The Intrinsic Ferromagnetism in a MnO2 Monolayer

Surface Vacancy-Induced Switchable Electric Polarization and Enhanced Ferromagnetism in Monolayer Metal Trihalides

Monolayer chromium triiodide (CrI₃), as the thinnest ferromagnetic material demonstrated in experiment [ Huang et al. Nature 2017 , 546 , 270 ], opens up new opportunities for the application of two-dimensional (2D) materials in spintronic nanodevices. Atom-thick 2D materials with switchable electric polarization are now urgently needed for their rarity and important roles in nanoelectronics. Herein, we unveil that surface I vacancies not only enhance the intrinsic ferromagnetism of monolayer CrI₃ but also induce switchable electric polarization. I vacancies bring about an out-of-plane polarization without breaking the nonmetallic nature of CrI₃. Meanwhile, the induced polarization can be reversed in a moderate energy barrier, arising from the unique porosity of CrI₃ that contributes to the switch of I vacancies between top and bottom surfaces. Engineering 2D switchable polarization through surface vacancies is also applicable to many other metal trihalides, which opens up a new and general way toward pursuing low-dimensional multifunctional nanodevices.

MBene (MnB): a new type of 2D metallic ferromagnet with high Curie temperature

We extend the 2D MXene family into the boride world, namely, MBenes. High-throughput calculations screen twelve MBenes with excellent stability. Among them, 2D MnB MBene exhibits robust metallic ferromagnetism (∼3.2 μ_B per Mn atom) and high Curie temperature (345 K). After functionalization with the -F and -OH groups, the ferromagnetic ground state of 2D MnB is well preserved. The Curie temperature is increased to 405 and 600 K, respectively, providing a novel and feasible strategy to tailor the T_C of 2D magnetic materials.

Metal-Free Half-Metallicity in B-Doped gh-C3N4 Systems

Half-metallicity rising from the s/p electrons has been one of the hot topics in spintronics. Based on the first-principles of calculation, we explore the magnetic properties of the B-doped graphitic heptazine carbon nitride (gh-C3N4) system. Ferromagnetism is observed in the B-doped gh-C3N4 system. Interestingly, its ground state phase (BC1@gh-C3N4) presents a strong half-metal property. Furthermore, the half-metallicity in BC1@gh-C3N4 can sustain up to 5% compressive strain and 1.5% tensile strain. It will lose its half-metallicity, however, when the doping concentration is below 6.25%. Our results show that such a metal-free half-metallic system has promising spintronic applications.

Electronic and magnetic properties of monolayer α-RuCl3: a first-principles and Monte Carlo study

Recent experiments revealed that monolayer α-RuCl₃ can be obtained by a chemical exfoliation method and exfoliation or restacking of nanosheets can manipulate the magnetic properties of the materials. Thermal variations of magnetization and specific heat curves indicate that monolayer α-RuCl₃ exhibits a phase transition between ordered and disordered phases at the Curie temperature of 14.21 K.

Theoretical prediction of LiScO2 nanosheets as a cathode material for Li–O2 batteries

The electrochemical reaction producing crystalline LiO₂ on the LiScO₂ nanosheet can deliver a high discharge voltage of 3.50 V.

Large magnetic anisotropy and its strain modulation in two-dimensional intrinsic ferromagnetic monolayer RuO2 and OsO2

The magnetic anisotropy energy (MAE) of monolayer 1T-RuO₂ and 1T-OsO₂ under −4%, −2%, 0%, 2% strains.

Redox properties of birnessite from a defect perspective

Birnessite, a layered-structure MnO₂, is an earth-abundant functional material with potential for various energy and environmental applications, such as water oxidation. An important feature of birnessite is the existence of Mn(III) within the MnO₂ layers, accompanied by interlayer charge-neutralizing cations. Using first-principles calculations, we reveal the nature of Mn(III) in birnessite with the concept of the small polaron, a special kind of point defect. Further taking into account the effect of the spatial distribution of Mn(III), we propose a theoretical model to explain the structure-performance dependence of birnessite as an oxygen evolution catalyst. We find an internal potential step which leads to the easy switching of the oxidation state between Mn(III) and Mn(IV) that is critical for enhancing the catalytic activity of birnessite. Finally, we conduct a series of comparative experiments which support our model.

