First-principles calculation of the magnetocrystalline anisotropy energy of iron, cobalt, and nickel

Rational design of a two-dimensional high-temperature ferromagnet from HCP cobalt

Cobalt has the highest Curie temperature (Tc) among the elemental ferromagnetic metals and has a hexagonal close-packed (HCP) structure at room temperature. In this study, HCP Co was thinned to the thickness of several (n) unit cells along the c-axis and then passivated by halogen atoms, thus being named Co2nX2 (X = F, Cl, Br and I). For Co2X2 and Co3X2, all of them are not only kinetically but also thermodynamically stable from the viewpoint of the phonon spectra and molecular dynamics. Similar to HCP Co, two-dimensional (2D) Co2F2, Co2Cl2 and Co3X2 (X = Cl, Br and I) are still ferromagnetic metals within the Stoner model but Co2X2 (X = Br and I) is a ferromagnetic half-metal with the coexistence of the metallic behavior for one spin and the insulating behavior for the other spin. Taking into account the spin-orbital coupling (SOC), the easy-magnetization axis is within the plane where the magnetization is isotropic, making it look like a 2D XY magnet. Applying a critical biaxial strain could lead to an easy-magnetization axis changing from the in-plane to the out-of-plane direction. Finally, we use classical Monte Carlo simulations to estimate the Curie temperature (Tc) which is as high as 957 and 510 K for Co2F2 and Co2Cl2, respectively, because of the strong direct exchange interaction. Different from being obtained by mechanical or liquid exfoliation from van der Waals layered structures, our study opens up new possibilities to search for novel 2D ferromagnets from the elemental ferromagnets and provides opportunities for realizing realistic ultra-thin spintronic devices.

Mn2C MXene functionalized by oxygen is a semiconducting antiferromagnet and an efficient visible light absorber

Manganese-based MXenes are promising two-dimensional materials due to the broad palette of their magnetic phases and the possibility of experimental preparation because the corresponding MAX phase was already prepared. Here, we systematically investigated geometrical conformers and spin solutions of oxygen-terminated Mn2C MXene and performed subsequent many-body calculations to obtain reliable electronic and optical properties. Allowing energy-lowering using the correct spin ordering via supercell magnetic motifs is essential for the Mn2CO2 system. The stable ground-state Mn2CO2 conformation is antiferromagnetic (AFM) with zigzag lines of up and down spins on Mn atoms. The AFM nature is consistent with the parent MAX phase and even the clean depleted Mn2C sheet. Other magnetic states and geometrical conformations are energetically very close, providing state-switching possibilities in the material. Subsequent many-body GW and Bethe-Salpeter equation (BSE) calculations provide indirect semiconductor characteristics of AFM Mn2CO2 with a fundamental gap of 2.1 eV (and a direct gap of 2.4 eV), the first bright optical transition at 1.3 eV and extremely strongly bound (1.1 eV) first bright exciton. Mn2CO2 absorbs efficiently the whole visible light range and near ultraviolet range (between 10 and 20%).

Valley polarization and magnetic anisotropy of two-dimensional Ni2Cl3I3/MoSe2 heterostructures

Two-dimensional (2D) Janus trihalides have attracted widespread attention due to their potential applications in spintronics. In this work, the valley polarization of MoSe2 at the K' and K points can be modulated by Ni2Cl3I3, a new 2D Janus trihalide. The Ni2Cl3I3/MoSe2 heterostructure has an in-plane magnetic anisotropy energy (IMA) and is characterized by three distinct electronic structures: metallic, semiconducting, and half-metallic. It is noted that the semiconducting state features a band gap of 0.07 eV. When spin-orbit coupling (SOC) is considered, valley polarization is exhibited in the Ni2Cl3I3/MoSe2 heterostructure, with the degree of valley polarization varying across different configurations and reaching a maximum value of 4.6 meV. The electronic properties, valley polarization and MAE of the system can be tuned by biaxial strains. The application of a biaxial strain ranging from -6% to +6% can enhance the valley polarization value from 0.9 meV to 12.9 meV. The directions of MAE of the Ni2Cl3I3/MoSe2 heterostructure can be changed at biaxial strains of -6%, +2%, +4% and +6%. The above calculation results show that the heterostructure system possesses rich electronic properties and tunability, with extensive potential applications in the fields of spintronic and valleytronic devices.

Topological Spin Textures: Basic Physics and Devices

In the face of escalating modern data storage demands and the constraints of Moore's Law, exploring spintronic solutions, particularly the devices based on magnetic skyrmions, has emerged as a promising frontier in scientific research. Since the first experimental observation of skyrmions, topological spin textures have been extensively studied for their great potential as efficient information carriers in spintronic devices. However, significant challenges have emerged alongside this progress. This review aims to synthesize recent advances in skyrmion research while addressing the major issues encountered in the field. Additionally, current research on promising topological spin structures in addition to skyrmions is summarized. Beyond 2D structures, exploration also extends to 1D magnetic solitons and 3D spin textures. In addition, a diverse array of emerging magnetic materials is introduced, including antiferromagnets and 2D van der Waals magnets, broadening the scope of potential materials hosting topological spin textures. Through a systematic examination of magnetic principles, topological categorization, and the dynamics of spin textures, a comprehensive overview of experimental and theoretical advances in the research of topological magnetism is provided. Finally, both conventional and unconventional applications are summarized based on spin textures proposed thus far. This review provides an outlook on future development in applied spintronics.

Boosting the Curie temperature of GaN monolayer through van der Waals heterostructures

The pursuit of van der Waals (vdW) heterostructures with high Curie temperature and strong perpendicular magnetic anisotropy (PMA) is vital to the advancement of next generation spintronic devices. First-principles calculations are used to study the electronic structures and magnetic characteristics of GaN/VS2vdW heterostructure under biaxial strain and electrostatic doping. Our findings show that a ferromagnetic ground state with a remarkable Curie temperature (477 K), much above room temperature, exists in GaN/VS2vdW heterostructure and 100% spin polarization efficiency. Additionally, GaN/VS2vdW heterostructure still maintains PMA under biaxial strain, which is indispensable for high-density information storage. We further explore the electron, magnetic, and transport properties of VS2/GaN/VS2vdW sandwich heterostructure, where the magnetoresistivity can reach as high as 40%. Our research indicates that the heterostructure constructed by combining the ferromagnet VS2and the non-magnetic semiconductor GaN is a promising material for vdW spin valve devices at room temperature.

Enhanced Direct Exchange Interaction and Hybridization by Single-Atom Linkers for High Curie Temperature and Superior Visible-Light Harvesting in Cr3(CN3)2

Designing two-dimensional (2D) ferromagnetic (FM) semiconductors with elevated Curie temperature, high carrier mobility, and strong light harvesting is challenging but crucial to the development of spintronics with multifunctionalities. Herein, we show first-principles computation evidence of the 2D metal-organic framework Kagome ferromagnet Cr3(CN3)2. Monolayer Cr3(CN3)2 is predicted to be an FM semiconductor with a record-high Curie temperature of 943 K owing to the use of a single-atom linker (N), which results in strong direct d-p exchange interaction and hybridization between dyz/xz and pz of Cr and N, as well as excellent matching characteristics in energy and symmetry. The single-atom linker structural feature also leads to notable band dispersion and a relatively high carrier mobility of 420 cm2 V-1 s-1. Moreover, under the in-plane strain, 2D Cr3(CN3)2 can be tuned to possess a strong visible-light-harvesting functionality. These novel properties render monolayer Cr3(CN3)2 a distinct 2D ferromagnet with high potential for the development of multifunctional spintronics.

