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

TL;DR

The paper addresses the weak spin splitting in 2D altermagnets by introducing a hydroxyl-rotation design in non-vdW MBene Mn2B2(OH)2. It combines spin-group symmetry analysis with first-principles calculations to realize altermagnetism without SOC, achieving a giant spin splitting up to ~1130 meV in the α60 state and revealing node-line semimetal behavior. The work also demonstrates ferroelastic coupling: rotating hydroxyl groups toggles the altermagnetic state and modulates magnetocrystalline anisotropy, while yielding sizable spin-polarized transport (~1×10^19 Ω^-1 m^-1 s^-1) and anisotropic spin polarization. This design provides a versatile pathway to robust, tunable 2D altermagnets with potential spintronic applications.

Abstract

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.

Figures (5)

• Figure 1: Schematic illustration of antiferromagnetic configurations and corresponding band structures. a) FM-AF, classified as AAFM, with the intralayer FM coupling and interlayer AFM coupling. b) AF-AF, with the intralayer and interlayer AFM coupling. c) AF-FM, with the intralayer AFM coupling and interlayer FM coupling. Blue and red arrows: the spins of upper and lower magnetic layers, respectively. Yellow lobes represent the charge polarity, with $+$/$-$ indicating the polar orientation. The transformation $C_2$ refers to the spin-space operation to flip spin up/down, while the transformations ($R$ and $t$) are operations in the real space. $R$: the rotational symmetry operation. $t$: the translational symmetry operation.
• Figure 2: Properties of $\alpha$-Mn$_2$B$_2$(OH)$_2$ monolayer. a) The top view. The primitive cell is indicated by the black-line rhombus. b) Electronic band structure without spin splitting. Inset: the Brillouin zone and high symmetry points. c) Side view of valence electron distribution, integrated within [$-0.5$, $0$] eV.
• Figure 3: a) Top view of rotation of one side hydroxyl groups, which leads to the $\alpha_{60}$-Mn$_2$B$_2$(OH)$_2$ monolayer. Left: the upper layer. Right: the lower layer. The primitive cell is indicated by the black-line rhombus. b) The energy evolution during the in-plane rotation of one layer OH around the $c$ axis, characterized by the rotation angle $\varphi$. When $\varphi=180^{\circ}$, it reaches the $\alpha_{180}$-Mn$_2$B$_2$(OH)$_2$, namely the orientations of hydroxyl groups on the upper and lower surfaces are completely opposite. The energy of $\alpha_{180}$-Mn$_2$B$_2$(OH)$_2$ is very close to (only $0.1$ meV/u.c. higher) that of $\alpha$-Mn$_2$B$_2$(OH)$_2$. c) The in-plane MAE as a function of spin orientation in the $\alpha_{60}$-Mn$_2$B$_2$(OH)$_2$ monolayer. d) Side view of electron density near the Fermi level in the $\alpha_{60}$-Mn$_2$B$_2$(OH)$_2$ monolayer, integrated within [$-0.5$, $0$] eV. The [110] direction corresponds to the diagonal between the $a$-axis and $b$-axis. e) The spin-resolved band structure without SOC of the $\alpha_{60}$-Mn$_2$B$_2$(OH)$_2$ monolayer. f) The spin-up and spin-down Fermi surfaces.
• Figure 4: The values of maximum spin splitting and band gaps for reported altermagnetic candidates. 2D and 3D materials are labeled in red and blue, respectively. tb is the abbreviation for twisted bilayer. sb stands for the stacking bilayer.
• Figure 5: a) The spin-resolved band structure without SOC in the $\alpha_{60}'$-Mn$_2$B$_2$(OH)$_2$ monolayer. b) Schematic diagram of the ferroelastic transition of Mn$_2$B$_2$(OH)$_2$ monolayer, along with the transformation of the magnetic properties. c) Energy barriers for two possible paths for the ferroelastic switching. The horizontal axis is defined by the sum of two angles of upper and lower hydroxyl groups ($\varphi_1$ and $\varphi_2$). Path I: rotate one layer of hydroxyl groups first ($\varphi_1$=$60^{\circ}$ and $\varphi_2$=$0^{\circ}$), then the other layer ($\varphi_1$=$60^{\circ}$ and $\varphi_2$=$60^{\circ}$). Path II: simultaneously rotate both the upper and lower hydroxyl groups ($\varphi_1$=$60^{\circ}$ and $\varphi_2$=$60^{\circ}$). d) Spin-resolved conductivity of the altermagnetic $\alpha_{60}$-Mn$_2$B$_2$(OH)$_2$ monolayer. For comparison, the spin-resolved conductivity of the conventional collinear antiferromagnetic $\alpha$-Mn$_2$B$_2$(OH)$_2$ monolayer has also been calculated, denoted as the orange dashed line. e) Spin polarization $SP$ at 0 eV as a function of $\varphi$. Here, $SP$= $\sigma_{xy}^\uparrow$sin(2$\varphi$)/($\sigma_{xx}^\uparrow$cos$^2$$\varphi$+$\sigma_{yy}^\uparrow$sin$^2$$\varphi$). f) Spin polarization $SP$ at $\varphi$=45$^\circ$ as a function of energy.