The Study Of Half-metallic 2D TMN₂ With Room Temperature Ferromagnetism

Since the discovery of graphene,two-dimensional(2D)materials have became the hotspots in the fields of physics,materialogy,chemistry,etc.With the development of experimental technology,an increasing number of graphene-like materials,such as silicene,black phosphorus,transition metal dichalcogenides(TMDs),have been synthesized in recent years.These monolayer materials hold promising application in numerous fields due to their fantastic physical and chemical properities.However,most of these 2D materials are neither intrinsically nonmagnetic,or magnetic with weak ferromagnetic coupling.Thus their practical applications are being formidably hindered by the difficulty in achieving room-temperature ferromagnetism from their pristine structure.In this context,computational search for 2D intrinsic room-temperature ferromagnetic materials,particularly the full spin-polarized half-metals,is considerably significant via first-principles calculations.This will not only shorten the experimental period,and speed up the development process of spintronic devices,but also provide perfect platform for deeper theoretical research.In this work,taking the new 2D transition-metaldinitries(TMN2)monolayers as research object,we have carried out the following calculation by used the spin-polarized method.(i)First-principles computational search has been used in thirty possible structures of TMN,(TM=Ti-Fe,Zr-Ru,Hf-Os)to develop intrinsic half-metallic nanomaterials with robust ferromagnetic ground state.Our comprehensive calculations of stability and magnetic properties show that the octahedral coordinated 1T-TaN2 monolayer is not only dynamically,thermally(500 K)and mechanically stable,but also possesses a robust FM ground state with Curie temperature of-339 K due to the strong N-N direct exchange interaction.Moreover,its half-metallic gap of 0.72 eV obtained from HSE06 method is large enough to efficiently prevent the thermally agitated spin-flip transition.Unlike the reported 2D half-metallic TM compounds,the half-metallicity and magnetic moments of 1T-TaN2 are mainly attributed to the p orbitals of non-metal atoms(N)instead of d orbitals of TM atoms(Ta),which is beneficial for overcoming the issue of short spin-relaxation-time caused by large spin coupling of TM atoms.The 2D 1T-TaN2 monolayer is looking forward to being the ideal materials for next-generation nanospintronics devices.(ii)In spintronics,it is highly desirable to find a new material holding completespin-polarization,high speed spin electrons,large Curie temperature and robust ferromagnetic ground state at the same time.Using first-principles calculations,we demonstrate that the stableYN2 monolayerwith octahedral coordinationis a novel p-state Dirac half metal(DHM),which not only has fully spin-polarized Dirac state,but also possesses the highest Fermi velocity(3.74×105 m/s)reported to datein the DHMs.In addition,being similar to 1T-TaN2,its half metallic gap of 1.53 eVis large enough to prevent spin flip transition.The estimate based on mean-field theory shows that the Curie temperature of YN2 monolayer is higher than room tepperature due to the strong nonlocal N-p direct exchange interaction.Thus,ferromagnetism in the YN2 monolayer is robust enough against electron and hole doping,as well as the external strain that might be induced by the interaction with the substrate.