Room-Temperature Ferromagnetism in Two-Dimensional Fe2Si Nanosheet with Enhanced Spin-Polarization Ratio

Searching experimental feasible two-dimensional (2D) ferromagnetic crystals with large spin-polarization ratio, high Curie temperature and large magnetic anisotropic energy is one key to develop next-generation spintronic nanodevices. Here, 2D Fe₂Si nanosheet, one counterpart of Hapkeite mineral discovered in meteorite with novel magnetism is proposed on the basis of first-principles calculations. The 2D Fe₂Si crystal has a slightly buckled triangular lattice with planar hexacoordinated Si and Fe atoms. The spin-polarized calculations with hybrid HSE06 function method indicate that 2D Fe₂Si is a ferromagnetic half metal at its ground state with 100% spin-polarization ratio at Fermi energy level. The phonon spectrum calculation and ab initio molecular dynamic simulation shows that 2D Fe₂Si crystal has a high thermodynamic stability and its 2D lattice can be retained at the temperature up to 1200 K. Monte Carlo simulations based on the Ising model predict a Curie temperature over 780 K in 2D Fe₂Si crystal, which can be further tuned by applying a biaxial strain. Moreover, 2D structure and strong in-plane Fe-Fe interaction endow Fe₂Si nanosheet sizable magnetocrystalline anisotropy energy with the magnitude of at least two orders larger than those of Fe, Co and Ni bulks. These novel magnetic properties render the 2D Fe₂Si crystal a very promising material for developing practical spintronic nanodevices.

Exploration of new ferromagnetic, semiconducting and biocompatible Nb3X8 (X = Cl, Br or I) monolayers with considerable visible and infrared light absorption

Ferromagnetic character and biocompatible properties have become key factors for developing next-generation spintronic devices and show potential in biomedical applications. Unfortunately, the Mn-containing monolayer is not biocompatible though it has been extensively studied, and the Cr-containing monolayer is not environmental friendly, although these monolayers are ferromagnetic. Herein, we systematically investigated new types of 2D ferromagnetic monolayers Nb₃X₈ (X = Cl, Br or I) by means of first principles calculations together with mean field approximation based on the classical Heisenberg model. The small cleavage energy and high in-plane stiffness have been calculated to evaluate the feasibility of exfoliating the monolayers from their layered bulk phase. Spin-polarized calculations together with self-consistently determined Hubbard U were utilized to assess a strong correlation energy, which demonstrated that Nb₃X₈ (X = Cl, Br or I) monolayers are ferromagnetic. The calculated Curie temperatures for Nb₃Cl₈, Nb₃Br₈ and Nb₃I₈ were 31, 56 and 87 K, respectively, which may be increased by external strain, or electron or hole doping. Moreover, the Nb₃X₈ (X = Cl, Br or I) monolayers exhibited strong visible and infrared light absorption. The biocompatibility, ferromagnetism and considerable visible and infrared light absorption render the Nb₃X₈ (X = Cl, Br or I) monolayers with great potential application in next-generation biocompatible spintronic and optoelectronic devices.

Tuning of intrinsic antiferromagnetic to ferromagnetic ordering in microporous α-MnO2 by inducing tensile strain

By employing first principles density functional calculations, we investigated an α-MnO₂ compound with a tunnel framework, which provides an eminent platform to alter the intrinsic antiferromagnetic (AFM) to ferromagnetic (FM) ordering, through the introduction of chemical or mechanical tensile strain. Our calculations further showed that the strength of FM ordering increases until 10% triaxial tensile strain. Since long range FM ordering is induced, it is realized to be superior as compared to the experimentally observed short-range FM ordering in oxygen-deficient compound. The driving force behind this superior effect is understood from the unusual electron occupancy in Mn atoms as a result of tetrahedral distortion in the MnO₆ octahedra and an increase in the sp³ character of the oxygen atoms. Thus, the α-MnO₂ compound belongs to a class of materials that exhibit good potential for piezomagnetic applications.

NiX2 (X = Cl and Br) sheets as promising spin materials: a first-principles study

In order to achieve paper-like spin devices, we explored two promising two-dimensional (2D) spin materials, namely NiCl₂ and NiBr₂.

