tuning to control interlayer magnetic order and spin polarization.

Experimental results

Research questions

• RQ1 Can hidden half-metallicity be realized in symmetry-enforced net-zero-magnetization magnets where global half-metallicity is forbidden by symmetry?
• RQ2 Can a bilayer built from a half-metallic monolayer host layer-resolved half-metallicity that cancels in total spin polarization?
• RQ3 How do interlayer coupling and stacking influence hidden half-metallicity, and can external electric fields enable tunable, layer-specific spin polarization?
• RQ4 Is hidden half-metallicity a general paradigm applicable to altermagnets and other symmetry-protected magnetic states?

Key findings

• AC-stacked PT-symmetric CrS2 bilayer with AFM interlayer coupling displays global spin-degeneracy and no conventional half-metallicity, yet each layer (A and B) is individually half-metallic with opposite spin polarization.
• Under interlayer AFM coupling and appropriate interlayer spacing, layer-resolved projections show spin-up crossing E_F in the lower layer and spin-down crossing E_F in the upper layer, realizing hidden half-metallicity.
• Applying an out-of-plane electric field breaks PT symmetry and lifts spin degeneracy, yielding a ferrimagnetic metal with zero net magnetization and distinct spin polarization per layer; spin-up mainly from lower layer and spin-down from upper layer.
• Ground-state AC-stacked bilayer CrS2 has interlayer AFM order at certain spacings; reducing spacing can favor FM couplings, enabling a switch between magnetic orders and preserving hidden half-metallicity in the appropriate phase.
• The concept is extended to bilayer altermagnets (e.g., AB’-stacked VI3) where layer-resolved bands remain half-metallic despite a globally non-half-metallic appearance.
• Overall, hidden half-metallicity provides a robust route to 100% spin polarization via layer degree of freedom, with electric-field tunability and potential generalization to other symmetry-protected magnetic states.