Electronic Structure of Above-Room-Temperature van der Waals Ferromagnet Fe3GaTe2

Fe3GaTe2, a recently discovered van der Waals ferromagnet, demonstrates intrinsic ferromagnetism above room temperature, necessitating a comprehensive investigation of the microscopic origins of its high Curie temperature (TC). In this study, we reveal the electronic structure of Fe3GaTe2 in its ferromagnetic ground state using angle-resolved photoemission spectroscopy and density functional theory calculations. Our results establish a consistent correspondence between the measured band structure and theoretical calculations, underscoring the significant contributions of the Heisenberg exchange interaction (Jex) and magnetic anisotropy energy to the development of the high-TC ferromagnetic ordering in Fe3GaTe2. Intriguingly, we observe substantial modifications to these crucial driving factors through doping, which we attribute to alterations in multiple spin-splitting bands near the Fermi level. These findings provide valuable insights into the underlying electronic structure and its correlation with the emergence of high-TC ferromagnetic ordering in Fe3GaTe2.

Enriching 2D transition metal borides via MB XMenes (M = Fe, Co, Ir): Strong correlation and magnetism

Recently, two-dimensional (2D) FeSe-like anti-MXenes (or XMenes), composed of late d-block transition metal M and p-block nonmetal X elements, have been both experimentally and theoretically investigated. Here, we select three 2D borides FeB, CoB and IrB for a deeper investigation by including strong correlation effects, as a fertile ground for understanding and applications. Using a combination of Hubbard corrected first-principles calculations and Monte Carlo simulations, FeB and CoB are found to be ferro- and anti-ferro magnetic, contrasting with the non-magnetic nature of IrB. The metallic FeB XMene monolayer, superior to most of the MXenes or MBenes, exhibits robust ferromagnetism, driven by intertwined direct-exchange and super-exchange interactions between adjacent Fe atoms. The predicted Curie temperature (TC) of the FeB monolayer via the Heisenberg model reaches an impressive 425 K, with the easy-axis oriented out-of-plane and high magnetic anisotropic energy (MAE). The asymmetry in the spin-resolved transmission spectrum induces a thermal spin current, providing an opportunity for spin filtration. This novel 2D FeB material is expected to hold great promise as an information storage medium and find applications in emerging spintronic devices.

Electronic and magnetic properties of 2D vanadium-based transition metal dichalcogenides

In this paper, electronic and magnetic properties of monolayers and bilayers of Vanadium-based transition metal dichalcogenides VX2 (X = S, Se, Te) in the H phase are investigated theoretically using methods based on DFT calculations as well as analytical methods based on effective spin Hamiltonians. The band structure has been computed for all systems, and then the results have been used to determine exchange parameters and magnetic anisotropy constants. These parameters are subsequently used for the determination of the Curie temperatures, hysteresis curves, and energy of spin-wave excitations. In the latter case, we compare analytical results based on effective spin Hamiltonian with those determined numerically by Quantum ATK software and find a good agreement. The determined Curie temperature for VTe2 monolayers and bilayers is below the room temperature (especially that for bilayers), while for the other two materials, i.e. for VS2 and VSe2, it is above the room temperature, in agreement with available experimental data.

A novel two-dimensional NiCl2O8 lattice with negative Poisson's ratio and magnetic modulation

Two-dimensional (2D) materials with simultaneous magnetic semiconducting properties and a negative Poisson's ratio are crucial for fabricating multifunctional electronic devices. However, progress in this area has been generally constrained. Based on first-principles calculations, we engineered a 2D Ni-based oxyhalide with a honeycomb lattice structure. It was observed that the NiCl2O8 monolayer exhibits both high- and low-buckling states in its geometry, along with intrinsic magnetic semiconductor properties in its electronic structure. Importantly, we demonstrated that the magnetic ordering of the NiCl2O8 lattice is susceptible to applied strain, which resulted in a phase transition from paramagnetic to ferromagnetic under biaxial strain. The Curie temperature was also evaluated using Monte Carlo simulations within the Ising model. Additionally, our research uncovered that the 2D NiCl2O8 lattice chain displays a negative Poisson's ratio (NPR) along the z-direction. The triangular hinge structure in its centrosymmetric configuration was identified as the origin of this unique phenomenon. The coexistence of NPR and magnetic phase transition properties in the NiCl2O8 lattice makes it quite promising for applications in nanoelectronic and spintronic devices.

Dirac semimetallic Janus Ni-trihalide monolayer with strain-tunable magnetic anisotropy and electronic properties

Two-dimensional (2D) ferromagnetic (FM) semiconductors have been paid much attention due to the potential applications in spintronics. Here, the electronic and magnetic properties of 2D Janus Ni-trihalide monolayer Ni2X3Y3 (X, Y = I, Br, Cl; X ≠ Y) are investigated by first-principle calculations. The properties of Ni2X3Y3 (X, Y = I, Br, Cl; X ≠ Y) monolayers are compared by selecting the NiCl3 monolayer as the reference material. Ni2X3Y3 monolayers have two distinct magnetic ground states of ferromagnetic (FM) and antiferromagnetic (AFM). In the Ni2X3Y3 monolayer, two different orbital splits were observed, one semiconductor state and the other semimetal state. The semimetal state of Ni2X3Y3 can be tuned to semiconductor or metallic state when biaxial strain is applied. The magnetic anisotropy energy (MAE) of the Ni2X3Y3 monolayer can display variations compared to that of the NiCl3 monolayer, with the direction of easy magnetization being influenced by the specific halogen elements present. The easy magnetization direction of Ni2X3Y3 can also be changed by applying biaxial strain. The Tc of Ni2X3Y3 is predicted to be about 100 K according to the calculation of the EAFM-EFM model. The design of the Janus Ni2X3Y3 structure has expanded the range of 2D magnetic materials, a significant contribution has been made to the advancement of spintronics and its applications.

Nonvolatile magnetoelectric coupling in two-dimensional van der Waals sandwich heterostructure CuInP2S6/MnCl3/CuInP2S6

Electrical control of magnetism is of great interest for low-energy-consumption spintronic applications. Due to the recent experimental breakthrough in two-dimensional materials, with the absence of hanging bonds on the surface and strong tolerance for lattice mismatch, heterogeneous integration of different two-dimensional materials provides a new opportunity for coupling between different physical properties. Here, we report the realization of nonvolatile magnetoelectric coupling in vdW sandwich heterostructure CuInP2S6/MnCl3/CuInP2S6. Using first-principles calculations, we reveal that when interfacing with ferroelectric CuInP2S6, the Dirac half-metallic state of monolayer MnCl3 will be destroyed. Moreover, depending on the electrically polarized direction of CuInP2S6, MnCl3 can be a half-metal or a ferromagnetic semiconductor. We unveil that the obtained ferromagnetic semiconductor in MnCl3 can be attributed to the different gain and loss of electrons on the two adjacent Mn atoms due to the sublattice symmetry broken by interlayer coupling. The effects of interfacial magnetoelectric coupling on magnetic anisotropy and ferromagnetic Curie temperature of MnCl3 are also investigated, and a multiferroic memory based on this model is designed. Our work not only provides a promising way to design nonvolatile electrical control of magnetism but also renders monolayer MnCl3 an appealing platform for developing low-dimensional memory devices.

