A large spin-splitting altermagnet designed from the hydroxylated MBene monolayer

The development of altermagnets is fundamentally important for advancing spintronic device technology, but remains unpractical for the weak spin splitting in most cases, especially in two-dimensional materials. Based on spin group symmetry analysis and first-principles calculations, a novel hydroxyl rotation strategy in collinear antiferromagnets has been proposed to design altermagnets. This approach achieves a large chirality-reversible spin splitting exceeding $1130$ meV in $α_{60}$-Mn$_2$B$_2$(OH)$_2$ monolayer. The system also exhibits intrinsic features of a node-line semimetal in the absence of spin-orbit coupling. Besides, the angles of hydroxyl groups serve as the primary order parameter, which can switch on/off the altermagnetism coupled with the ferroelastic mechanism. The corresponding magnetocrystalline anisotropy have also been modulated. Moreover, an interesting spin-related transport property with the spin-polarized conductivity of 10$^{19}$ $Ω^{-1}m^{-1}s^{-1}$ also emerges. These findings uncover the hydroxyl rotation strategy as a versatile tool for designing altermagnetic node-line semimetals and opening new avenues for achieving exotic chemical and physical characteristics associated with large spin splitting.

💡 Research Summary

The manuscript introduces a novel “hydroxyl rotation strategy” to engineer altermagnetism in two‑dimensional transition‑metal boride (MBene) monolayers, achieving an unprecedented spin‑splitting of ~1.13 eV. Altermagnets are magnetic materials that exhibit momentum‑space spin polarization without net magnetization, making them attractive for spin‑tronic devices. However, most reported 2D altermagnets display only modest spin‑splittings (tens to a few hundred meV), limiting their practical use.

The authors focus on Mn₂B₂(OH)₂, a hydroxyl‑terminated MBene that can be synthesized by thermal etching of Mn₂AlB₂ precursors followed by surface hydroxylation. Two structural variants exist: a “g‑state” (more stable) and an “α‑state” (meta‑stable) with space group Amm2. Both adopt an A‑type antiferromagnetic (AAF​M) order, i.e., ferromagnetic layers stacked antiferromagnetically. First‑principles density‑functional theory (DFT) calculations show that the α‑state is a viable platform for further manipulation.

The key idea is to rotate the hydroxyl groups on one side of the monolayer around the out‑of‑plane (c) axis by an angle φ. This generates a series of metastable configurations: α₆₀ (φ≈±60°), α₁₂₀ (φ≈±120°) and α₁₈₀ (φ≈180°). The α₆₀ configuration is energetically comparable to the original α‑state (energy difference <0.1 eV per unit cell) and remains AAFM after full structural relaxation. Mapping DFT energies onto a Heisenberg model yields exchange constants J₁ = −11.1 meV (nearest‑neighbor intralayer ferromagnetic), J₂ = 63.3 meV (interlayer antiferromagnetic) and J₃ = −11.9 meV (next‑nearest intralayer). These non‑frustrated exchanges stabilize the collinear AAFM ground state.

In the α‑state the spin‑group symmetry is