Study On Preparation,electrical And Magnetic Properties Of Mn And Ni Doped BiFeO₃ Thin Films

Multiferroelectric materials,which possess abundant physical properties and wide application prospect due to is unique coexistance of ferroelectric and magnetic orders and their coupling effect,has been an important research hotspot in recent years.Among them,bismuth ferrite?BiFeO₃,short for BFO?,possessing a typical ABO₃ peroviskite structure,has been attracting increasing interest due to its specific property that the ferroelectric and antiferromagnetic are coexisted at the room temperature.However,owing to easy Bi volatilization in the preparation process and therefore resulted defects,it is very difficult to synthesize the pure phase perovskite BFO thin film with high quality,which significantly limits its practical application.Factors such as oxygen vacancy,impurity and scaling effect,etc.,play an important role in modifying the microstructure and property of BFO thin film.Therefore,effects of the oxygen vacancy concentration,impurity type and content,film scaling and doping effect?various elements and their contents?on the modifying method and mechanism of microstructure,electrical and magnetic properties of BFO thin films are investigated and explored.It is well known that impurity is easily generated in BFO thin film.Aiming at solving this problem,the influence of O2/Ar ratio on the phase composition,microstructure and property of BFO thin films was investigated in this theis.The results demonstrate that the generation of impurity phase could be effectively modified by controling O2/Ar ratio,and when the O2/Ar ratio is 1:10,the content of the pure phase in BFO thin film reaches over 98%.Meanwhile,a proper O2/Ar ratio not only benefits the grain growth and makes the grain shape and surface roughness become more uniform and smoother respectively,but also decreases the concentration of oxygen vacancy and the leakage current of BFO thin film?when the O2/Ar ratio is 1:10,the leakage current density is 7.34×10-6 A/cm2?.Oxygen vacancy concentration could directly influence the electrical and magnetic property of BFO thin film.In this thesis,the oxygen vancancy of BFO thin film was modified by changing the different annealing atmosphere.The results reveal that the O2 annealing not only restrains the generation of impurity phase and oxygen vacancy,increases the film density and reduces leakage current density of BFO thin film,but also improves the ferroelectric,dielectric and magnetic properties of BFO thin film.The perovskite phase of BFO thin film could only be stable in a narrow temperature range.In this thesis,the effect of annealing temperature on the microstructure and property of BFO thin films was investigated.The results suggest that proper improvement of annealing temperature could promote the grain growth,decreases the surface roughness,decreases the leakage current density.Moreover,the ferroelecric remnant polarization,coercive electric field,dielectric constant and magnetization of BFO thin film are generally increased.Film thickness is also one of the main methods to modify the property of BFO thin film.Thus,in this thesis,the film thickness was controlled by change of depostion time and the effect of film thickness on the microstructure and property of BFO thin films was investigated.The results demonstrate that with increasing of film thickness,the chemical composition of BFO thin film is much closer to the chemical stoichiometric ratio and the phase purity becomes much higher.With the increasing of film thickness,the grain size and surface roughness of BFO thin film are obviously improved and the ferroelectric remnant polarization,dielectric constant and both the magnetic remnant polarization and coercive magnetic filed of BFO thin films are improved.Doping is a critical way to modify the property of BFO thin film.In this thesis,the microstructure and property of BFO thin film was modified by doping of Mn and Ni elements.The results demonstrate that: with increasing of Mn doping,the oxygen vacancy concentration gradually decreases,the grain size,ferroelectric remnant polarization,coercive electric field,dielectric constant,piezoelectric coefficient and magnetic remnant polarization of BFO thin films were obviously improved;however,with increasing of Mn doping,the transmittance of BFO thin film slightly decreases and the band gap is decreased?when the Mn content x=0.08,a lowest band gap,Eg=2.58 eV?.The Ni doping could induce a lattice distortion,leading to a formation of morphotropic phase boundary region in Ni doped BFO thin film;with increasing of Ni doping,the concentration of Fe2+ ion,grain size,crystallinity,ferroelectric remnant polarization and coercive electric field,dielectric and magnetic remnant polarization are obviously increased;the ferroelectric remnant polarization,coercive electric field and dielectric constant reach the maximum?Pr=11.45 ?C/cm2,Ec=200.5 kV/cm and 262,respectively?when the Ni doping content x=0.1;moreover,the band gap of Ni doped BFO thin film has a decrease from 2.58 to 2.48 eV due to the increased grain size caused by Ni doping.