Voltage control of magnetism in Fe3-xGeTe2/In2Se3 van der Waals ferromagnetic/ferroelectric heterostructures

We investigate the voltage control of magnetism in a van der Waals (vdW) heterostructure device consisting of two distinct vdW materials, the ferromagnetic Fe3-xGeTe2 and the ferroelectric In2Se3. It is observed that gate voltages applied to the Fe3-xGeTe2/In2Se3 heterostructure device modulate the magnetic properties of Fe3-xGeTe2 with significant decrease in coercive field for both positive and negative voltages. Raman spectroscopy on the heterostructure device shows voltage-dependent increase in the in-plane In2Se3 and Fe3-xGeTe2 lattice constants for both voltage polarities. Thus, the voltage-dependent decrease in the Fe3-xGeTe2 coercive field, regardless of the gate voltage polarity, can be attributed to the presence of in-plane tensile strain. This is supported by density functional theory calculations showing tensile-strain-induced reduction of the magnetocrystalline anisotropy, which in turn decreases the coercive field. Our results demonstrate an effective method to realize low-power voltage-controlled vdW spintronic devices utilizing the magnetoelectric effect in vdW ferromagnetic/ferroelectric heterostructures.

Strain-dependent magnetic ordering switching in 2D AFM ternary V-based chalcogenide monolayers

The lack of macroscopic magnetic moments makes antiferromagnetic materials promising candidates for high-speed spintronic devices. The 2D ternary V-based chalcogenides (VXYSe4; X, Y = Al, Ga) monolayers are investigated based on the density-functional theory and Monte Carlo simulations. The results reveal that the Néel temperature of the VGa2Se4 monolayer is 18 K with zigzag2-antiferromagnetic (AFM) spin ordering. Also, the magnetic ordering of V ions in VAl2Se4 and VAlGaSe4 monolayers prefer zigzag1-AFM coupling with Néel temperature of 47 K and 33 K, respectively. The magnetic anisotropy calculations demonstrate that the easy magnetization axis of the VXYSe4 monolayers is parallel to the y axis. In addition, the VXYSe4 monolayers can be adjusted from the AFM state to the ferromagnetic (FM) state under biaxial stretching, which can be attributed to the competition between d-p-d superexchange and d-d direct exchange caused by the variation of bond length. The transition temperature of VXYSe4 monolayers can be elevated above room temperature with the help of compression strain. In particular, the in-plane magnetic anisotropy is a robust characteristic regardless of the magnitude of the applied biaxial strain. These explorations not only enrich the family of AFM monolayers with excellent stability but also provide distinctive ideas for the performance control of AFM materials and their applications in nanodevices.

High-Entropy Layered Double Hydroxides with Highly Adjustable Components for Enhancing Electrocatalytic Oxygen Evolution

The main obstacle to the development of large-scale electrochemical hydrogen production based on water splitting is the slow four-electron kinetics of OER (oxygen evolution reaction). The most efficient method is to create sophisticated and effective OER catalysts. Here, we proposed the controlled synthesis of high-entropy layered double hydroxides (HELDH) for wide component regulation and the component design of high OER activity to make up for the restricted component regulation in conventional catalysts. Through the use of coprecipitation and hydrothermal synthesis, the representative sample (MgCoNi)3(FeAl)-LDH is created and systematically characterized. Significantly, this technique of preparation may generically synthesize a variety of HELDH with various component combinations, demonstrating the remarkable adaptability of the HELDH components. Subsequently, (FeCoNi)3(FeCr)-LDH with high OER activity is designed and synthesized. (FeCoNi)3(FeCr)-LDH shows excellent OER activity (overpotential is only 230 mV at 10 mA cm-2). A new platform for the creation of high-performance catalysts and high-entropy materials was established by the synthesis and design of HELDH.

MXene-based Ti2C/Ta2C lateral heterostructure: an intrinsic room temperature ferromagnetic material with large magnetic anisotropy

Two-dimensional (2D) lateral heterostructures (LH) combining Ti₂C and Ta₂C MXenes were investigated by means of first-principles calculations. Our structural and elastic properties calculations show that the lateral Ti₂C/Ta₂C heterostructure results in a 2D material that is stronger than the original isolated MXenes and other 2D monolayers such as germanene or MoS₂. The analysis of the evolution of the charge distribution with the size of the LH shows that, for small systems, it tends to distribute homogeneously between the two monolayers, whereas for larger systems electrons tend to accumulate in a region of ∼6 Å around the interface. The work function of the heterostructure, one crucial parameter in the design of electronic nanodevices, is found to be lower than that of some conventional 2D LH. Remarkably, all the heterostructures studied show a very high Curie temperature (between 696 K and 1082 K), high magnetic moments and high magnetic anisotropy energies. These features make (Ti₂C)/(Ta₂C) lateral heterostructures very suitable candidates for spintronic, photocatalysis, and data storage applications based upon 2D magnetic materials.

Magnetization reversal of perpendicular magnetic anisotropy regulated by ferroelectric polarization in CoFe3N/BaTiO3 heterostructures: first-principles calculations

Exploring the electric-field switching of perpendicular magnetic anisotropy (PMA) in multiferroic heterostructures has important physical significance, which attracts great interest due to its promising application for energy-efficient information storage. Herewith, we investigate the effect of ferroelectric polarization on magnetic anisotropy in CoFe₃N/BaTiO₃ heterostructures using first-principles calculations. The calculations reveal that the magnetic anisotropy of CoFe₃N can be regulated by ferroelectric polarization of BaTiO₃. When the ferroelectric polarization reverses, the PMA of FeCo-TiO₂ and FeN-BaO configurations remains, but in the FeN-TiO₂ and FeCo-BaO cases, magnetic anisotropy inverses between out-of-plane and in-plane direction. Further orbital-resolved analysis indicates that the transition of magnetic anisotropy is mainly attributed to the orbital hybridization of interfacial Fe/Co atoms with O atoms induced by the magnetoelectric effect. This study may open an effective approach toward modulating PMA and lays a foundation to the development of low energy consumption memory devices.

Theoretical studies on electronic, magnetic and optical properties of two dimensional transition metal trihalides

Two dimensional transition metal trihalides have drawn attention over the years due to their intrinsic ferromagnetism and associated large anisotropy at nanoscale. The interactions involved in these layered structures are of van der Waals types which are important for exfoliation to different thin samples. This enables one to compare the journey of physical properties from bulk structures to monolayer counterpart. In this topical review, the modulation of electronic, magnetic and optical properties by strain engineering, alloying, doping, defect engineering etc have been discussed extensively. The results obtained by first principle density functional theory calculations are verified by recent experimental observations. The relevant experimental synthesis of different morphological transition metal trihalides are highlighted. The feasibility of such routes may indicate other possible heterostructures. Apart from spintronics based applications, transition metal trihalides are potential candidates in sensing and data storage. Moreover, high thermoelectric figure of merit of chromium trihalides at higher temperatures leads to the possibility of multi-purpose applications. We hope this review will give important directions to further research in transition metal trihalide systems having tunable band gap with reduced dimensionalities.

Two-Dimensional Iron Silicide (FeSix) Alloys with Above-Room-Temperature Ferromagnetism

Two-dimensional (2D) materials with intrinsic room-temperature ferromagnetism have gathered tremendous interest as promising candidates for next-generation spintronics. Here, on the basis of first-principles calculations, we report a family of stable 2D iron silicide (FeSi_x) alloys via dimensional reduction of their bulk counterparts. Our results demonstrate that 2D Fe₄Si₂-hex, Fe₄Si₂-orth, Fe₃Si₂, and FeSi₂ nanosheets are lattice-dynamically and thermally stable, confirmed by the calculated phonon spectra and Born-Oppenheimer dynamic simulation up to 1000 K. 2D FeSi_x nanosheets are ferromagnetic metals with estimated Curie temperatures ranging from 547 to 971 K due to strong direct exchange interaction between Fe sites. In addition, the electronic properties of 2D FeSi_x alloys can be maintained on silicon substrates, providing an ideal platform for spintronics applications in the nanoscale.