Ferromagnetic and Half-Metallic FeC2 Monolayer Containing C2 Dimers

Ferromagnetism and half-metallicity are two vital properties of a material for realizing its potential in spintronics applications. However, none of the two-dimensional (2D) pristine metal-carbide sheets synthesized experimentally exhibits half-metallicity with ferromagnetic coupling. Here, a ferromagnetic and half-metallic FeC₂ sheet containing isolated C₂ dimers rather than individual carbon atoms is predicted to be such a material. Based on state-of-the-art theoretical calculations, we show that the FeC₂ sheet is dynamically, thermally, and mechanically stable and can be chemically exfoliated from the bulk phase of layered ThFeC₂. Due to its unique atomic configuration, the FeC₂ sheet exhibits ferromagnetism with a Curie temperature of 245 K. This is in contrast to its bulk counterpart, ThFeC₂, which is paramagnetic. We also find that, unlike the metallic metal-carbide sheets, the FeC₂ sheet is half-metallic with semiconducting spin-up and metallic spin-down channels. Moreover, half-metallicity can remain until an equi-biaxial strain of 10%. In addition, we provide the Raman and infrared spectra which can be used to identify this new 2D structure experimentally in the future.

Mechanistic Evaluation of LixOy Formation on δ-MnO2 in Nonaqueous Li-Air Batteries

Transition metal oxides are usually used as catalysts in the air cathode of lithium-air (Li-air) batteries. This study elucidates the mechanistic origin of the oxygen reduction reaction catalyzed by δ-MnO2 monolayers and maps the conditions for Li2O2 growth using a combination of first-principles calculations and mesoscale modeling. The MnO2 monolayer, in the absence of an applied potential, preferentially reacts with a Li atom instead of an O2 molecule to initiate the formation of LiO2. The oxygen reduction products (LiO2, Li2O2, and Li2O molecules) strongly interact with the MnO2 monolayer via the stabilization of Li-O chemical bonds with lattice oxygen atoms. As compared to the disproportionation reaction, direct lithiation reactions are the primary contributors to the stabilization of Li2O2 on the MnO2 monolayer. The energy profiles of (Li2O2)2 and (Li2O)2 nucleation on δ-MnO2 monolayer during the discharge process demonstrate that Li2O2 is the predominant discharge product and that further reduction to Li2O is inhibited by the high overpotential of 1.21 V. Interface structures have been examined to study the interaction between the Li2O2 and MnO2 layers. This study demonstrates that a Li2O2 film can be homogeneously deposited onto δ-MnO2 and that the Li2O2/MnO2 interface acts as an electrical conductor. A mesoscale model, developed based on findings from the first-principles calculations, further shows that Li2O2 is the primary product of electrochemical reactions when the applied potential is smaller than 2.4 V.

Transition-metal embedded carbon nitride monolayers: high-temperature ferromagnetism and half-metallicity

High-temperature ferromagnetic materials with planar surfaces are promising candidates for spintronics applications. Using state-of-the-art density functional theory (DFT) calculations, transition metal (TM = Cr, Mn, and Fe) incorporated graphitic carbon nitride (TM@gt-C3N4) systems are investigated as possible spintronics devices. Interestingly, ferromagnetism and half-metallicity were observed in all of the TM@gt-C3N4 systems. We find that Cr@gt-C3N4 is a nearly half-metallic ferromagnetic material with a Curie temperature of ∼450 K. The calculated Curie temperature is noticeably higher than other planar 2D materials studied to date. Furthermore, it has a steel-like mechanical stability and also possesses remarkable dynamic and thermal (500 K) stability. The calculated magnetic anisotropy energy (MAE) in Cr@gt-C3N4 is as high as 137.26 μeV per Cr. Thereby, such material with a high Curie temperature can be operated at high temperatures for spintronics devices.

Intrinsic quantum spin Hall and anomalous Hall effects in h-Sb/Bi epitaxial growth on a ferromagnetic MnO2 thin film

Exploring a two-dimensional intrinsic quantum spin Hall state with a large band gap as well as an anomalous Hall state in realizable materials is one of the most fundamental and important goals for future applications in spintronics, valleytronics, and quantum computing. Here, by combining first-principles calculations with a tight-binding model, we predict that Sb or Bi can epitaxially grow on a stable and ferromagnetic MnO2 thin film substrate, forming a flat honeycomb sheet. The flatness of Sb or Bi provides an opportunity for the existence of Dirac points in the Brillouin zone, with its position effectively tuned by surface hydrogenation. The Dirac points in spin up and spin down channels split due to the proximity effects induced by MnO2. In the presence of both intrinsic and Rashba spin-orbit coupling, we find two band gaps exhibiting a large band gap quantum spin Hall state and a nearly quantized anomalous Hall state which can be tuned by adjusting the Fermi level. Our findings provide an efficient way to realize both quantized intrinsic spin Hall conductivity and anomalous Hall conductivity in a single material.

