Preparation And Doping Of BiFeO₃ Film By Sol-gel Process And Study On The Electric And Magnetic Properties

Mutiferroics are also called magneto-electric materials, which shows coexistent ferroelectric order and magnetic order and was considered to have important potential application in ferroelectric and magnetic devices. The coexistence of magnetism and ferroelectricity in this kind of materials supplies them magneto-electric effects and the potential for applications as sensors, waveguides, switches, and modulators. The coexistence also deduces magneto-electric couple. The property is suggested to be applied to magnetically recording ferroelectric memories. For the above reasons, multiferroics are extensively focused in recent years.BiFeO3 is a perovskite typed mutiferriocs(TC=1103 K,TN=643 K), which shows coexistent ferroelectricity and magnetism at room temperature. In recent years, epitaxial BiFeO3 films have been prepared by pulsed laser deposition method and galactic remnant polarization was observed, which attracted extraordinary attentions. In this paper, we studied the sol–gel preparation method of BiFeO3 film, and the improvements in physic properties by the substitutions in A and B site have been studied.The main contents of this paper are as the following:(1) Bi(NO3)3.5H2O and Fe(NO3)3.9H2O are adopted as starting materials, 2-methoxyethanol as solvent, ethanolamine was used to adjust the viscosity to preparation the precursor solutions and phase pure BiFeO3 films are successfully deposited on ITO/glass, LaNiO3/Si(111) and Pt/Ti/SiO2/Si substrates at different annealing temperatures from 450℃to 600℃. Moreover, (110) preferred oriented and randomly oriented BiFeO3 films were prepared on LaNiO3/SiO2/Si substrates by controlling the annealing technology at annealing temperature of 500℃.The BiFeO3 films deposited on ITO/glass substrates adopt different orientation according to the annealing temperatures. The film annealed at 500℃and 600℃adopt (110) preferred orientation and random orientation, respectively. The remnant polarizations of the two films are 2.0μC/cm2 and 1.76μC/cm2 respectively. Debye typed relaxation was observed in the films by measuring the dielectric property. The intense dielectric dispersion is thought to be originated from the inertia of dipole, while the loss peaks are thought to be from the resonance of dipole.The remnant polarizations of the BiFeO3 films deposited on Pt/Ti/SiO2/Si substrates are 0.51μC/cm2, 0.72μC/cm2, 4.54μC/cm2 and 5.94μC/cm2 respectively under annealing temperatures of 450℃, 500℃, 550℃, 600℃. Compared with the films on ITO/glass substrates, the films on Pt/Ti/SiO2/Si substrates show small dielectric dispersion and is attributed to better interface between the BiFeO3 film and Pt bottom electrode. The films on LaNiO3/Si substrates show largest remnant polarizations under the same annealing temperatures, the films annealed at 450℃to 600℃show double remnant polarizations of 6.4μC/cm2, 8.2μC/cm2, 12.8μC/cm2 and 2.3μC/cm2. Interface reaction was observed by the cross-section TEM measuring. The interface layer bore large measuring field, which reduced measuring fields supplied to the BiFeO3 film and deteriorated the dielectric property. In addition, the leakage current increased with annealing temperature and which is suggested to be caused by opening of grain boundary at higher annealing temperature.As for the films deposited on LaNiO3/SiO2/Si substrates, the orientation is determined by the annealing technology. Annealing layer by layer makes for eptaxial growth and thus getting preferred orientation, while annealing by one time is in favor of random orientation. The ferroelectricity and dielectric property are determined by the crystallization the preferred orientation. The analysis of leakage conduction property showed that the films have the same transition field from ohmic conduction to space charge limited conduction.(2)A site La substituted Bi1-xLaxFeO3 films was prepared by adding La(NO3).6H2O as starting material and referring to the preparation technology of pure BiFeO3 film. The tolerance factors are 0.8479 and 0.8486 before and after La substitution, respectively, therefore we expect a stable pervoskite structure through the La substitution. It has been found that all films show well R3m structure until x increases to 0.20 and no structural disorder are observed, moreover, all films adopt random orientation. The measurement of ferroelectricity shows that the film with x = 0.10 has most intense ferroelectricity, the 2Pr of it was 4.40μC/cm2. The ferroelectricity is enhanced through the La substitution on A site since the double remnant polarizations for the film with x=0.10 was enhanced to 4.40μC/cm2 from 3.35μC/cm2 for the film with x=0. The enhancement of ferroelectric is suggested originated from the change of orientation and distortion of lattice by the La substitution. More over, dielectric property was enhanced since the dielectric property for the film with x=0.10 was 165 while the film with x=0 was 120 under a measuring frequency 10 kHz. Furthermore, leakage currents were reduced substantially since the leakage current density for the film with x=0.20 is 3～5 magnitude orders lower than the undopted one.