Two-dimensional magnetic Janus monolayers and their van der Waals heterostructures: a review on recent progress

A magnetic Janus monolayer, a special type of material which has asymmetric arrangements of its surface at the nanoscale, has been shown to present rather exotic properties for applications in spintronics and its intersections. This review aims to offer a comprehensive review of the emergent physical properties of magnetic Janus monolayers and their van der Waals heterostructures from a theoretical point of view. The review starts by introducing the theoretical methodologies composed of the state-of-the-art methods and the challenges and limitations in validations for the descriptions of the magnetic ground states and thermodynamic properties in magnetic materials. The built-in polarization field induced physical phenomena of magnetic Janus monolayers are then presented. The tunable electronic and magnetic properties of magnetic Janus monolayer-based van der Waals heterostructures are discussed. Finally, the conclusions and future challenges in this field are prospected. This review serves as a complete summary of the two-dimensional magnetic Janus library and emergent electronic and magnetic properties in magnetic Janus monolayers and their heterostructures, and provides guidelines for the design of electronic and spintronic devices based on Janus materials.

Nonvolatile electrical control of valley splitting by ferroelectric polarization switching in a two-dimensional AgBiP2S6/CrBr3 multiferroic heterostructure

The generation and controllability of valley splitting are the major challenge in effectively utilizing valley degrees of freedom in valleytronics. Using first-principles calculations, we propose a novel multiferroic system, a AgBiP₂S₆/CrBr₃ van der Waals heterostructure, with ferromagnetism, ferroelectricity and ferrovalley behaviors. The ferroelectric monolayer AgBiP₂S₆ originally has two degenerate valleys with a large spin splitting (∼423.1 meV) at the conduction band minimum of K/K' points, due to inversion symmetry breaking combined with strong spin orbit coupling. Magnetic proximity coupling with the ferromagnetic layer CrBr₃ breaks the time-reversal symmetry, damaging the degeneracy of K/K' valleys and causing valley splitting (∼30.5 meV). The transition energy barrier between two ferroelectric states with opposite polarization direction of the heterostructure is sufficient to prevent the spontaneous transition at room temperature, and the large intermediate barrier suggests that the ferroelectric state should be observed experimentally under ambient conditions. Nonvolatile electrical control of the valley degrees of freedom is achieved by switching the polarization direction of the ferroelectric layer in the heterostructure. The modulation of valley splitting can also be achieved by applying an external electric field and biaxial strain, as well as changing the magnetization direction. The research of nonvolatile electrical control of valley splitting in the two-dimensional AgBiP₂S₆/CrBr₃ multiferroic heterostructure is crucial for designing all-in-one valleytronic devices, and has important theoretical significance and practical value.

Exploring a novel class of Janus MXenes by first principles calculations: structural, electronic and magnetic properties of Sc2CXT, X = O, F, OH; T = C, S, N

The already intriguing electronic and optical properties of the MXene Sc₂C family can be further tuned through a wide range of possible functionalizations. Here, by means of density functional theory, we show that the 36 possible elements of the Janus MXT (M: Sc₂C, X: O, F, OH, T: C, N, S) family, built by considering the four possible structural models (i) FCC, (ii) HCP, (iii) FCC + HCP, and (iv) HCP + FCC, are all potentially stable. The analysis of their mechanical properties shows the excellent mechanical flexibility of functionalized MXenes (f-MXenes) under large strain, making them more suitable for applications where stress could be an issue. Interestingly, while Sc₂C presents a metallic character, Sc₂COS, Sc₂CFN and Sc₂COHN are found to be semiconductors with bandgaps of 2.5 eV (indirect), 1.67 eV (indirect) and 1.1 eV (direct), respectively, which presents promising applications for nano- and optoelectronics. Moreover, Sc₂CFC presents a ferromagnetic ground state with the 2 × 2 × 1 supercell magnetic moment of 3.99 μ_B, while the ground state of Sc₂COHC might be antiferromagnetic with a magnetic moment of 3.98 μ_B, depending on the environment. Remarkably, the band structures of Sc₂CFC and Sc₂COHC present a half-metallic character with an HSE06 fundamental band gap of 0.60 eV and 0.48 eV, respectively. Our results confirm the extraordinary potential of the Janus MXT (M: Sc₂C, X: O, F, OH, T: C, N, S) family for novel applications in 2D nano-,opto- and spintronics.

Role of the Magnetic Anisotropy in Atomic-Spin Sensing of 1D Molecular Chains

One-dimensional metal-organic chains often possess a complex magnetic structure susceptible to modification by alteration of their chemical composition. The possibility to tune their magnetic properties provides an interesting playground to explore quasi-particle interactions in low-dimensional systems. Despite the great effort invested so far, a detailed understanding of the interactions governing the electronic and magnetic properties of the low-dimensional systems is still incomplete. One of the reasons is the limited ability to characterize their magnetic properties at the atomic scale. Here, we provide a comprehensive study of the magnetic properties of metal-organic one-dimensional (1D) coordination polymers consisting of 2,5-diamino-1,4-benzoquinonediimine ligands coordinated with Co or Cr atoms synthesized under ultrahigh-vacuum conditions on a Au(111) surface. A combination of integral X-ray spectroscopy with local-probe inelastic electron tunneling spectroscopy corroborated by multiplet analysis, density functional theory, and inelastic electron tunneling simulations enables us to obtain essential information about their magnetic structures, including the spin magnitude and orientation at the magnetic atoms, as well as the magnetic anisotropy.

Intrinsic spin, valley and piezoelectric polarizations in room-temperature ferrovalley Janus TiXY (XY = SCl and SeBr) monolayers

Two-dimensional room-temperature Janus ferrovalley semiconductors with large spin, valley and piezoelectric polarizations provide fertile platforms for designing multifunctional nanodevices. Little research has been reported to date on such materials. Here, using first-principles calculations, we predict two dynamically stable Janus titanium chalcohalide (TiSCl and TiSeBr) monolayers, which are excellent piezoelectric ferrovalley semiconductors with in-plane magnetization and high magnetic transition temperatures (738 and 884 K). When an extrinsic magnetic field is used to force the magnetization along the out-of-plane direction, a large valley polarization (64 and 146 meV) can be generated in the highest valence band with a large spin-orbit coupling by the breaking of time-reversal and space-inversion symmetry, which can be further clarified by a two-band k·p model. This robust valley-contrasting physics characterized by the valley-dependent Berry curvature leads to the anomalous valley Hall effect. It can be observed by suitable hole doping or light irradiation under an in-plane electric field. Besides, we find that the missing mirror symmetry results in giant out-of-plane piezoelectric polarization (2.05 and 2.04 pm V^-1). These outstanding properties give the Janus TiSCl and TiSeBr monolayers potential for a wide variety of applications in nanoelectronics, spintronics, valleytronics, piezoelectrics and other demanding areas.

Origin of metamagnetism in skyrmion host Cu[Formula: see text]OSeO[Formula: see text]

Skyrmion host chiral Cu[Formula: see text]OSeO[Formula: see text] has attracted researchers due to several intriguing properties. Observation of metamagnetism in low-temperature and low-field makes the magnetic properties of Cu[Formula: see text]OSeO[Formula: see text] more complex. Here, we present an investigation on metamagnetism in Cu[Formula: see text]OSeO[Formula: see text] by analyzing its structural and magnetic properties. Study of magnetic properties reveal spin-flip of one of the Cu[Formula: see text] ions, embedded in square pyramidal CuO[Formula: see text] polyhedra, due to the development of strain in low-temperature and low-field regime. The spin-flip is found to be the main reason for field-induced first-order metamagnetic transition. Magnetic phase diagram of Cu[Formula: see text]OSeO[Formula: see text] has been constructed with the help of magnetization analyses. It is argued that the metamagnetic hysteretic field region may be low-temperature skyrmion phase with additional spiral and tilted-conical phases. A tricritical point has been observed in the phase diagram at which first-order metamagnetic hysteretic field range ceases to exist.

