Characterization And Application Research Of BFCO/BFCNT Multiferroic Heterojunction Thin Film

Multiferroic materials have attracted much attention for their novel coupling effect of multiple fields,showing great application prospects in modern optoelectronic materials and devices,and have been widely used in energy conversion,information storage,high-frequency communication,etc.Especially in recent years,the emergence of the layered perovskite-like system provides infinite design possibilities for researchers.Therefore in this paper,the narrow-band gap Bi₂Fe Cr O₆（BFCO）and Bi₆Fe_2（1-x）Co_xNi_xTi₃O₁₈（x BFCNT,x=0.0,0.2,0.4,0.6）with excellent properties in layered perovskite-like oxides were selected as the focus of this dissertation,and the application of BFCO/BFCNT heterojunction in ferroelectric photovoltaic and resistive switching effects was studied as emphases,providing an experimental and theoretical basis for designing next-generation high-performance multiferroic oxide intelligent devices.The four main innovative works are as follows,1.Double perovskite BFCO and layered perovskite-like Bi₆Fe_2（1-x）Co_xNi_xTi₃O₁₈（x BFCNT,x=0.0,0.2,0.4,0.6）nanofilms were successfully fabricated employing sol-coating and rapid thermal process（RTP）,and the phase composition,microstructure,electrical,magnetic and optical properties were studied in detail.It illustrated that both BFCO and x BFCNT exhibit excellent electrical and magnetic properties at room temperature,and the residual polarization（Pr）of BFCO is 53.9μC·cm^-2,the saturation magnetization（Ms）is 2.23 emu·cm^-3.The Pr of x BFCNT reduces from 48.6μC·cm^-2to 29.5μC·cm^-2,and the Ms increases from 0.07 emu·cm^-3to 2.15 emu·cm^-3with the increase of x.The excellent electrical characteristics are mainly attributed to the orbital hybridization of 6s²lone pair electrons of Bi³⁺ions and2p electrons of O^2-,while the magnetism of BFCO is ascribed to the parallel spin state of 180°Fe³⁺（d⁵）-O-Cr³⁺（d³）,and the magnetism of x BFCNT is assigned to the spin tilt caused by DM interaction.The optical properties showed that the optical band gap（E_g）of BFCO is 1.74±0.04 e V,but the E_gof x BFCNT gradually decreases from 1.98 e V to 1.26 e V,caused by the raising of the HOMO resulting from the hybridization of the3d orbitals of transition metal ions at B-site with the 2p orbital of O^2-.Especially for0.4BFCNT（abbr.BFCNT）,it presents not only good electrical and magnetic properties,but an appropriate E_gof 1.62±0.04 e V,which is more suitable for solar cells.2.The multiferroic BFCO/BFCNT heterojunction was successfully prepared by sol-coating and RTP,and the regulation of macro and micro（inverse-）magnetoelectric coupling effect was studied detailly.It suggested that the ferromagnetic（electric）domain structure of the heterojunction films changes significantly and reversibly with the applied electric（magnetic）field changing,leading to a decrease of Ms from 2.98 emu·cm^-3to 2.05 emu·cm^-3and Pr from 38.4μC·cm^-2reduced to 23.7μC·cm^-2,which means that in the superposition field of electricity and magnetism,the stress-strain,the interaction of the Lorentz and electrostatic forces,and the Hall effect are the important factors causing the changes in electrical and magnetic domain structure.Nevertheless,the reduced residual polarization increases to 27.5μC·cm^-2with the continuous increase of the external field intensity,depending on the recovery of the deflected dipoles led by the gradual decrease of the contraction coefficient after reaching its saturation decided by the restriction of non 180°domain activity.3.Three novel multiferroic solar cells were prepared by using BFCO/BFCNT heterojunction as the light absorption layer.The performance of the cells was gradually improved from 3.1%to 3.9%by introducing Zn₂Sn O₄,Ti O₂,Ni O_x:Cu,FTO,graphene,and black silicon,respectively,indicating that selecting an active layer with appropriate optical band gap,changing the material and the shape of the electrodes,surface roughness,and introducing an anti-reflective layer can improve the conversion efficiency of the cells.The regulation of the external electric and magnetic fields on the performances showed that a forward ferroelectric polarization will worsen the photovoltaic response,while a reverse one will enhance the performance.And meanwhile,the current density of the cells increases first and then decreases with the increase of the external magnetic field intensity,which is consistent with the variation of the magnetoelectric coupling effect of heterojunction thin films.These changes are directly related to the changes in carrier migration and band alignment caused by the changes in the built-in electric field caused by magnetoelectric coupling,Zeeman,and Rashba effects.4.Novel multiferroic memristor array was obtained by introducing BFCO/BFCNT heterojunction and Ag as the active layer and the electrodes,respectively.After a detailed study of the memristive and artificial synapse performances,the influence of external electricity,magnetic field,and illumination on device performances was reported.Finally,the memristor model and the corresponding Pavlov memristor neural network were designed by Spice.It demonstrated that such multiferroic heterojunction memristor arrays exhibit good bipolar resistance switching at room temperature.The resistance window is large and the difference between the HRS and LRS is about two orders of magnitude,leading from the connection and disconnection of conductive filament appearing in the active layer by the redox reaction of Ag electrodes in the electric field.No performance degradation was found in the 50 cycle tests,which showed that the memristor array had high stability.The illumination can increase the number of carriers in the active layer of multiferroic BFCO/BFCNT heterojunction,to increase the number of non-equilibrium carriers（electron-hole pairs）which will gather at the ferroelectric domain walls,changing the depolarization electric field and the redistributions of the domains.The Pavlov neural network based on multiferroic BFCO/BFCNT heterojunction films exhibits obvious associative memory（learning and forgetting）process,which can be used to simulate the characteristics of biological neurons and synapses,thus providing an experimental and theoretical basis for realizing the integration of storage and computing and reducing the cost of data migration.