First-Principles Studies Of Several Two-Dimensional Ferroic/Multiferroic Materials

Ferroic materials,mainly including ferroelectrics,ferromagnets and ferroelastics with order parameters（electric polarization,spin polarization and strain）switchable by external stimuli（electric field,magnetic field and stress）.Multiferroic materials simultaneously exist two or more ferroic orders.Two-dimensional（2D）materials exhibit ultra-thin thickness,dangling bond-free surfaces and other exotic properties.2D ferroic/multiferroic materials have attracted research enthusiasm from researchers in recent years,which have excellent promising performance for electronics devices like memories,sensors,actuators and spin electron devices.In recent years,several nanometers thick films or even monolayer 2D ferroelectric,ferromagnetic,multiferroic materials have experimentally synthesized.However,until now,it is still necessary to explore more 2D ferroic/multiferroic materials with excellent properties and potential for experimental synthesis from the perspective of theoretical prediction.Based on the above backgrounds,this work investigates the structures and properties of several families of 2D ferroic/multiferroic materials by using first-principles calculations based on density functional theory.The main contents include:1)prediction of a family of new 2D MX ferroelectric materials from phosphorus-analogue structure,the periodic trend and correlation between chemical composition and ferroelectricity and piezoelectricity,a in-plane ferroelectric tunnel junction device constructed by newly predicted 2D ferroelectric materials is designed;2)prediction of an extraordinary combination half-auxetic effect and ferroelasticity in Ti Se monolayer;3)Moreover,a family of 2D M₂RX₂multiferroic materials and their multiferroic origin are investigated.The main research results are as follows:1.Ferroelectric materials,displaying spontaneous electric polarization that can be switched by an electric field,with relatively low switching barrier and large polarization,have great potential applications in nonvolatile memory devices,sensors,field effect transistors.Among them,the binary compounds with the MX stoichiometry with 10valence electrons have multiple monolayer phases in broken inversion symmetry,and possess excellent properties,such as multiferroicity and piezoelectricity.In this work,39 new 2D ferroelectric materials,MX（M:Group Ⅲ-Ⅴ elements;X:Group Ⅴ-Ⅶ elements）,in three phosphorus-analogue phases including black phosphorene-likeα-phase,blue phosphorus-likeβ-phase,and Ge Se-likeγ-phase have been predicted,by using high-throughput calculations.Seven materials（α-Sb P,α-Sb As,α-Bi As,α-Bi Sb,γ-As P,γ-Bi As,andγ-Bi Sb）exhibit FE switching barriers lower than0.3 e V/f.u.,ferroelectric polarization larger than 2×10^-10C/m,and energy above hull smaller than 0.2 e V/atom.Both ferroelectricity and in-plane piezoelectricity are rationalized on the basis of dynamical charge transfer influenced by electronegativity.Further calculations predict a giant tunneling electroresistance in ferroelectric tunnel junctionα-Sb（Sn）P/α-Sb P/α-Sb（Te）P（TER=1.26×10⁴%）and large piezoelectric strain coefficient inα-Sn Te(d₁₁₌396 pm/V),providing great opportunities to the design of non-volatile resistive memories,and high-performance piezoelectric devices.2.When longitudinal compressive（tensile）strain is applied,common materials undergo a transverse expansion（contraction）,while auxetic materials with negative Poisson’s ratio undergo a transverse contraction（expansion）.Different from the common materials and traditional auxetic materials,which have a linear mechanical response to a uniaxial strain close to the equlibrium state,this work predicts that P2mm-type Ti Se monolayer undergoes a transverse expansion no matter longitudinal stretched or compressed under small strain.The unique half-auxetic effect is caused by considering both the nearest and the next-nearest interactions of Ti-Se atoms and Ti-Ti atoms based on first-principles calculations.Moreover,Ti Se undergoes mechanical switching between two crystal structures by external stress.The origin of the ferroelasticity in Ti Se is the Jahn–Teller effect,which breaks the degeneracy of partially occupied d_xz and d_yz orbitals of Ti atom.More electrons occupy the lower orbital and less electrons occupy the higher orbital,then lower the total electronic energy.Thus,the ferroelastic transition of Ti Se from high symmetric transition state to low symmetric ferroelastic structure is realized,and the ferroelastic switching barrier is 0.25 e V/atom.This work find a single-phase material Ti Se with exotic combination of half-auxetic effect and ferroelasticity,and provide a candidate material for the future design of novel2D multiple functional materials at the nanoscale.3.Magnetoelectric materials that combine ferroelectricity and magnetism have novel properties,such as magnetoelectric coupling,topological order and thermal Hall effect.A family of stable 2D materials,FE-ZB′V₂NR₂（R=I,Br）has been theoretically proposed in this work,by using high-throughput calculations.Magnetism is derived from the antiferromagnetic coupling between d-orbitals of V atom.Arising from the inversion symmetry broken and strong spin-orbit coupling,a large Dzyaloshinskii-Moriya interaction is obtained in the FE-ZB′V₂NBr₂ monolayer.The DMI gives rise to a domain-like magnetic spin configuration and a skyrmion-like spin configuration in the presence of an external magnetic field.Moreover,due to the structural distortion and charge redistribution caused by Jahn-Teller effect,the intrinsic ferroelectricity and4%strain-induced ferroelectricity are predicted in V₂NI₂ and V₂NBr₂ monolayers,respectively.This work extends the 2D strong magnetoelectric coupling multiferroic systems,and greatly enriches the research on antiferromagnetic spin configurations.