Preparation Of BiFeO₃ Based Thin Films And Research On Their Electrical,Magnetic Properties

Bismuth ferrite(BiFeO₃,BFO)has high ferroelectric Curie temperature and antiferromagnetic Neel temperature,as well as high theoretical remnant polarization and low optical band gap.Therefore,BFO has great application promise for magnetoelectric memories,ferroelectric memories,photoelectric sensors and so on.However,the application of BFO is limited by its small actual remnant polarization,large leakage current and weak magnetism.Therefore,the modification of bismuth ferrite is of great significance.Construction of bismuth ferrite based solid solution and adjust its microstructure is an important means to improve the electrical and magnetic properties.In this thesis,novel bismuth ferrite based solid solution films were prepared by A and B sites co-substitution.The effects of different element contents on the microstructure and electric,magnetic properties were studied.In addition,a barium titanate(BTO)layer,which shows lower leakage,is added to improve the electrical properties.In order to improve the magnetic properties,the magnetic properties of bismuth based laminated composite films was studied.The specific research contents of this thesis are as follows:(1)The binary 0.72Bi Ti_0.27Fe_0.46Mg_0.27O₃-0.28La FeO₃(0.72BTFM-0.28LFO)and the ternary 0.625Bi Ti_0.27Fe_0.46Mg_0.27O₃-0.25La FeO₃-0.125La₂Mg TiO₆(0.625BTFM-0.25LFO-0.125LMT)bismuth ferrate based solid solution films were prepared on Pt/Ti/SiO₂/Si substrates by sol-gel method.The solid solution films show polycrystalline structure.Due to the difference of element content,content of oxygen vacancy and valence state of iron ions in the films were different.Dielectric tests show that the ternary film has higher permittivity,lower dielectric loss and leakage.Both films exhibit macroscopic and microscopic ferroelectric properties.The binary films have symmetrical I-V curves,while the ternary films exhibit obvious resistive switching behavior.Both the solid solution films have weak ferromagnetism.(2)A BTO layer,which shows lower leakage,was added on the 0.72BTFM-0.28LFO solid solution film by sol-gel method to improve the electrical properties.The composite film is smooth and compact,showing uniform grain size and clear two-phase interface.Compared with the single-phase solid solution films,the permittivity of composite film is significantly increased,the leakage is reduced by about one order of magnitude,and the ferroelectric property is enhanced.In addition,the composite film shows resistive switching behavior,nonetheless,the on/off ratio and stability are low.(3)The high-quality epitaxial bismuth ferrate based films,including single-phase La_0.2Bi_0.8FeO₃(LBFO)film and La_0.2Bi_0.8FeO₃&La_0.67Sr_0.33Mn O₃(LBFO&LSMO)composite films with 2,3 and 6 layers respectively,were prepared on the surface-treated STO(001)substrates by pulsed laser deposition.LBFO film has relatively high permittivity and low leakage,meanwhile shows ferroelectric property and the direction of ferroelectric polarization can be changed by applied electric field.The LBFO/LSMO and(LBFO/LSMO)₃ films show relatively stronger magnetic properties.