Alloying Driven Antiferromagnetic Skyrmions on NiPS3 Monolayer: A First-Principles Calculation.

Topological magnetic states are promising information carriers for ultrahigh-density and high-efficiency magnetic storage. Recent advances in two-dimensional (2D) magnets provide powerful platforms for stabilizing various nanometer-size topological spin textures within a wide range of magnetic field and temperature. However, non-centrosymmetric 2D magnets with broken inversion symmetry are scarce in nature, making direct observations of the chiral spin structure difficult, especially for antiferromagnetic (AFM) skyrmions. In this work, it is theoretically predicted that intrinsic AFM skyrmions can be easily triggered in XY-type honeycomb magnet NiPS3 monolayer by alloying of Cr atoms, due to the presence of a sizable Dzyaloshinskii-Moriya interaction. More interestingly, the diameter of the AFM skyrmions in Ni3/4Cr1/4PS3 decreases from 12 to 4.4 nm as the external magnetic field increases and the skyrmion phases remain stable up to an external magnetic field of 4 T. These results highlight an effective strategy to generate and modulate the topological spin texture in 2D magnets by alloying with magnetic element.

Assembly-induced spin transfer and distance-dependent spin coupling in atomically precise AgCu nanoclusters.

Nanoparticle assembly paves the way for unanticipated properties and applications from the nanoscale to the macroscopic world. However, the study of such material systems is greatly inhibited due to the obscure compositions and structures of nanoparticles (especially the surface structures). The assembly of atomically precise nanoparticles is challenging, and such an assembly of nanoparticles with metal core sizes strictly larger than 1 nm has not been achieved yet. Here, we introduced an on-site synthesis-and-assembly strategy, and successfully obtained a straight-chain assembly structure consisting of Ag77Cu22(CHT)48 (CHT: cyclohexanethiolate) nanoparticles with two nanoparticles separated by one S atom, as revealed by mass spectrometry and single crystal X-ray crystallography. Although Ag77Cu22(CHT)48 bears one unpaired shell-closing electron, the magnetic moment is found to be mainly localized at the S linker with magnetic isotropy, and the sulfur radicals were experimentally verified and found to be unstable after disassembly, demonstrating assembly-induced spin transfer. Besides, spin nanoparticles are found to couple and lose their paramagnetism at sufficiently short inter-nanoparticle distance, namely, the spin coupling depends on the inter-nanoparticle distance. However, it is not found that the spin coupling leads to the nanoparticle growth.

FeSi2: a two-dimensional ferromagnet containing planar hexacoordinate Fe atoms.

As an unconventional bonding pattern different from conventional chemistry, the concept of planar hypercoordinate atoms was first proposed in the molecular system, and it has been recently extended to 2D periodic systems. Using first-principles calculations, herein we predict a stable FeSi2 monolayer with planar hexacoordinate Fe atoms. Due to its abundant multicenter bonds, the FeSi2 monolayer shows excellent thermal and kinetic stability, anisotropic mechanical properties and room-temperature ferromagnetism (T C ∼360 K). Furthermore, we have demonstrated the feasibility of directly growing an FeSi2 monolayer on a Si (110) substrate while maintaining the novel electronic and magnetic properties of the freestanding monolayer. The FeSi2 monolayer synthesized in this way would be compatible with the mature silicon semiconductor technology and could be utilized for spintronic devices.

Transition-Metal Interlink Neural Network: Machine Learning of 2D Metal-Organic Frameworks with High Magnetic Anisotropy.

Two-dimensional (2D) metal-organic framework (MOF) materials with large perpendicular magnetic anisotropy energy (MAE) are important candidates for high-density magnetic storage. The MAE-targeted high-throughput screening of 2D MOFs is currently limited by the time-consuming electronic structure calculations. In this study, a machine learning model, namely, transition-metal interlink neural network (TMINN) based on a database with 1440 2D MOF materials is developed to quickly and accurately predict MAE. The well-trained TMINN model for MAE successfully captures the general correlation between the geometrical configurations and the MAEs. We explore the MAEs of 2583 other 2D MOFs using our trained TMINN model. From these two databases, we obtain 11 unreported 2D ferromagnetic MOFs with MAEs over 35 meV/atom, which are further demonstrated by the high-level density functional theory calculations. Such results show good performance of the extrapolation predictions of TMINN. We also propose some simple design rules to acquire 2D MOFs with large MAEs by building a Pearson correlation coefficient map between various geometrical descriptors and MAE. Our developed TMINN model provides a powerful tool for high-throughput screening and intentional design of 2D magnetic MOFs with large MAE.

Robust Dirac spin gapless semiconductors in a two-dimensional oxalate based organic honeycomb-kagome lattice.

Two-dimensional (2D) ferromagnetic materials with intrinsic and robust spin-polarized Dirac cones are of great interest in exploring exciting physics and in realizing spintronic devices. Using comprehensive ab initio calculations, herein we reveal a family of 2D oxalate-based metal-organic frameworks (MOFs) that possess the desired characteristics. We propose that these 2D oxalate-based MOFs may be assembled by oxalate ions (C2O42-) and two homo-transition metal atoms. We demonstrate that 2D MOFs of Ni2(C2O4)3 and Re2(C2O4)3 are intrinsic Dirac spin gapless semiconductors with linear band dispersion, low energy dissipation and high electron carrier velocity. As robust ferromagnets, they also possess large magnetic moments, large perpendicular magnetic anisotropy, and high Curie temperatures, e.g. 208 K for Ni2(C2O4)3. In particular, spin-orbit coupling triggers a topologically nontrivial band gap of 143 meV in Re2(C2O4)3, which is promising to realize the quantum anomalous Hall effect at high temperatures.