Spin-flip-driven reversal of the angle-dependent magnetic torque in layered antiferromagnetic Ca0.9Sr0.1Co2As2

Spin-flip transition can occur in antiferromagnets with strong magnetocrystalline anisotropy, inducing a significant modification of the anisotropic magnetic properties through phase conversion. In contrast to ferromagnets, antiferromagnets have not been thoroughly examined in terms of their anisotropic characteristics. We investigated the magnetic-field and angle-dependent magnetic properties of Ising-type antiferromagnetic Ca_0.9Sr_0.1Co₂As₂ using magnetic torque measurements. An A-type antiferromagnetic order emerges below T_N = 97 K aligned along the magnetically easy c-axis. The reversal of the angle-dependent torque across the spin-flip transition was observed, revealing the strong influence of the magnetocrystalline anisotropy on the magnetic properties. Based on the easy-axis anisotropic spin model, we theoretically generated torque data and identified specific spin configurations associated with the magnetic torque variation in the presence of a rotating magnetic field. Our results enrich fundamental and applied research on diverse antiferromagnetic compounds by shedding new light on the distinct magnetic features of the Ising-type antiferromagnet.

Synthesis, Characterization, and First-Principles Analysis of the MAB-Like Ternary Transition-Metal Boride Fe(MoB)2

Novel transition-metal borides have attracted considerable attention because they exhibit high stability under extreme conditions. Compared with binary borides, ternary transition-metal borides (TTMBs) exhibit novel boron substructures and diverse properties, which result in excellent designability. In this study, we synthesized the MAB-like (where M = iron, A = molybdenum, and B = boron) phase Fe(MoB)₂ using a high-pressure and high-temperature method. Fe(MoB)₂ exhibited ferromagnetic metastable characteristics with a saturation magnetization of 8.35 emu/g at room temperature. Microhardness measurement revealed an indentation hardness of 10.72 GPa, which was higher than those of conventional magnetic materials. First-principles calculations revealed excellent mechanical properties, which mainly originated from the strong covalent short B₂ chains. Furthermore, magnetism was attributed to the Fe 3d electrons. Numerous d-d hybridizations existed between the Fe 3d e_g and Mo 4d orbitals, and the antibonding/nonbonding state difference for up/down-spin electrons in the hybridization orbitals led to the local magnetic moment of Fe(MoB)₂. The magnetic anisotropy energy analyses reveal that Fe(MoB)₂ prefers the easy magnetization axis along the z direction, and Mo atom acts as a medium to realize the exchange action between two Fe atoms. The B-B and Fe-B bonds were considerably stronger than the Fe-Mo and Mo-B bonds, and Fe(MoB)₂ exhibited a class of atomically laminate composed of FeB₂ and Mo layers. These results may provide guidance for the design of novel multifunctional TTMBs by adjusting the interactions between binary metal components.

AuCo nanoparticles: ordering, magnetisation, and morphology trends predicted by DFT

The rapid development of applications relying on magnetism at the nanoscale has put a spotlight on nanoparticles with novel morphologies that are associated with enhanced electronic and magnetic properties. In this quest, nanoalloys combining highly magnetic cobalt and weakly reactive gold could offer many application-specific advantages, such as strong magnetic anisotropy. In the present study, we have employed density functional theory (DFT) calculations to provide a systematic overview of the size- and morphology-dependence of the energetic order and magnetic properties of AuCo nanoparticles up to 2.5 nm in diameter. The core-shell icosahedron was captured as the most favourable morphology, showing a small preference over the core-shell decahedron. However, the magnetic properties (total magnetic moments and magnetic anisotropy) were found to be significantly improved within the L1₀ ordered structures, even in comparison to monometallic Co nanoparticles. Atom-resolved charges and orbital moments accessed through the DFT analysis of the electronic level properties permitted insight into the close interrelation between the AuCo nanoparticle morphology and their magnetism. These results are expected to assist in the design of tailored magnetic AuCo nanoalloys for specific applications.

Giant magnetoresistance in spin valves realized by substitutingY-site atoms in Heusler lattice

'All-Heusler' spin-valve constructed by two half-metallic Heusler electrodes and a non-magnetic Heusler spacer contains two interfaces that have a crucial influence on the magnetoresistance. In order to reduce the disorder at the interface and protect the half metallicity of the electrode at the same region, we propose a scheme to construct a spin valve by replacing theY-site atoms in the half-metallic Heusler electrode to obtain the corresponding non-magnetic spacer based on the Slater-Pauling rule. In this way, the lattice and band match of the two materials can be ensured naturally. By using Co₂FeAl as electrode and Co₂ScAl as the spacer materials, we construct the Co₂FeAl/Co₂ScAl/Co₂FeAl(001)-spin valve. Based on the first-principles calculation, the most stable FeAl/CoCo-interface is determined both from the phonon spectra and the formation energy when the spacer Co₂ScAl grows on the FeAl-terminated (001) surface of electrode material Co₂FeAl. By comparing the projected density of states of the interfacial atoms with the corresponding density of states of the bulk electrode material, only the value of spin-up state of Al changes from 0.17 states/atom/eV to 0.06 states/atom/eV before and after substitution, the half metallicity at the interface is maintained. As a result, the spin-dependent transport properties show significant theoretical magnetoresistance MR_opwhich can reach up to 10¹⁰% and much larger than 10⁶% reported before.

Enhancement of the magnetic anisotropy in single semiconductor nanowires via surface doping and adatom deposition

The magnetic anisotropy of single semiconductor (ZnO and GaN) nanowires incorporating both a transition metal (Co and Mn, respectively) as a substitutional surface dopant and a heavy metal (Au, Bi, or Pt) adatom is studied by performing density-functional supercell calculations with the HubbardUcorrection. It is found that a substantial enhancement in the magnetic anisotropy energy is obtained through the deposition of Bi; the deposition of Au and Pt leads to significant variation in other magnetic properties, but not in the magnetic anisotropy energy. An analysis within a band description shows that the coexistence of Bi adatom and a surface dopant with large spin moment activates a mechanism involving reorientation and readjustment of the spin moments of electrons in occupied bands in response to the change of magnetization direction, which promotes giant magnetic anisotropy. Our results for adsorption energetics indicate that the accommodation of Bi in the neighborhood of the surface dopant is more likely in GaN nanowires, because the Bi adatom does (not) tend to be closer to the Mn (Co) dopant on the surface of GaN (ZnO) nanowire. The stability of GaN nanowire with giant magnetic anisotropy owing to the incorporation of both Mn and Bi is demonstrated by performingab initiomolecular dynamics simulations at temperatures considerably higher than room temperature. These results suggest that adatom deposition and surface doping can be used complementarily to develop single nanowire-based spintronic devices.

Valley splitting and magnetic anisotropy in two-dimensional VI3/MSe2 (M = W, Mo) heterostructures

As a new van der Waals ferromagnetic material, VI₃ can be used to lift the valley degeneracy of transition metal dichalcogenides at the K' and K points. Here, the electronic structure and magnetic anisotropy of the VI₃/MSe₂ (M = W, Mo) heterostructures are studied. The VI₃/WSe₂ heterostructure is semiconducting with a band gap of 0.26 eV, while the VI₃/MoSe₂ heterostructure is metallic. Considering the spin-orbit-coupling, a maximum valley splitting of 3.1 meV appears in the VI₃/WSe₂ heterostructure. The biaxial strain can tune the valley splitting and magnetic anisotropy of VI₃/MSe₂ heterostructures. In the VI₃/WSe₂ heterostructure, which has the most stable stacking, valley splitting can be increased from 1.8 meV at 4% compressive strain to 3.1 meV at 4% tensile strain. At a biaxial strain of -2% to 4%, the VI₃/WSe₂ heterostructure maintains a small perpendicular magnetic anisotropy, while the VI₃/MoSe₂ heterostructure shows in-plane magnetic anisotropy under different strains. The significantly tunable electronic structure and magnetic anisotropy under biaxial strain suggest that the VI₃/MSe₂ heterostructures have potential applications in spintronic devices.

