First-principles Study On The Structure And Magnetic Properties Of Two-dimensional Transition Metal Carbide Materials

Spintronic devices have become an important development direction of information technology due to their advantages of non-volatility,low energy consumption and high speed.The emergence of two-dimensional materials makes spintronic devices based on two-dimensional materials have the characteristics of miniaturization and low energy consumption.Magnetic materials are the basis of spintronic devices.The emerging two-dimensional magnetic materials rely on rich and novel magnetic phenomena,and provide a platform for the construction of a new generation of spintronic devices.Therefore,the search for novel two-dimensional magnetic materials with intrinsic ferromagnetism,especially stable existence at room temperature,is particularly critical for the preparation of a new generation of spintronic devices.Among them,as a new type of environmentally friendly and pollution-free two-dimensional material,two-dimensional magnetic transition metal carbide has been widely used in spintronic devices because of its atomic thickness,flat surface and rich and adjustable magnetic properties.Studies have shown that it is possible to design smaller and more efficient spintronic devices by modulating the magnetic properties of two-dimensional transition metal carbides.In this paper,the electronic structure and magnetic regulation of Fe-and Co-based carbides with intrinsic ferromagnetism have been systematically studied using first principles.This work will provide a theoretical reference for the application of two-dimensional materials in spintronics devices.The main research contents are summarized as follows:（1）Fe₂C monolayer is ferromagnetic metal with the Curie temperature（T_C）of 510 K.A phase transition from ferromagnetic（FM）to antiferromagnetic（AFM）ordering under large biaxial strain.Introducing the tensile strain,the magnetic moments,spin polarization（P）,magnetic anisotropy energy（MAE）and Néel temperature（T_N）can be significantly enhanced.A tensile strain applied on Fe₂C monolayer with the value less than 8%facilitates the enhancement of MAE and P,and maintaining the T_Cabove room temperature.Fe₂C was modified symmetrically and asymmetrically by functional groups（H、F、Cl、Br）.The results show that surface functional group modification can improve the P of Fe₂C monolayer.The introduction of H group will reduce the ferromagnetism of the system without changing its metal properties.It is worth noting that Fe₂CCl Br are ferromagnetic half-metals,and Fe₂CF₂and Fe₂CFCl are bipolar ferromagnetic semiconductors,and the T_Cis 353,988 and 995 K,respectively.Hole doping can improve the magnetic moment,P and magnetization direction of Fe₂C monolayer.A small amount of hole doping can increase the T_C,which reaches a maximum of 616 K at a doping concentration of-0.5e per unit cell.（2）Co₂C monolayer is a two-dimensional material with intrinsic magnetism,and T_Cis lower than room temperature.Biaxial strain and hole doping had no significant effect on the regulation of Co₂C.Replacing a Co atomic layer in Co₂C with V,Cr and Mn forms a double transition metal（DTM）carbide Co MC（M=V、Cr、Mn）.Co Mn C is a ferromagnetic metal with a high T_Cof 1366 K,which is an ideal for spintronics material.While Co VC and Co Cr C are antiferromagnetic metals,and T_Nare much lower than room temperature.Applying biaxial strain to a monolayer Co MC,the results indicate the AFM-FM phase transition occurs for Co VC and Co Cr C under the stress of±2%and 2%,respectively.And the T_Ccan be raised above room temperature by applying tensile strain greater than 4%.The T_Cof Co Mn C increased to2374 K under the 8%tensile strain.In addition,tensile strain can induce Co VC to transform into ferromagnetic half-metal or semiconductor.Hole doping can also effectively regulate the properties of Co MC.At a doping concentration of-0.1e per unit cell,the AFM-FM phase transition occurs for Co VC and Co Cr C.Co VC and Co Mn C are converted to ferromagnetic semimetals at hole-doping concentrations of-0.2e and-0.4e per unit cell,respectively.Under hole doping,Co VC and Co Cr C obtain a large in-plane MAE,while the magnetization direction of Co Mn C changes.More importantly,hole doping can significantly the T_C/Nof Co MC monolayer,and the T_Cof Co VC and Co Cr C is higher than that of room temperature at doping concentrations greater than-0.5e per unit cell.The above results show that tensile strain and hole doping are beneficial to DTM carbide Co MC to achieve room temperature ferromagnetism.In conclusion,based on the room-temperature ferromagnetism of two-dimensional transition metal carbides,the magnetic properties of them are regulated through functional group modification,biaxial strain and hole doping,making monolayer Fe₂C and Co₂C more suitable for spintronic devices,and provides theoretical guidance for the preparation of spintronic devices.