First-principles Study On Electronic,Optical And Magnetic Properties Of BiFeO₃ Thin Film

With the continuous development of science and technology,multiferroic materials have been widely used in electronic devices and various smart devices.In Multiferroic materials two or more ferroic properties coexisting.As a single-phase multiferroic material,bismuth ferrite（BiFeO₃,BFO）has a perovskite structure with the general formula ABO₃.BFO has received extensive attention of the scientific field due to its coupling effect between ferroelectricity,ferromagnetism and ferro-elasticity.In recent years,researchers have found that BFO not only has a small optical band gap（2.2-2.9 eV）,but possess a favorable nonlinear optical properties,this widened the application of BFO.However,BFO still has the shortcomings such as weak magnetism and high leakage current,which reduces the magnetoelectric coupling effect and lowering the practical application efficiency.Therefore,improving the electrical,optical and magnetic properties of BFO on the basis of stability is one of the effective ways to improve the application efficiency.The researchers proposed that by doping different types of elements at the A/B sites of bismuth ferrite and changing the doping proportion can improve the material property and make more practical application.Therefore,in present work the first principle numerical simulation method was used to study the effects of rare earth elements and transition metal element doping on the mechanical,electronic,magnetic and optical properties of BFO.At the first step A site is doped with La（BiLaFe₂O₆,BLFO）and B site doped with Mn（Bi₂FeMnO₆,BFMO）.Then mechanical,electronic,magnetic and optical properties of BFO、BLFO and BFMO were calculated,and effect of doping elements on the BFO are discussed.In the second step,25%,50%Ni and 25%Mn are doped B site respectively,corresponding calculation are carried out for the electronic and magnetic properties and then effect of transition metal elements doping on the electronic and magnetic properties of BFO are discussed.Results are as following:（1）Firstly,the elastic constant is calculated to study the mechanical properties.results show that the elastic constants of the three kinds of compounds BFO,BLFO and BFMO met the mechanical stability criteria,thereby confirmed the possibility of industrial processing of these three kinds of compounds.In order to further study other mechanical characteristics,the bulk modulus B,shear modulus G,Young s modulus E,Poisson ratio v,Pough ratio were calculated for each compound.It is found that the Poisson ratios of BFO,BLFO and BFMO are 0.338,0.323 and 0.340 respectively and larger than 0.26,the Pough ratios 2.70,2.47 and 2.87 are larger than 1.75.,results indicated that all of these compounds exhibit ductile nature.Calculated elastic anisotropy index（A^U）,（AB）and（AG）,angular dependent Young’s modulus and Poisson s ratio indicate the anisotropic nature of these compounds.At the same time,the bulk modulus of BFO increases with the La and Mn doping,which indicates the increased hardness.（2）In terms of electronic property,band gap,full density of state（DOS）and partial density of state（PDOS）are calculated,and results shows that BFO,BLFO and BFMO exhibit indirect band with the band gap values of 0.58 eV,0.56 eV and 0.45 eV,respectively.Spin-up and spin-down energy bands separated at the bottom of the conduction bands,which indicates that they possess certain magnetism.The DOS and PDOS calculation results show that the magnetic properties of these three compounds are contributed by the 3d orbital electrons of Fe atoms.（3）By calculating the electron spin density,the effect of La and Mn doping on the magnetic properties of BFO were analyzed.Results show that after the La and Mn doping,clear spin polarization appeared around the Fe atoms.This indicates that the magnetic properties were improved with the La and Mn doping·From the spin density it can be inferred the magnetic property changing tendency was BFO<BLFO<BFMO.（4）The optical property calculation results show that BFO,BLFO and BFMO possess favorable Second Harmonic Generation response（SHG）.The maximum SHG coefficients of BFO,BLFO,and BFMO are d₂₂ =73.6 pm/V,do₂₂= 53.6 pm/V,and d₃₃ = 72.4 pm/V,respectively.And they exhibit a highly polarized SHG response.（5）When B site doped with 25%,50%Ni,BiFe_1-xNi_xO₃（x= 0.25,0.5）,the band gaps are 0.899 eV and 0.659 eV respectively.When doped with the 25%Mn,the band gap of BiFe_0.5Mn_0.5O₃ is 0.686 eV.The spin-up and spin-down energy bands of these three compounds are cleaved at the bottom of the conduction band and the total density of states is asymmetrical,indicating that they have certain magnetic properties.（6）There is obvious spin polarization around the Fe and Ni atoms in BiFe_0.75Ni_0.25O₃.When BFO is doped with 50%Ni there is clear spin polarization around Ni atom,when doped with 25%Mn,there is obvious spin polarization around Fe atoms,which indicates that Ni and Mn doping improve the magnetic properties of BFO.