Multiferroic Propreties And Ferroelectric Photovoltaic Effect Of Bi₅Ti₃FeO₁₅ Based Films

In recent decades,with the increasing in population and the development of industrialization,people have put forward higher requirements for the functionality of electronic devices,and the demand for more energy use.Multiferroic materials with multiple ferroic order parameters can achieve mutual regulation among multiple physical responses,injecting new vitality into the design of new magnetoelectric functional devices.In addition,ferroelectric materials are expected to play a potential role in solving energy demand problems due to their unrestricted open-circuit voltage and anomalous photovoltaic response.Among them,Bi5Ti3FeO15（BTFO）has been widely studied in magnetoelectric and photoelectric fields because of its unique Aurivillius phase structure and multi-iron properties.In this thesis,high quality BTFO polycrystalline films were prepared by sol-gel spinning coating method,and the multiferroicity,magnetoelectric coupling properties and ferroelectric photovoltaic properties were modified and regulated via doping and recombination experimental methods.The specific research work and experimental results are as follows:1.Bi_5-xLa_xTi₃FeO₁₅（x=0,0.25,0.5,1）films doped with La were prepared.All BLTFO samples have Aurivillius phase structure and the grain distribution is relatively uniform.Compared with pure BTFO films,15%La-doped films obtain a larger residual polarization value of about 21 μC/cm²,which is attributed to the favorable orientation of the films derived from the inhibition of the formation of A-site ion and oxygen vacancy by the introduction of La.The dielectric properties of the films change obviously with the concentration of La doping,that is,the sensitivity of dielectric constant and dielectric loss to temperature and frequency changes increases.The reduction of dielectric constant at high frequencies can be explained by Maxwell-Wagner mechanism.Moreover,the magnetization of the film increases from 5.8 emu/cc to 10.4 emu/cc with increasing La concentration,which is related to the increased uncompensated spin on the surface.The surface-volume ratio of the film is enhanced,and the Fe³⁺ magnetic moment caused by the octahedral tilt in the multi-iron nanoparticles inhibits the spin structure due to the decrease of grain size,which increases the weak ferromagnetism.In addition,the significant magnetic exchange bias effect is generated by La doping and reaches its maximum value of 2.53 emu/cc at 15%doping concentration,which may result from the coupling interaction between the antiferromagnetic layer and the ferromagnetic layer interface in the BTFO base phase.The sample obtained a large magnetoelectric coupling coefficient with value of 35.1 mV/cm·Oe under the bias field Hbias of 8 kOe due to the enhancement of ferroelectric and ferromagnetic properties,which is almost 4 times that of pure BTFO films.2.Bi₅Ti₃FeO₁₅-NiFe₂O₄（BTFO-NFO）and Bi₅Ti₃FeO₁₅-graphene（BTFO-GO）films with 0-3 type composite structures were prepared,and the magnetoelectric coupling coefficient of BTFO-NFO film and the ferroelectric photovoltaic performance of BTFO-GO film were studied.The test results show that the ferroelectricity of BTFO-NFO is improved,and the remnant polarization and saturation polarization are 25 μC/cm² and 52 μC/cm²,respectively.Under the magnetic field of 8 kOe,in-plane and out-of-plane magnetoelectric coupling coefficients were obtained,with values of 80 mV/cm·Oe and 52 mV/cm·Oe,respectively.The strain generated by the magnetostrictive effect of NFO particles is transmitted through the BFO/BTFO interface to the BTFO layer,enhancing the magnetoelectric coupling response.And the enhanced magnetoelectric coupling effect in the 0-3 type composite film was simulated by using finite element analysis software（ANSYS）.In addition,BTFO-GO composite films were prepared.The advantage of the high carrier mobility of graphite oxide enhances the carrier lifetime generated by light excitation and improves the carrier density in the films.Compared to the photovoltaic performance of BTFO thin films,the short-circuit current of BTFO-GO films has increased by four times.3.BTFO films with good properties were successfully prepared by sol-gel method,and two different ions Ti⁴⁺ and Fe³⁺ at B-site were doped with Zr⁴⁺ and Mn³⁺ ions respectively.The substitution of Zr⁴⁺ ion with Ti⁴⁺ ion at B-site changes the hybrid level structure between the central atom and oxygen atom,which narrows the band gap and shows a outstanding visible light response.At the same time,the open circuit voltage and short circuit current are increased by 44 times and 1.38 times respectively,resulting in a significant photovoltaic effect.The substitution of Mn³⁺ ions for Fe³⁺ ions at B-site generates impurity levels in the lattice and a narrow band gap of 2.37 eV is obtained,which enables the carrier to transition effectively and exhibit good open circuit voltage and short circuit current.It provides a feasible idea to improve the ferroelectric photovoltaic performance of layered perovskite by doping at different B sites.