Two-Dimensional AXenes: A New Family of Room-Temperature d0 Ferromagnets and Their Structural Phase Transitions

Two-dimensional half-metals MSi2N4 (M = Al, Ga, In, Tl) with intrinsic p-type ferromagnetism and ultrawide bandgaps

Intrinsic half-metallic nanomaterials with 100% spin polarization are highly demanded for next-generation spintronic devices. Here, by using first-principles calculations, we have designed a class of new two-dimensional (2D) p-type half-metals, MSi2N4 (M = Al, Ga, In and Tl), which show high mechanical, thermal and dynamic stabilities. MSi2N4 not only have ultrawide electronic bandgaps for spin-up channels in the range of 4.05 to 6.82 eV but also have large half-metallic gaps in the range of 0.75 to 1.47 eV, which are large enough to prevent the spin-flip transition. The calculated magnetic moment is 1 μB per cell, resulting from polarized N1-px/py orbitals. Moreover, MSi2N4 possess robust long-range ferromagnetic orderings with Curie temperatures in the range of 35-140 K, originating from the interplay of N1-M-N1 superexchange interactions. Furthermore, spin dependent electronic transport calculations reveal 100% spin polarization. Our results highlight new promising 2D ferromagnetic half-metals toward future spintronic applications.

Genuine Dirac Half-Metals in Two-Dimensions

When spin-orbit coupling (SOC) is absent, all proposed half-metals with twofold degenerate nodal points at the K (or K') point in 2D materials are classified as "Dirac half-metals" owing to the way graphene is utilized in the earliest studies. Actually, each band crossing point at the K or K' point is described by a 2D Weyl Hamiltonian with definite chirality; hence, it should be a Weyl point. To the best of its knowledge, there have not yet been any reports of a genuine (i.e., fourfold degenerate) 2D Dirac point half-metal. In this work, using first-principles calculations, it proposes for the first time that the 2D d0 -type ferromagnet Mg4 N4 is a genuine 2D Dirac half-metal candidate with a fourfold degenerate Dirac point at the S high-symmetry point, intrinsic magnetism, a high Curie temperature, 100% spin polarization, topology robust under the SOC and uniaxial and biaxial strains, and spin-polarized edge states. This work can serve as a starting point for future predictions of intrinsically magnetic materials with genuine 2D Dirac points, which will aid the frontier of topo-spintronics research in 2D systems.

Intrinsic half-metallicity in two-dimensional Cr2TeX2 (X = I, Br, Cl) monolayers

Two-dimensional (2D) materials with intrinsic half-metallicity at or above room temperature are important in spin nanodevices. Nevertheless, such 2D materials in experiment are still rarely realized. In this work, a new family of 2D Cr2TeX2 (X = I, Br, Cl) monolayers has been predicted using first-principles calculations. The monolayer is made of five atomic sublayers with ABCAB-type stacking along the perpendicular direction. It is found that the energies for all the ferromagnetic (FM) half-metallic states are the lowest. The phonon spectrum calculations and molecular dynamics simulations both demonstrate that the FM states are stable, indicating the possibility of experimentally obtaining the 2D Cr2TeX2 monolayers with half-metallicity. The Curie temperatures from Monte Carlo simulations are 486, 445, and 451 K for Cr2TeI2, Cr2TeBr2, and Cr2TeCl2 monolayers, respectively, and their half-metallic bandgaps are 1.72, 1.86 and 1.90 eV. The corresponding magnetocrystalline anisotropy energies (MAEs) are about 1185, 502, 899 μeV per Cr atom for Cr2TeX2 monolayers, in which the easy axes are along the plane for the Cr2TeBr2 and Cr2TeCl2 monolayers, but being out of the plane in the Cr2TeI2. Our study implies the potential application of the 2D Cr2TeX2 (X = I, Br, Cl) monolayers in spin nanodevices.

A new concept of atomically thin p-n junction based on Ca2N/Na2N donor-acceptor heterostructure: a first-principles study

In atomically thin p-n junctions, traditional strategies such as doping and implantation for realizing a p- or n-region will fail at the nanoscale, and the Schottky barrier and Fermi level pinning effect taking place in metal-semiconductor contacts seriously suppress the transport properties. In this work, based on first-principles calculations, we propose a new strategy for realizing an ultrathin p-n junction by vertically stacking nonstoichiometric Ca₂N and Na₂N monolayers, which represents a kind of donor-acceptor heterostructure with a natural Ohmic contact. It is of great interest to find that the tunneling barrier can be eliminated and the charge transfer quantity is one order of magnitude higher than that between polar monolayers by adjusting the interlayer distance. In addition, at equilibrium the interlayer tunneling can be turned into resonant transport due to the quasi-bonding, thus enabling excellent transmission performance. In accordance with the results, we believe that our new concept of an atomically thin p-n junction will provide an unprecedented possibility for the development of nanodevices.