(3)Zr and Ti substitution on B siteZr(NO3)3.5H2O and Ti(C4H9O)4 were added as starting materials to prepared BiFe1-xTixO3 +δand BiFe1 - xZrxO3 +δfilms on LaNiO3/SiO2/Si, the detailed preparation technology referred to pure BiFeO3 films on ITO/glass substrates.BiFe1-xTixO3+δfilms was prepared on LaNiO3/SiO2/Si substrates at an annealing temperature of 600℃. The film with x = 0.00 show throughly random orientation, while the film with x= 0.05 show (101) preferred orientation. The film with x=0.10 show randomly orientation anew. The crystal grains were observed finer and the compaction of the films increased through the substitution and which caused higher measuring fields. The remnant polarization under different measuring electric fields for the films with x=0.00, 0.05 and 0.10 are 1.2μC/cm2 , 0.58μC/cm2 and 3.8μC/cm2. New loss mechanisms and enhanced dielectric property were also identified. The leakage currents were also reduced. The measurements of magnetism shows that all films indicate weak antimagneitsm and the magnetism are dominated by the destruction of spiral spin structure and the reduction ofμB in unit volume caused by the substitution.The BiFe1-xTixO3+δ(x = 0.00 to 0.20) films deposited on ITO/glass substrates were annealed at 600℃, which were identified adopted random orientation. The (012) and (110) peaks were observed have the largest intensity and the (012) and (110) peaks were observed widened in the film with x=0.20, it means that the film has poor crystallization. XRD subtle structure shows that the lattice constant increased through the Ti substitution. The film with x=0.05 has the largest double remnant polarization of 8.30μC/cm2, while the double remnant polarization for the films with x=0.0, 0.01 and 0.2 are 2.12μC/cm2, 2.63μC/cm2 and 0.44μC/cm2. Comparing with the films on LaNiO3/Si substrates, their showed more intense ferroelectricity. More over, the BiFe1-xTixO3+δfilms on ITO/glass indicated enhanced dielectric property since compared with BiFeO3 film, the dielectric constant increased 150 for the film with x=0.10 in the frequency range from 0.1 kHz to 100 kHz. The film with x=0.20 has different dielectric and dispersion mechanism from the others and is attributed to the abundant space charges in it caused by the substitution. In addition, the VΩ-TFL were substantially promoted by the substitution of Ti in the films with x=0.10 and 0.20.BiFe1-xZrxO3+δfilms were prepared on ITO/glass substrates at an annealing temperature 600℃, in which x=0.00 to 0.40 and all films showed random orientation. Pyrochlore phase was observed in the film with x≤0.20. XRD subtle structure shows that the lattice constant increased through the substitution of Zr. The film with x=0.20 showed largest remnant polarization of 3.9μC/cm2. While the film with x=0.10 has the largest dielectric constant under the same measuring electric field. The leakage currents of all films show well symmetry under positive and negative bias applied field. The leakage currents were substantially reduced because of the pyrochlore phase. The currents for the film with x=0.20 was five magnitude order smaller that the one with x=0.00.(4) The preparation technology of BiFeO3-PbTiO3, PZT-BiFeO3 solid solution films was studied. The structure, ferroelectricity, dielectric property, and leakage current property were measured and studied.Pb(CH3COO)2.3H2O,Zr(NO3)3.5H2O,Ti(C4H9O)4 were adopted to prepared Pb(Zr0.5Ti0.5)O3(PZT) and PbTiO3 precursor solutions. The BiFeO3 precursor solution was mixed with it and stablizer was added to obtain the final BiFeO3-PbTiO3 and PZT-BiFeO3 precursor solutions. The (BiFeO3)1-x-(PbTiO3)x and (PZT)1-x-(BiFeO3)x solid solution films were deposited on LaNiO3 bottom electrodes. In the above solid solution systems, x=0.0, 0.05, 0.10 and 0.20 for (BiFeO3)1-x-(PbTiO3)x films and x=0.05, 0.10, 0.20 and 0.30 for (PZT)1-x-(BiFeO3)x system. A structural phase transition was observed in the film with x=0.10 for the (BiFeO3)1-x-(PbTiO3)x films, through which, the rhombohedral structure transferred to cubic structure. The analyse to the radii of Bi3+, Pb2+, Fe3+, Ti4+ and Zr4+ showed that Bi3+, Pb2+ occupy A site, while Fe3+,Ti4+, Zr4+ occupy B site. A site and B site co-substitution was realized through the solid solution. Meanwhile, the ferroelectricity and dielectric property were enhanced through the substitution and magnetism was also enhanced. In PZT films, the ferroelectricity of it increased first and then decreased with the increasing of x. The film with x=10% showed largest ferroelecity while the film with x=20% show largest coercive.