Monolayer gadolinium halides, GdX2 (X = F, Cl, Br): intrinsic ferrovalley materials with spontaneous spin and valley polarizations

Two-dimensional (2D) intrinsic ferrovalley semiconductors provide unprecedented opportunities to investigate valley physics as well as providing promising device applications due to their exceptional combination of spontaneous spin and valley polarizations. Here, we have predicted from first-principles calculations and Monte Carlo simulations that monolayers (MLs) GdX₂ are such extremely rare excellent materials. Apart from their robust stabilities energetically, dynamically, thermally, and mechanically, these 2D materials are found to be semiconducting intrinsic ferromagnets where the magnetic coupling is ascribed to 5d-electron-mediated 4f-4f exchange interactions. Moreover, MLs GdX₂ (X = F, Cl, Br) not only exhibit significant magnetic anisotropy energy of 351, 268, and 30 μeV per Gd, but also have a high Curie temperature of 300, 245, and 225 K, respectively. In particular, spontaneous valley polarization in three systems occurs due to the cooperative interplay between the spin-orbit coupling and magnetic exchange interactions, whose magnitude is as sizable as 55, 38, and 82 meV for MLs GdF₂, GdCl₂, and GdBr₂, respectively. Under the action of an in-plane longitudinal electrical field, the valley-contrasting Berry curvatures arising from the broken space-inversion and time-reversal symmetries in MLs GdX₂ could yield opposite transverse velocities of the carriers, giving rise to the occurrence of a spin-polarized anomalous valley Hall effect. Overall, these findings render 2D GdX₂ a class of promising candidate materials for experimental studies and practical spintronics and valleytronics applications.

Theoretical scheme of the nonvolatile strain switchable high/low resistance based on the novel strain-tunable magnetic anisotropy in Mn2.25Co0.75Ga0.5Sn0.5/MgO superlattice

It is great desirable to effectively tune electronic and magnetic properties by simple means such as applying external electrical fields or strains in spintronics research. Here, we investigate the strain...

Spontaneous magnetic merons in a half-metallic Mn2I3Br3 monolayer with easy-plane anisotropy

Spontaneous magnetic merons are found to exist in a wide magnetic field range (0–6 T) stabilized by the large in-plane magnetic anisotropy and strong Dzyaloshinskii–Moriya interaction in a half-metallic Mn2I3Br3 monolayer.

Large magnetic anisotropy in Co-Fe-Ni-N ordered structures: a first-principles study

Material design of promising rare-earth free permanent magnet requires tailoring and controlling the intrinsic magnetic properties namely large saturation magnetizationμ₀M_s, giant uniaxial magnetic anisotropyK_u, and high Curie temperatureT_C. Based on first-principles electronic structure calculations, we present a detailed analysis for the intrinsic magnetic properties of Co_xFe_1-xNi and Co_xFe_1-xNiN_0.25ordered structures. We predict an enhanced structural stability with improvedK_uranging from 1.53-2.29 MJ m^-3for Co_xFe_1-xNiN_0.25ordered structures, with the exception of CoNiN_0.25having planar anisotropy. Detailed analysis of the predicted largeK_u, based on perturbation theory and electronic structure calculations, is attributed to the cumulative effect of contribution from the increased tetragonal distortion and induced orbital distortion from the simultaneous Co substitution and interstitial N-doping. By tailoring theK_u, we may create efficient and affordable PMs, bridging the gap between commonly used ferrite and high-performance Nd-Fe-B magnets.

Reduction in magnetic coercivity of Co nanomagnets by Fe alloying

We measured the magnetic hysteresis and coercivity of individual Co and Co_0.8Fe_0.2 bilayer nano-sized island structures formed on Cu (111) substrate using spin-polarized scanning tunneling microscopy. From the hysteresis taken on various sizes of islands, we found that the alloyed islands are ferromagnetic with out-of-plane magnetic anisotropy, same as the pure islands. Coercivity of the alloy islands, which is dependent on their size, was significantly reduced to ≈40% of that of the pure islands. Based on the Stoner-Wohlfarth model, we evaluated the amount of magnetic anisotropic energy and anisotropy constant for both pure and alloy islands. Since tunneling spectra taken on the alloy islands show upward shifts of the valence electronic states as compared to the pure ones, fewer electrons populated in the valence band of the alloy islands are presumably responsible for the reduction in the magnetic anisotropic energy.

Effects of Interfacial Termination, Oxidation, and Film Thickness on the Magnetic Anisotropy in Mn2.25Co0.75Ga0.5Sn0.5/MgO Heterostructures

Perpendicular magnetic anisotropy (PMA) is a determining factor for the realization of nonvolatile information storage devices with high efficiency and thermal stability. In this work, a new spin gapless semiconductor Mn_2.25Co_0.75Ga_0.5Sn_0.5 Heusler alloy with an inter-spin zero gap was first designed theoretically. The Mn_2.25Co_0.75Ga_0.5Sn_0.5 bulk was prepared successfully in experiment. The effects of interfacial termination, oxidation, and film thickness on the magnetic anisotropy of Mn_2.25Co_0.75Ga_0.5Sn_0.5/MgO (MCGS/MgO) heterostructures are investigated systematically by first-principles calculations. The results show that all the Mn(A)Mn(C)GaSn-, Mn(A)Mn(C)CoGaSn-, Mn(B)GaSn_I-, and Mn(B)GaSn_II-terminated MCGS/MgO heterostructures (called as AC1, AC2, BD1, and BD2 models, respectively) present PMA, which mainly derives from the interfacial and surficial MCGS layers. Furthermore, the PMA of MCGS/MgO heterostructures can be preserved in a large range of interfacial oxidization (up to ±50%). With MCGS thickness increasing from 5 to 16 monolayers, the PMA of MCGS/MgO heterostructures with an AC-type surface decreases significantly. However, the PMA of BD-type surface models is relatively robust to the thickness of the MCGS layer, and the magnetic anisotropy always points to the out-of-plane direction. Therefore, MCGS Heusler alloy is a new promising spin gapless semiconductor candidate for spintronics applications. The robust and tunable PMA in MCGS/MgO heterostructures offers the possibility for developing nonvolatile data memory devices.

Induced half-metallic characteristics and enhanced magnetic anisotropy in the two-dimensional Janus V2I3Br3 monolayer by graphyne adsorption

The recent emergence of two-dimensional (2D) Janus materials has opened a new avenue for spintronic and optoelectronic applications. However, 2D magnetic Janus materials and Janus monolayer-based magnetic heterostructures are yet to be fully studied. Herein, the stability and electronic structure of 2D Janus V₂I₃Br₃ and V₂I₃Cl₃ monolayers, and the electronic and magnetic properties of 2D graphyne/Janus V₂I₃Br₃ (γ-GY/V₂I₃Br₃) heterostructures are calculated based on the density functional theory. Janus V₂I₃Br₃ and V₂I₃Cl₃ monolayers are ferromagnetic semiconductors with good stability and direct band gap. By combing the graphyne layer, the Janus V₂I₃Br₃ monolayer shows half-metallic characteristics. The electrical conductivity of the Janus V₂I₃Br₃ monolayer in γ-GY/V₂I₃Br₃ heterostructures is further improved, which is very favorable for the applications of the γ-GY/V₂I₃Br₃ heterostructure in battery anodes. Moreover, the Janus V₂I₃Br₃ monolayer possesses a smaller perpendicular magnetic anisotropy (PMA), and the PMA can be effectively enhanced by combing γ-GY. Herein, the enhanced PMA was discovered to depend on the stacking patterns of γ-GY and V₂I₃Br₃ monolayers. Biaxial strains can further affect the PMA of the γ-GY/V₂I₃Br₃ heterostructure. Meanwhile, at a compressive strain, the Janus V₂I₃Br₃ monolayer in the γ-GY/V₂I₃Br₃ heterostructure realizes the transition from the magnetic half-metallic to the magnetic metal state. These results can enrich the applications and designs of γ-GY/V₂I₃Br₃ magnetic heterostructures in spintronic devices and energy fields.