Spin-orbital effects in metal-dichalcogenide semiconducting monolayers

Metal-dioxide & metal-dichalcogenide monolayers are studied by means of Density Functional Theory. For an accurate reproduction of the electronic structure of transition metal systems, the spin orbit interaction is considered by using fully relativistic pseudopotentials (FRUP). The electronic and spin properties of MX₂ (M = Sc, Cr, Mn, Ni, Mo & W and X = O, S, Se & Te) were obtained with FRUP, compared with the scalar relativistic pseudopotentials (SRUP) and with the available experimental results. Among the differences between FRUP and SRUP calculations are giant splittings of the valence band, substantial band gap reductions and semiconductor to metal or non-magnetic to magnetic “transitions”. MoO₂, MoS₂, MoSe₂, MoTe₂, WO₂, WS₂ and WSe₂ are proposed as candidates for spintronics, while CrTe₂, with μ ~ 1.59 μ_B, is a magnetic metal to be experimentally explored.

First-principles design of spintronics materials

Spintronics is one of the most promising next generation information technology, which uses the spins of electrons as information carriers and possesses potential advantages of speeding up data processing, high circuit integration density, and low energy consumption. However, spintronics faces a number of challenges, including spin generation and injection, long distance spin transport, and manipulation and detection of spin orientation. In solving these issues, new concepts and spintronics materials were proposed one after another, such as half metals, spin gapless semiconductors, and bipolar magnetic semiconductors. Topological insulators can also be viewed as a special class of spintronics materials, with their surface states used for pure spin generation and transportation. In designing these spintronics materials, first-principles calculations play a very important role. This article attempts to give a brief review of the basic principles and theoretical design of these materials. Meanwhile, we also give some attentions to the antiferromagnetic spintronics, which is mainly based on antiferromagnets and has aroused much interest in recent years.

Evidencing the existence of exciting half-metallicity in two-dimensional TiCl3 and VCl3 sheets

Half-metallicity combined with wide half-metallic gap, unique ferromagnetic character and high Curie temperature has become a key driving force to develop next-generation spintronic devices. In previous studies, such half-metallicity always occurred under certain manipulation. Here, we, via examining a series of two-dimensional transition-metal trichlorides, evidenced that TiCl₃ and VCl₃ sheets could display exciting half-metallicity without involving any external modification. Calculated half-metallic band-gaps for TiCl₃ and VCl₃ sheets are about 0.60 and 1.10 eV, respectively. Magnetic coupled calculation shows that both sheets favor the ferromagnetic order with a substantial collective character. Estimated Curie temperatures can be up to 376 and 425 K for TiCl₃ and VCl₃ sheets, respectively. All of these results successfully disclose two new promising two-dimensional half-metallic materials toward the application of next-generation paper-like spintronic devices.

Exfoliating biocompatible ferromagnetic Cr-trihalide monolayers

Cr-trihalide monolayers CrX₃ (X = Cl, Br, I) can be exfoliated from their layered bulk phase, exhibiting high in-plane stiffness, tunable Curie temperatures and biocompatibility.

The static and dynamic magnetic properties of monolayer iron dioxide and iron dichalcogenides

The electronic structures, and static and dynamic magnetic properties of monolayer iron dioxide and iron dichalcogenides are investigated using first-principle calculations in conjunction with MC simulation and atomic spin dynamics simulation.

Atomically Thin Transition-Metal Dinitrides: High-Temperature Ferromagnetism and Half-Metallicity

High-temperature ferromagnetic two-dimensional (2D) materials with flat surfaces have been a long-sought goal due to their potential in spintronics applications. Through comprehensive first-principles calculations, we show that the recently synthesized MoN2 monolayer is such a material; it is ferromagnetic with a Curie temperature of nearly 420 K, which is higher than that of any flat 2D magnetic materials studied to date. This novel property, made possible by the electron-deficient nitrogen ions, render transition-metal dinitrides monolayers with unique electronic properties which can be switched from the ferromagnetic metals in MoN2, ZrN2, and TcN2 to half-metallic ones in YN2. Transition-metal dinitrides monolayers may, therefore, serve as good candidates for spintronics devices.

