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.

Figures (5)

• Figure 1: (Color online)(a):this system belongs to the class of symmetry-enforced net-zero-magnetization magnets, encompassing primarily $PT$-antiferromagnet and altermagnet. The system is composed of three sectors: A, B, and C. The electronic states near the Fermi level are primarily contributed by A and B, while those from C lie far from the Fermi energy. (b): the sectors A and B are half-metallic, exhibiting opposite spin polarizations. (c): viewed globally, the energy band is either spin-degenerate or exhibits altermagnetic spin-splitting. In (b, c), the blue, red, and purple curves denote the spin-up, spin-down, and spin-degenerate bands, respectively, while the horizontal black dashed line indicates the Fermi level.
• Figure 2: (Color online) For $\mathrm{CrS_2}$, (a, b , c): the crystal structures and energy band structures of monolayer; (d, e, f): the AA-, AB-, and AC-stacked bilayer crystal structures. In (c), the spin-up and spin-down channels are depicted in blue and red.
• Figure 3: (Color online) For AC-stacked bilayer $\mathrm{CrS_2}$, (a): the schematic illustration of tuning the interlayer spacing between the lower and upper layers; (b): the energy difference between FM and AFM interlayer couplings versus $d-d_0$, where the $d_0$ denotes the equilibrium interlayer distance; (c, d, e): the total energy band structure along with the spin-resolved projections onto the lower and upper layers with the equilibrium interlayer distance; (f, g, h): the total energy band structure along with the spin-resolved projections onto the lower and upper layers with $d-d_0$ being 0.48 $\mathrm{{\AA}}$. In ( c, d, e, f, g, h), the blue, red, and purple curves denote the spin-up, spin-down, and spin-degenerate bands, respectively. In (d, e, g, h), the weighting coefficient is proportional to the circle size.
• Figure 4: (Color online) For AC-stacked bilayer $\mathrm{CrS_2}$ with $d-d_0$ being 0.48 $\mathrm{{\AA}}$, (a, b, c): the total energy band structure along with the spin-resolved projections onto the lower and upper layers at $E$=0.05 $\mathrm{V/{\AA}}$; (d): the total DOS at $E$= 0.00 and 0.05 $\mathrm{V/{\AA}}$. In (a, b, c), the blue, red, and purple curves denote the spin-up, spin-down, and spin-degenerate bands, respectively. In (b, c), the weighting coefficient is proportional to the circle size.
• Figure 5: (Color online) For AB'-stacked bilayer $\mathrm{VI_3}$, (a): the crystal structures; (b, c, d): the total energy band structure along with the spin-resolved projections onto the lower and upper layers. In (a), the red and blue balls represent V and I atoms, respectively. In (b, c, d), the blue, red, and purple curves denote the spin-up, spin-down, and spin-degenerate bands, respectively. In (c, d), the weighting coefficient is proportional to the circle size.