A valence balancing rule for the design of bimetallic phosphides targeting high thermoelectric performance.

Two-dimensional (2D) materials with outstanding electronic and mechanical properties have attracted considerable attention as efficient thermoelectric materials. Here, we propose a generalized eight-valence electron rule for designing 2D semiconductor materials, i.e., metal-shrouded bimetallic phosphides ABP (A: group IA element, B: group IIA element). Following this rule, we screen out ten stable semiconductors (LiMgP, LiCaP, LiSrP, NaBeP, NaMgP, KMgP, KCaP, RbMgP, RbCaP and RbSrP) with tunable bandgaps in the range of 0.35-2.40 eV by comprehensive first-principles calculations. Among them, the electron mobility of RbMgP can be as high as 2.3 × 104 cm2 V-1 s-1, and the hole mobility of KMgP is estimated to be 9.9 × 103 cm2 V-1 s-1. Moreover, KMgP, KCaP, RbCaP and RbSrP exhibit an ultralow thermal conductivity of 0.02, 0.14, 0.08 and 0.14 W m-1 K-1, respectively. As a result, KMgP and RbCaP monolayers are p-type or n-type thermoelectric materials with a figure of merit of 2.25 and 1.13 at room temperature, respectively. The underlying mechanism of high electron conductivity and low thermal conductivity has been correlated with their unique bonding characteristics, narrow phonon band gap and the scattering from low-frequency phonons. This work demonstrates not only a guiding electron principle to design stable 2D semiconductors, but also a powerful metal-shrouded strategy for discovering high performance thermoelectric materials by decoupling electronic and thermal transport properties.

2D tetragonal transition-metal phosphides: an ideal platform to screen metal shrouded crystals for multifunctional applications.

Two-dimensional (2D) metal shrouded crystals, a new kind of conceptional material, have attracted remarkable attention due to their unique properties. Here, we propose a novel class of 2D metal shrouded materials, tetragonal transition-metal phosphides (TM2Ps), which show peculiar features of coexistence of in-plane TM-P covalent bonds and TM-TM interlayer metallic bonds. From a combination of high throughput searching and first-principles calculations, Fe2P, Co2P, Ni2P, Ru2P, and Pd2P monolayer sheets stand out because they simultaneously have high thermal, dynamical, and mechanical stability. All these five TM2P materials are metals, especially Pd2P, which can be a promising catalyst for the hydrogen evolution reaction with a very low overpotential. Moreover, these 2D TM2Ps show good ductility since they can withstand a tensile strain of up to 45%. Even in the large strain range, the strengthened interlayer TM-TM metallic bonds dominate the deformation behavior, and the corresponding metallicity of 2D TM2Ps is well preserved. Due to the competition between the d-d direct exchange and d-p-d superexchange interactions, Fe2P behaves as an antiferromagnetic material with a TN of 23 K, while Co2P is a ferromagnetic material with a TC of 580 K. Our results not only enrich the database of 2D metal shrouded crystals, but also provide novel 2D materials as promising candidates for multifunctional applications in nanoelectronics, spintronics and electrocatalysis.

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

The recent discovery of two-dimensional (2D) magnetic order in monolayer CrI3 and bilayer Fe3GeTe2 has stimulated intense experimental and theoretical activities to expand the family of 2D magnets. Most 2D magnets reported to date are transition metal compounds with unpaired d electrons. Novel 2D intrinsic magnets with long-range p state coupling are also highly desirable. Here, we propose that nonstoichiometry is a feasible and universal strategy to realize long-range p electron magnetic order in 2D metal-shrouded AXenes (Na2N, K2N, and Rb2N), supported by our first-principles calculations. Taking K2N as a representative, three series of cation-deficient K2N (T, H, and I phases) have been predicted as stable ferromagnetic half-metal/metal with a Curie temperature of 480-1180 K. Their robust ferromagnetism is ascribed to the coexistence of carrier-mediated exchange and N-K-N superexchange interaction. Moreover, mechanical deformation can trigger reversible phase transformation by choosing their 3D layered counterpart as the intermediate phase.

Screening and Design of Novel 2D Ferromagnetic Materials with High Curie Temperature above Room Temperature.

Two-dimensional (2D) intrinsic ferromagnets with high Curie temperature ( TC) are desirable for spintronic applications. Using systematic first-principles calculations, we investigate the electronic and magnetic properties of 22 monolayer 2D materials with layered bulk phases. From these candidates, we screen out five ferromagnetic monolayer materials belonging to three types of structures: type i (ScCl, YCl, LaCl), type ii (LaBr2), and type iii (CrSBr). Type i is a kind of metallic ferromagnetic material, whereas LaBr2 and CrSBr of type ii and iii are small-bandgap ferromagnetic semiconductors with TC near room temperature. Moreover, the ferromagnetic CrSBr monolayer possesses a large magnetic moment of ∼3 µB per Cr atom, originating from its distorted octahedron coordination. The robust ferromagnetism of the CrSBr monolayer is ascribed to the halogen-mediated (Cr-Br-Cr) and chalcogen-mediated (Cr-S-Cr) superexchange interactions; then, an isoelectronic substitution strategy is proposed to tailor the magnetic coupling strength. Hence, monolayer structures of CrSI, CrSCl, and CrSeBr with notably enhanced Curie temperature up to 500 K as well as favorable formation energy are designed. The moderate interlayer binding energy and high TC make these monolayer ferromagnetic materials feasible for experimental synthesis and attractive as 2D spintronic devices.