Preparation And Characterization Of Bi₅Ti₃FeO₁₅ Based Composite Film

Pentabismuth trititanium iron pentadecaoxide(Bi₅Ti₃FeO₁₅,BTFO),which is a multiferroic four-layer Aurivillius phase compound,has received much attention in recent years.The bismuth-based Aurivillius compound BTFO is a very important compound.It is an environmentally friendly lead-free multiferroic material with excellent ferroelectro-magnetic properties,low band gap,and highly flexible chemical properties.Previous researches have confirmed the large spontaneous polarization of BTFO with a high curie point,and ferromagnetism at room temperature.Therefore,BTFO is regarded as a promising candidate for applications that involve both magnetoelectric coupling and piezoelectricity.However,the pure BTFO phase exhibits weak magnetism and magnetoelectric coupling at room temperature,which limits their use in practical devices.A much larger room-temperature magnetoelectric effect is observed for artificially developed multiferroics,which are composites of ferroelectric and ferromagnetic thin films.Several studies have focused on artificial multiferroic composite thin films prepared by use of different ferroelectric(e.g.,Bi_5-xA_xTi₃Fe_1-yB_yO₁₅)and magnetic(e.g.,La_1-xSr_xMnO₃,Pr_1–xCa_xMnO₃,La_1-xCa_xMnO₃,NiFe₂O₄and CoFe₂O₄)materials.In this paper,2-2 layered heterostructure magnetoelectric composite films were prepared on Pt（200 nm）/TiO₂（50 nm）/SiO₂（500 nm）/Sisubstrates by pulsed laser deposition.Firstly,the BTFO film was systematically modified by doping,such as A-site doping,B-site doping and AB site co-doping,and the multiferroic properties at room temperature were explored.Using doped BTFO film as ferroelectric phase and NiFe₂O₄（NFO）as ferromagnetic phase,high-quality 2-2 layered heterostructure magnetoelectric composite film was prepared by PLD to prepare environment-friendly magnetoelectric composite film materials.The microstructure,morphology,piezoelectric properties,ferroelectric properties and ferromagnetic properties of the composite films were studied at room temperature.The results are as follows:1.BTFO、Bi₅Ti₃Fe_0.5Ni_0.5O₁₅（BTFNO）、Bi₄DyTi₃FeO₁₅（BDTFO）、Bi₄DyTi₃Fe_0.5Ni_0.5O₁₅（BDTFNO）、NFO ceramic targets were prepared by traditional high temperature solid-state method.X-ray Diffractometer（XRD,D/max-2500V Rigaku）was used to analyze the phase of the ceramic target.The diffraction peaks of all the targets were in good correspondence with the standard card,indicating that the preparation process of the target was good and there were no impurities and secondary phases in the target.2.The BTFO-based ferroelectric thin films were successfully prepared.The effects of pluse laser frequency,growing temperature,growing time,oxygen pressure and annealing time on the properties of BTFO thin films were systematically studied on the basis of relevant research,and the growth process was optimized.The film prepared under various conditions was tested by using the variable-temperature ferroelectric testing system,and its ferroelectric properties were compared.The optimal film preparation conditions were obtained as follows:The temperature was maintained at 750℃,the distance between the targets and the substrates is 50 mm.The targets are bombarded for 45 min under 13 Pa oxygen partial pressure by a laser energy of 330 mJ and a frequency of 2 Hz.Finally,all the samples were annealed at 750℃for 1 h under 1000 Pa oxygen pressure.In this way,films with good properties can be prepared.The surface of the film is compact and the cross section is clear.XRD results are in good agreement with the standard card,and there is no hetero-phase and secondary phase.3.Improve the properties of BTFO film by doping modification.BDTFNO lead-free ferroelectric thin films with good performance were prepared by PLD on Pt（200 nm）/TiO₂（50nm）/SiO₂（500 nm）/Sisubstrates by Dy²⁺and Ni³⁺substitution doping of BTFO at A and B sites respectively.BTFO,BDTFO,BTFNO and BDTFNO films were characterized respectively.The phase structure,surface and interface of the thin films were characterized by XRD,Scanning Electron Microscopy（SEM,Hitachi SU-8100）and Atomic Force Microscope（AFM,Bruker Multi Mode 8）.The element distribution of the samples was characterized by Energy Dispersive Spectroscopy（EDS,HSTND-9751-V）.The ferroelectric properties and leakage current characteristics of the thin film were tested with a variable temperature Multiferroic Test System（Multiferroic 200 V,Radiant Technologies）.The ferromagnetic properties of the films,such as hysteresis loops and M-T curves,were measured using a Comprehensive Physical Property Measurement System（PPMS,Dyna Cool-9T）.The results show that the ferroelectric and ferromagnetic properties of doped BTFO films are better than those of pure BTFO films.The maximum polarization 2P_s=68.08μC/cm²,the remnant polarization 2P_r=38.59μC/cm²,the leakage current density J_s=2.6×10^-7A/cm²,the saturation magnetization 2M_s=10.02emu/cm³.4.The effect of deposition process on the magnetoelectric properties of BTFO based 2-2layered heterostructure magnetoelectric composite films was studied.BDTFNO films were deposited with pluse laser energy 330 mJ,pluse laser frequency 2 Hz,target-substrate separation50mm,deposition temperature 750℃,deposition oxygen pressure 13 Pa.Without annealing and the target position was adjusted to the NFO target,then the NFO films were deposited with under the same experimental conditions as BDTFNO films.Finally,the composite films annealing at750℃for 60 min.After comparing the SEM cross sections,it was found that the film interface of NFO layer,BDTFNO layer and Pt layer were clear and distinct.5.The magnetoelectric properties of BTFO based magnetoelectric composite films were studied.The phase structure,microstructure,piezoelectric properties,ferroelectric properties,leakage current characteristics and ferromagnetic properties were characterized.The maximum polarization 2P_s=50.39μC/cm²,the coercive field 2E_c=250.87 kV/cm,the saturation magnetization 2M_s=98.5 emu/cm³,the coercive field 2H_c=970 Oe.