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

Stacking induced symmetry breaking and gap opening in Dirac half-metal MnF3

Two-dimensional ferromagnetic materials have a broader development prospect in the field of spintronics. In particular, the high spin polarization system with half-metallic characteristics can be used as an efficient spin injection electrode. Via first-principles calculations, we predict that monolayer MnF3 has Dirac half-metallic properties. The formation mechanism of this Dirac half-metallic state is mainly attributed to the local symmetry of magnetic Mn3+ in the sublattice. It is interesting to note that the local symmetry can be broken in bilayer MnF3 through different stacking configurations. Therefore, different stacking models of bilayer MnF3 are established, and the calculation of their magnetic ground states shows that all the systems maintain a ferromagnetic ground state. The AA-stacking holds the symmetry and Dirac electronic states. Under interfacial Coulomb repulsion, the Dirac electronic states of the top layer and bottom layer of MnF3 are shifted relative to each other and overlap to form a nodal-ring state. Interestingly, in the AB-stacking model, the inversion symmetry exists, while the sublattice symmetry of Mn is broken, resulting in different orbital filling of Mn and forming a large insulating gap (732.2 meV). Additionally, the inversion symmetry of the system is broken in AC-stacking, while the intralayer sublattice symmetry is preserved. Therefore, under the effect of broken inversion symmetry, the Dirac electronic states of both top and bottom layer MnF3 will have a small gap opening (24.6 meV). The topological properties of all three systems have been analyzed. Based on the above research results, the electronic states of the system can be regulated by changing the stacking model between the 2D magnetic homostructure, which provides an ideal platform for the design and development of spin logic devices.

First-Principles Calculations of the Phonon, Elastic, and Thermoelectric Properties of a Ti2CO2 Monolayer

The phonon, elastic, and thermoelectric properties of Ti2CO2 are investigated by first-principles calculations. The dynamic and mechanical stabilities of Ti2CO2 are confirmed. The Ti2CO2 monolayer exhibits strong acoustic-optical coupling with the lowest optical frequency of 122.83 cm-1. The TA mode originates from the contribution of Ti(XY) vibrations and has the largest gruneisen parameter at the Γ point; the LA mode has the main contribution of O(XY) and Ti(XY) vibrations and has the lowest gruneisen parameter at the M point. The analysis of the phonon spectrum indicates that the vibration contributions from C, O, and Ti atoms are mainly located in the low-, middle-, and high-energy regions, respectively. The Seebeck coefficient and electronic conductivity increase with increasing carrier concentration under room temperature. The analysis of mechanical properties shows that Ti2CO2 possesses a larger Young's modulus and bending modulus, which has a better ability to resist deformation. Thermal properties are further investigated.

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.

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 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.

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.