Multiferroic Properties Research Of BiFeO₃-based/CoFe₂O₄ Composite Thin Films Regulated By Doping Magnetic Layer

Ions-doping can enhance ferroelectric properties of BiFeO₃?BFO?thin films,but the problem of its weak magnetism has been not effectively improved.BFO combined with magnetic layer materials can enhance ferromagnetic properties,which leads to the deteriorating of ferroelectric properties of the composite films.In this thesis,doping in the magnetic layer is used to regulate the structure and multiferroic properties of the composite film.In this thesis,Bi_0.97-xSr_0.03Gd_xFe_0.94Mn_0.04Co_0.02O₃?BSG_xFMC?thin films,Bi_0.88Sr_0.03Gd_0.09Fe_0.94Mn_0.04Co_0.02O₃/Co_1-yMn_yFe₂O₄?BSGFMC/CM_yFO?composite thin films and Bi_0.88Sr_0.03Gd_0.09Fe_0.94Mn_0.04Co_0.02O₃/CoFe_2-zGd_zO₄?BSGFMC/CFG_zO?composite thin films have been fabricated by the sol-gel method.The effect of multi-doping on structure,defects,built-in electric field and multiferroic properties were investigated.And the effect of doping in the magnetic layer on structure and multiferroic properties were studied.The main conclusions are as follows:?1?The structure,morphology,defects,impedance and multiferroic properties of BSG_xFMC thin films were studied.BSG_xFMC thin films belong to the co-existence of trigonal-R3c:H phase and trigonal-R3m:R phase,but the content of the trigonal-R3m:R phase in the thin film is increased by Gd³⁺-doping.and the refined grain size with Gd³⁺-doping results in the increase of the resistance of grain and grain boundary,leakage current density of the thin films is reduced.The formation of oxygen vacancies is restrained,which lead to the decreasing of the built-in electric field at interface.The ferroelectric properties of the thin films are improved significantly by Gd³⁺-doping.the remnant polarization,the switching current and the squareness ratio of BSG_0.09FMC film are 108?C/cm²,1.4 mA and 1.18,respectively,and BSG_0.09FMC film also shows great ferroelectric stability.?2?The structure,defect complexes and multiferroic properties of BSGFMC/CM_yFO composite thin films are investigated.Mn²⁺-doping at A-site causes the structural distortion in the CM_xFO films,which induces a structural transition of BSGFMC layer in the composite film from the co-existence of trigonal-R3c:H and trigonal-R3m:R phase to a single trigonal-R3m:R phase.And Mn²⁺-doping suppress the formation of defect complexes in the composite films.The defect complexes for the restraining of the ferroelectric wall switching are weakened at a low applied voltage,which enables the composite film to obtain excellent saturated polarization at lower applied voltage.The remnant polarization,the coercive field and saturation magnetization of BSGFMC/CM_0.3FO composite film are 106?C/cm²,263 kV/cm and 45.91 emu/cm³,respectively.?3?The crystal structure,ferroelectric and ferromagnetic properties of BSGFMC/CFG_zO composite thin films are studied.The top BSGFMC layer of the composite film belongs to the co-existence of trigonal-R3c:H and trigonal-R3m:R phase,but Gd³⁺-doping at B-site in magnetic CFG_zO films results in increasing of the ratio of the trigonal-R3m:R phase from 4%?BSGFMC/CFO?to31%(BSGFMC/CFG_0.10O)in the BSGFMC layer.BSGFMC/CFG_0.10O composite film exhibits outstanding ferroelectric behavior and the remnant polarization and the coercive field are 92?C/cm² and 361 kV/cm,respectively.BSGFMC/CFG_0.02O composite film exhibits the improved ferromagnetic properties and the saturation magnetization and the squareness ratio are 68.8emu/cm³ and 0.66.