Alloying two-dimensional NbSi2N4: a new strategy to realize half-metallic antiferromagnets

Finding two-dimensional (2D) materials with both 100% spin polarization and zero net magnetic moment is essential for next-generation spintronics. Half-metallic antiferromagnets (HMAFs) are ideal materials to satisfy these exigent needs, but such a system has never been found among 2D inorganic materials. In this paper, we theoretically demonstrate that intrinsic 2D HMAFs can be realized by alloying Nb with Mn in 2D septuple-atomic-layer NbSi₂N₄. By continuously incorporating Mn, the stronger Mn-N hybridization relative to Nb-N induces a metal to half-metal to semiconductor transition. The competitive coupling between the Nb-d itinerant electron spin and the Nb-Mn d-d direct interaction drives the ferromagnetic to antiferromagnetic phase transition. For the first time in 2D inorganic materials, the exact cancellation of local magnetic moments and band gap opening in one spin channel is obtained simultaneously at a Nb/Mn ratio of 3 : 1, as demonstrated by our first-principles calculations. The present results would not only inspire materials design of more 2D HMAFs in the future but also impel the advancement of next-generation antiferromagnetic spintronic devices.

A multiferroic vanadium phosphide monolayer with ferromagnetic half-metallicity and topological Dirac states

Ferroelasticity, ferromagnetism, half-metallicity, and topological Dirac states are properties highly sought in two-dimensional (2D) materials for advanced device applications. Here, we report first-principles prediction of a dynamically and thermally stable tetragonal vanadium phosphide (t-VP) monolayer that hosts all these desirable properties. This monolayer is substantially ferromagnetic with polarized spins aligned in the in-plane direction via a d-p-d super-exchange coupling mechanism; meanwhile, its tetragonal lattice enables an intrinsic in-plane ferroelasticity with a reversible strain of 23.4%. As a result, the ferroelasticity is strongly coupled with ferromagnetism via spin-orbit coupling to enable deterministic control over the magnetocrystalline anisotropy by an applied elastic strain. More interestingly, this multiferroic t-VP monolayer possesses half-metallicity with an anisotropic, topological Dirac cone residing in the majority-spin channel. We also predict a multiferroic t-CrN monolayer, whose ferromagnetism features a high Curie temperature of up to 478 K but is weakly coupled to its in-plane ferroelasticity. These results suggest a tetragonal 2D lattice as a robust atomic-scale scaffold on the basis of which fascinating electronic and magnetic properties can be rationally created by a suitable combination of chemical elements.

Orbital Design of Two-Dimensional Transition-Metal Peroxide Kagome Crystals with Anionogenic Dirac Half-Metallicity

Assembling p orbital ferromagnetic half-metallicity and a topological element, such as a Dirac point at the Fermi level, in a single nanomaterial is of particular interest for long-distance, high-speed, and spin-coherent transportation in nanoscale spintronic devices. On the basis of the tight-binding model, we present an orbital design of a two-dimensional (2D) anionogenic Dirac half-metal (ADHM) by patterning cations with empty d orbitals and anions with partially filled p-type orbitals into a kagome lattice. Our first-principles calculations show that 2D transition-metal peroxides h-TM₂(O₂)₃ (TMO₃, TM = Ti, Zr, Hf), containing group IVB transition-metal cations [TM]⁴⁺ bridged with dioxygen anions [O₂]^8/3- in a kagome structure, are stable ADHMs with a Curie temperature over 103 K. The 2/3 filled π* orbitals of dioxygen anions are ferromagnetically coupled, leading to p orbital ferromagnetism and a half-metallic Dirac point right at the Fermi level with a Fermi velocity reaching 2.84 × 10⁵ m/s. We proposed that 2D h-TM₂(O₂)₃ crystals may be extracted from ABO₃ bulk materials containing 2D TMO₃ layers.

First-principles investigation of a new 2D magnetic crystal: Ferromagnetic ordering and intrinsic half-metallicity

The development of two-dimensional (2D) magnetic materials with half-metallic characteristics is of great interest because of their promising applications in spintronic devices with high circuit integration density and low energy consumption. Here, by using density functional theory calculations, ab initio molecular dynamics, and Monte Carlo simulation, we study the stability, electronic structure, and magnetic properties of a OsI₃ monolayer, of which crystalline bulk is predicted to be a van der Waals layered ferromagnetic (FM) semiconductor. Our results reveal that the OsI₃ monolayer can be easily exfoliated from the bulk phase with small cleavage energy and is energetically and thermodynamically stable at room temperature. Intrinsic half-metallicity with a wide bandgap and FM ordering with an estimated T_C = 35 K are found for the OsI₃ monolayer. Specifically, the FM ordering can be maintained under external biaxial strain from -2% to 5%. The in-plane magnetocrystalline anisotropy energy of the 2D OsI₃ monolayer reaches up to 3.89 meV/OsI₃, which is an order larger than that of most magnetic 2D materials such as the representative monolayer CrI₃. The excellent magnetic features of the OsI₃ monolayer therefore render it a promising 2D candidate for spintronic applications.