A Theoretical Study Of Two-Dimensional Multiferroic Materials

With the continuous development of modern information technology,the realization of small memory device with non-volatile,low power consumption and high memory density has become a new direction of the next generation of electronic devices.However,the size of traditional 3D materials limits their application in the field of nanoscale memory device,and it is difficult to thin their thickness by traditional means in experiments.In recent years,two-dimensional magnetic van der Waals crystals have been successfully fabricated.The regulation of two-dimensional magnetism by mechanical,electrical and optical means can promote the practical application of atomically thick layered magnetic materials and provide the possibility for the design of new spintronics devices with higher integration degree.In addition,two-dimensional ferroelectric materials are considered to have great application value in various fields,such as micro field effect transistors and information storage devices,due to their stable spontaneous polarization at nanoscale.In recent years,the theoretical researches on two-dimensional ferroelectric materials have made a series of important advances,which promoted the research of related experiments and provided a new way for the development of new nanoscale electronic storage devices.For the two-dimensional multiferroic materials,the coupling between the intrinsic ferroic properties,for instance,the magnetoelectric coupling effect,magnetostrictive effect and piezoelectric effect can be realized,and the mutual regulation between different physical degrees of freedom can be realized.Although very few 2D multiferroic materials have been discovered at present,this kind of materials are considered to have broad application prospects in the field of new information storage electronic devices,such as the realization of long-sought multi-state storage applications in 2D multiferroic materials.Based on the above research directions,the ferroelectric properties of single-layer Sc2P2Se6 and the introduction of multiferroicity after the transition metal element substitution,as well as the direction of spontaneous electric polarization and easy magnetization axis of antiferromagnetism of single-layer?-FeOOH that can be switched by ferroelasticity,are studied by first-principles calculations.The main outcome of the paper is as follows:(1)A single-layer Sc2P2Se6 material with stable ferroelectric polarization is studied theoretically.We demonstrate that the monolayer is dynamically and energetically stable.The calculated out-of-plane polarization intensity is considerable,which can be detected by current experimental methods,The energy barrier between the ferroelectric phase and antiferroelectric phase is 0.13 eV/f.u.,which indicates that the ferroelectric phase is stable.We further replaced 50%Sc atoms with Cr atoms in the unit cell to obtain a monolayer of ScCrP2Se6.The adjacent Cr atoms have super-exchange interactions bridged by other atoms,and the magnetic ground state is antiferromagnetism.The structural distortion during the transition between the ferroelectric phase and antiferroelectric phase causes the indirect exchange interaction changing,and the magnetic ground state of the system is changed into ferromagnetism,and the magnetic ground state of the system is controlled by the electric field in the vertical direction.The multiferroic properties of the material after element substitution provide a new candidate material for the study of magnetoelectric coupling effect.(2)We theoretically investigated that it is possible for a single layer of ?-FeOOH to be separated from its bulk material by means of mechanical stripping,and it is proved to be dynamically and thermodynamically stable.First-principles calculations show that ferroelectricity,antiferromagnetism,and ferroelasticity coexist in the monolayer y-FeOOH.The ferroelectric polarization intensity of the single layer reaches 77.5 pC/m,and the energy barrier is 0.11 eV/f.u.is calculated by the transition state calculations.The Monte Carlo simulations based on the Heisenberg model show that the phase transition temperature of antiferromagnetism is 126 K.In addition,we have also studied the possible ferroelasticity of the material,finding that there is a suitable ferroelastic switching barrier and considerable reversible deformation in this material.The direction of ferroelectric polarization and magnetization can be controlled by ferroelastic switching at the same time.All the results of this study provide a promising research strategy for novel multiferroic coupling effects and provide a new concept for the design of the next generation of electronic or spintronic devices.The structure of the paper is as follows:The first chapter is the introduction,which gives a brief overview of the theoretical and experimental research status of two-dimensional magnetic materials,two-dimensional ferroelectric materials and two-dimensional multiferroic materials,and summarizes the landmark materials in related fields.The second chapter introduces the theoretical research methods adopted in this paper.Topics include first-principles computing fundamentals,density functional theory,and computing package VASP.The third and fourth chapters give a detailed description of the work during the master's period.The fifth chapter summarizes this paper and looks forward to future development.