Layer-dependent dopant stability and magnetic exchange coupling of iron-doped MoS2 nanosheets

Using density-functional theory calculations including a Hubbard U term we explore structural stability, electronic and magnetic properties of Fe-doped MoS2 nanosheets. Unlike previous reports, the geometry and the stability of Fe dopant atoms in MoS2 nanosheets strongly depend on the chemical potential and the layer number of sheets. The substitution Fe dopant atoms at the Mo sites are energetically favorable in monolayer MoS2 and the formation of intercalated and substitutional Fe complexes are preferred in bilayer and multilayer ones under the S-rich regime that is a popular condition for the synthesis of MoS2 nanosheets. We find that the Fe dopants prefer to the ferromagnetic coupling in monolayer MoS2 and the antiferromagnetic coupling in bilayer and multilayer ones, suggesting the layer dependence of magnetic exchange coupling (MEC). The transition of MEC in Fe-doped MoS2 sheets induced by the change of layer number arises from the competition mechanism between the double-exchange and superexchange couplings. The findings provide a route to facilitate the design of MoS2-based diluted magnetic semiconductors and spintronic devices.

Room-temperature ferromagnetism of 2H-SiC-α-Al2O3 solid solution nanowires and the physical origin

In this work we report the first synthesis of 2H-SiC-α-Al2O3 solid solution (SS) nanowires with 2H-SiC as the host phase. The one dimensional (1D) fake binary-system exhibits interesting room-temperature ferromagnetism and spin-glass-like (SGL) behavior. This novel diluted magnetic semiconductor (DMS) was designed on the basis of SiC which is the most promising fundamental semiconductor used in next-generation electronics as the substitute for Si. A systematic investigation of the magnetic properties reveals the origin of the material's room-temperature ferromagnetism and spin-glass behavior. Spin-polarized density functional theory (DFT) calculations reveal that the net moment originates from a strong coupling between atoms around local Si vacancies produced by the SS defect reaction. Unlike random defects derived magnetic behavior, the SS resulted magnetism is significant to be utilized in functional devices since it belongs to a stable crystal structure that is possible to be prepared rationally in a controlled manner.

Tunable band gap and magnetism of the two-dimensional nickel hydroxide

The electronic structures and magnetic properties of two dimensional (2D) hexagonal Ni(OH)₂ are explored based on first-principles calculations.

Half-metallic and magnetic properties in nonmagnetic element embedded graphitic carbon nitride sheets

The B- and Al-doped triazine-based g-C₃N₄ monolayers exhibit long-range half-metallic ferromagnetic order, and are potential candidates for spintronics applications.

Stability and properties of 2D porous nanosheets based on tetraoxa[8]circulene analogues

Two-dimensional porous nanosheets based on tetraoxa[8]circulene and its analogue are theoretically studied using first principles calculations focusing on their thermal stability and mechanical, electronic, optical and thermoelectric properties. It is found that the nanosheets composed of tetraoxa[8]circulene (TO8C) and tetraaza[8]circulene (TA8C) are thermodynamically and kinetically stable. Both sheets show anisotropic Young's moduli corresponding to tetragonal symmetry. However, due to their porosity, the Young's moduli of both sheets are much smaller than that of graphene. Electronic structure calculations indicate that both TO8C and TA8C nanosheets are direct semiconductors with a band gap of 1.92 eV and 1.83 eV respectively, and they can adsorb strongly visible light and exhibit a huge Seebeck coefficient. Thus they can be promising candidates for solar and thermoelectric applications.

Robust ferromagnetism in monolayer chromium nitride

Design and synthesis of two-dimensional (2D) materials with robust ferromagnetism and biocompatibility is highly desirable due to their potential applications in spintronics and biodevices. However, the hotly pursued 2D sheets including pristine graphene, monolayer BN, and layered transition metal dichalcogenides are nonmagnetic or weakly magnetic. Using biomimetic particle swarm optimization (PSO) technique combined with ab initio calculations we predict the existence of a 2D structure, a monolayer of rocksalt-structured CrN (100) surface, which is both ferromagnetic and biocompatible. Its dynamic, thermal and magnetic stabilities are confirmed by carrying out a variety of state-of-the-art theoretical calculations. Analyses of its band structure and density of states reveal that this material is half-metallic, and the origin of the ferromagnetism is due to p-d exchange interaction between the Cr and N atoms. We demonstrate that the displayed ferromagnetism is robust against thermal and mechanical perturbations. The corresponding Curie temperature is about 675 K which is higher than that of most previously studied 2D monolayers.