Ferromagnetic Dirac half-metallicity in transition metal embedded honeycomb borophene

Exploring two-dimensional (2D) ferromagnetic materials with intrinsic Dirac half-metallicity is crucial for the development of next-generation spintronic devices. Based on first-principles calculations, here we propose a simple valence electron-counting rule to design such materials and endow them with good stability and desirable magnetic properties. Taking honeycomb borophene as a prototype, we demonstrate that embedding open-shell transition metal (like Cr) atoms in the hexagonal ring of boron atoms can provide two valence electrons to fully occupy the in-plane σ and out-of-plane π bands of B atoms. The remaining four valence electrons reside in d orbitals that split under C_6v symmetry, yielding a magnetic moment of ∼2 μ_B per Cr atom. The resulting CrB₂ monolayer exhibits a Dirac half-metal band structure, a high Curie temperature of 175 K, and a large out-of-plane magnetic anisotropy energy of 4 meV per Cr simultaneously. Our work establishes a feasible route for the experimental realization of ferromagnetic Dirac half-metallicity in 2D materials and provides new opportunity to realize high-speed devices with low consumption.

Effect of coverage on the magnetic properties of -COOH, -SH, and -NH2 ligand-protected cobalt nanoparticles

Implementation of magnetic nanoparticles in biomedicine requires their passivation, which often comes at a cost of diminished magnetic properties. For the design of nano-agents with targeted magnetic behaviour, it is important to distinguish between ligands which can improve desired performance, and those that reduce it. Carboxylic acid-, thiol-, and amine-protected cobalt nanoparticles were studied by density functional theory calculations to model the impact of ligand coverage on the magnetic properties. The simulations show that the functional group, arrangement, and coverage density of the ligand coating control both the total magnetic moment and magnetic anisotropy energy of the nanoparticle, as well as the distribution of local spin magnetic moments across the metallic core. Captured effects of ligand binding on the orbital moments of cobalt atoms were insignificant. Out of the three ligand families, only carboxylic acid coatings increased the magnetic moments of cobalt nanoparticles, while amines and thiols quenched them. Calculated anisotropy energies of protected nanoparticles consistently increased with the growing ligand density, reaching the highest values for a 100% coverage of both carboxylic acid and thiol coatings. However, the binding nature of the two functional groups showed opposite impacts on the d-states of interacting cobalt atoms. This study has thus established important principles for the design of biocompatible magnetic nanocomposites, highlighting different routes to achieve desired magnetic behaviour.

Conversation from antiferromagnetic MnBr2 to ferromagnetic Mn3Br8 monolayer with large MAE

A pressing need in low energy spintronics is two-dimensional (2D) ferromagnets with Curie temperature above the liquid-nitrogen temperature (77 K), and sizeable magnetic anisotropy. We studied Mn3Br8 monolayer which is obtained via inducing Mn vacancy at 1/4 population in MnBr2 monolayer. Such defective configuration is designed to change the coordination structure of the Mn-d5 and achieve ferromagnetism with sizeable magnetic anisotropy energy (MAE). Our calculations show that Mn3Br8 monolayer is a ferromagnetic (FM) half-metal with Curie temperature of 130 K, large MAE of - 2.33 meV per formula unit, and atomic magnetic moment of 13/3μB for the Mn atom. Additionally, Mn3Br8 monolayer maintains to be FM under small biaxial strain, whose Curie temperature under 5% compressive strain is 160 K. Additionally, both biaxial strain and carrier doping make the MAE increases, which mainly contributed by the magneto-crystalline anisotropy energy (MCE). Our designed defective structure of MnBr2 monolayer provides a simple but effective way to achieve ferromagnetism with large MAE in 2D materials.

Emergence of magnetic anisotropy by surface adsorption of transition metal dimers on γ-graphyne framework

In this paper a systematic study is carried out to demonstrate the structural stability and magnetic novelty of adsorbing transition metal (TM) dimers (A-B) on graphyne (GY) surface, GY@A-B. Our research points out that the dimers are strongly adsorbed onto GY due to their large natural pores and the electron affinity of the sp-hybridized carbon atoms. Electronic properties of these dimer-graphyne composite systems are of particular importance as they behave as degenerate semiconductors with partial occupation of states atE_F. Furthermore, their remarkable spin polarization (>80%) at Fermi energy (E_F) can be of paramount importance in spintronics applications. Most of the GY@A-B structures exhibit large magnetic anisotropies as well as magnetic moments along the out-of-plane direction with respect to the GY surface. Particularly, GY@Co-Ir, GY@Ir-Ir and GY@Ir-Os structures possess positive magnetic anisotropic energies (MAE) of 121 meV, 81 meV and 137 meV, respectively, which are comparable to other well-known TM dimer doped systems. The emergence of high MAE can be understood using the second-order perturbation theory on the basis of the strong spin-orbit coupling (SOC) between the two TMs and the degeneracy of their d-orbitals nearE_F. A close correspondence between the simulated and the analytical results has been established through our work. Further, a simple estimation shows that, GY@A-B structures have the potential to store data up to 64 PB m^-2. These intriguing electronic characteristics along with magnetism suggest GY@A-B to be a promising material for future magnetic storage devices.

A first-principles study on the electronic property and magnetic anisotropy of ferromagnetic CrOF and CrOCl monolayers

Two-dimensional ferromagnetic materials with large perpendicular magnetic anisotropy (PMA) hold great potential in realizing low critical switching current, high thermal stability and high density nonvolatile storage in magnetic random-access memories. Our first-principles calculations reveal that CrOF and CrOCl monolayers (MLs) are two-dimensional (2D) ferromagnetic semiconductors with out-of-plane magnetic easy axis, and PMAs of CrOF and CrOCl MLs are mainly contributed by Cr atoms. The magnetic anisotropy of CrOF and CrOCl MLs can be controlled and enhanced by applying biaxial strain. Tensile strain can further enhance PMAs of CrOF and CrOCl MLs by 82.9% and 161.0% higher than those of unstrained systems, respectively. In addition, appropriate compressive strain can switch the magnetic easy axis of CrOF and CrOCl MLs from out-of-plane direction to in-plane direction. The semiconductor natures of CrOF and CrOCl MLs robust against biaxial strain, the band gaps of these systems under biaxial strain are in the range of 1.26 eV to 2.40 eV. By applying biaxial strain, the Curie temperatures of CrOF and CrOCl MLs increase up to 282 K and 163 K, respectively. These tunable properties suggest that CrOF and CrOCl MLs have great application potentials for magnetic data storage.

Intrinsic ferromagnetic semiconductors in rhombohedral RMnO3 (R = Sc, Y, and Lu) with high critical temperature and large ferroelectric polarization

Ferromagnetic (FM) semiconductors have been recognized as the cornerstone for next-generation highly functional spintronic devices. However, the development in practical applications of FM semiconductors is limited by their low Curie temperatures (T _C). Here, on the basis of model analysis, we find that the FM super-exchange couplings in the d ⁵ - d ³ system can be significantly strengthened by reducing the virtual exchange gap (G _ex) between occupied and empty e _g orbitals. By first-principle calculations, we predict robust ferromagnetism in three rhombohedral RMnO₃ (R = Sc, Y, and Lu) compounds with the T _C that is as high as ∼1510 K (YMnO₃). The oxygen breathing motions open a band gap and create an unusual Mn²⁺/Mn⁴⁺ charge ordering of the Mn-d electrons, which play an important role in altering the G _ex. Interestingly, the rhombohedral RMnO₃ compounds are also ferroelectric (FE) with a large spontaneous polarization approaching that of LiNbO₃. These results not only deepen the understandings of magnetic couplings in d ⁵ - d ³ system, but also provide a way to design room-temperature FM-FE multiferroics.

Transition-metal adatoms on 2D-GaAs: a route to chiral magnetic 2D materials by design

Using relativistic density-functional calculations, we examine the magneto-crystalline anisotropy and exchange properties of transition-metal atoms adsorbed on 2D-GaAs. We show that single Mn and Mo atom (Co and Os) strongly bind on 2D-GaAs, and induce local out-of-plane (in-plane) magnetic anisotropy. When a pair of TM atoms is adsorbed on 2D-GaAs in a close range from each other, magnetisation properties change (become tunable) with respect to concentrations and ordering of the adatoms. In all cases, we reveal presence of strong Dzyaloshinskii-Moriya interaction. These results indicate novel pathways towards two-dimensional chiral magnetic materials by design, tailored for desired applications in magneto-electronics.

Electric field induced reversal of spin polarization, magnetic anisotropy and tailored Dzyaloshinskii-Moriya interaction in underoxidized SrRuO3/SrTiO3 heterostructures

Electric field tailored magnetic properties of the perovskite-type oxide heterostructures are important in spintronic devices with low energy consumption and small size. Here, the electric field modulated magnetic properties of underoxidized SrRuO3 (SRO)/SrTiO3 (STO) heterostructures are investigated using first-principles calculations. The spin polarization of underoxidized SRO/STO heterostructures turns from negative to positive as the electric field changes from -0.2 to 0.2 V nm-1. The underoxidized SRO/STO heterostructure with 7 SRO atomic layers turns from perpendicular magnetic anisotropy to in-plane magnetic anisotropy as the electric field turns from -0.2 to 0.2 V nm-1, which can be attributed to the in-plane dx2-y2 and out-of-plane dxz, dyz orbitals. The Dzyaloshinskii-Moriya interaction of underoxidized SRO/STO heterostructures can also be effectively tailored using an electric field. These results indicate that the use of electric field is an effective method to modulate magnetic properties of perovskite-type oxide heterostructures, which is beneficial for the development of the high-performance spintronic devices.

First-principles explorations on P8 and N2 assembled nanowire and nanosheet

‘Bottom-up’ method is a powerful approach to design nanomaterials with desired properties. The bottle neck of being oxidized of phosphorous structures may be conquered by cluster assembling method. Here, we used P8 and N2 as assembling units to construct one-dimensional (1D) nanowire (NW) and two-dimensional (2D) nanosheet (NS), the stability, electronic and magnetic properties of these assembled nanomaterials are investigated using density functional theory (DFT) calculations. The assembled 1D-P8N2 NW and 2D-P8N4 NS are identified to possess good stability, as demonstrated by their high cohesive energies, positive phonon dispersions, and structural integrity through molecular dynamics simulations at 300 and 500 K. Moreover, they also exhibit good anti-oxidization property. The 2D-P8N4 NS is a direct bandgap semiconductor with the HSE06 gap of 2.61 eV, and shows appropriate band-edge aliments and moderate carrier mobility for photocatalyzing water splitting. The 1D-P8N2 NW is an indirect bandgap semiconductor, and Mn doping could convert it into a dilute magnetic semiconductor (DMS) with one Dirac cone in the spin-up channel, while the vdW-type sheet composed of Mn1@1D-P8N2 NWs is a ferromagnetic metal. Our theoretical study is helpful to design stable phosphorus-based nanomaterials with diverse properties and potential applications.

Electric polarization related Dirac half-metallicity in Mn-trihalide Janus monolayers

A two-dimensional Dirac half-metal system, in which the 100% spin polarization and massless Dirac fermions can coexist, will show more advantages on the efficient spin injection and high spin mobility in spintronic devices. Moreover, it is attractive to achieve out-of-plane electric polarization in addition to the Dirac half-metal behavior, because this will open a new horizon in the field of multifunctional devices. In this work, a systematic study is made of Janus monolayers of Mn2X3Y3 (X, Y = Cl, Br and I, X ≠ Y) with asymmetric out-of-plane structural configurations, based on first-principles calculations. We demonstrate that monolayer Mn2X3Y3 freestanding films will remain stable experimentally by using the stability analysis. All the Janus monolayers show a ferromagnetic ground state and maintain their original DHM behavior. However, due to the large electric polarization, the hybridization intensities of Mn and the halogen atoms on both sides of Mn2Cl3I3 are very different, resulting in an obvious distortion of the spin-polarized Dirac cone. The distorted Dirac cone is repaired by the compression, indicating that strain can improve the orbital distortion induced by the electric polarization. All Mn2X3Y3 monolayer have in-plane magnetization anisotropy, which is mainly contributed by heavy halogen elements (Br and I), and the polarized substitution and biaxial strain will not change the easy magnetization orientation of the system. Thus, the electrically polarized Dirac half-metal system has potential for application in multifunctional spintronic devices.

Tunable valley polarization, magnetic anisotropy and Dzyaloshinskii-Moriya interaction in two-dimensional intrinsic ferromagnetic Janus 2H-VSeX (X = S, Te) monolayers

Two-dimensional (2D) Janus materials are a novel kind of 2D materials, which have potential applications in nanoelectronics, optoelectronics and spintronics. However, a 2D Janus material combined with intrinsic ferromagnetism, electric dipole moment, valley polarization and Dzyaloshinskii-Moriya interaction (DMI) remains rarely reported. Here, the electronic structure and magnetic properties of 2D intrinsic ferromagnetic Janus 2H-VSeX (X = S, Te) monolayers are investigated systematically using the density-functional theory. Janus 2H-VSeX (X = S, Te) monolayers are intrinsic ferromagnetic semiconductors with in-plane magnetic anisotropy (IMA). The valley splitting of Janus 2H-VSeX (X = S, Te) monolayers appears by considering the spin-orbit coupling (SOC) effect and out of plane magnetization. Additionally, spontaneous vertical electric dipole moment and a large DMI are also found in Janus 2H-VSeX (X = S, Te) monolayers due to the broken inversion symmetry. Moreover, the valley splitting and DMI can be significantly increased by applying in-plane biaxial strain. These results provide an interesting 2D intrinsic ferromagnetic Janus material, which has potential applications in spintronic and valleytronic devices.

Thickness and Ferroelectric Polarization Influence on Film Magnetic Anisotropy across a Multiferroic Material Interface

The ferroelectric switching effect on perpendicular magnetic anisotropy is examined for the case of the BaTiO₃/L1₀-CoFe interface through first-principles calculations of film magnetocrystalline anisotropy energy (MAE), both with the frozen-potential method and the second-order perturbation theory. The ferroelectric switching-MAE relationship is shown to have opposite trends for BaO- and TiO₂-terminated interfaces because of the distinct orbital interaction mechanisms predominant in each termination configuration. The ferroelectric switching effect, changes in Fe-O bond lengths, and termination constitute three different contributors to MAE change, each with a different penetration depth into the CoFe film. The top surface CoFe atoms are shown to feature a high density of minority-spin 3d_xz states, which could play a role in influencing the ferroelectric switching-MAE relationship in cases where the top surface undergoes modifications.