| BiFeO3(BFO)has ferroelectric,resistive switching effect(RS effect)and antiferromagnetism at room temperature,which provides a new possibility for multi-functional and miniaturized design of storage devices.However,there are many defects and defective dipoles in BFO films,which leads to the increase of leakage current and the deterioration of ferroelectric properties.Meanwhile,the weak ferromagnetic properties and interface effect of BFO films limit the application of BFO films in practice.The multielement co-doping is generally chosen to improve the multiferroic properties of BFO films.It is also found that the combination of BFO and magnetic films can obtain excellent ferromagnetic properties and RS effect,and regulate the interface effect.In this thesis,three-element co-doped Bi0.92-xHo0.08SrxFe0.97Mn0.03O3 thin films,Bi0.92-XHo0.08AXFe0.97Mn0.03O3/Zn0.5Ni0.5Fe2O4(A=Ca,Sr and Ba)composite thin films,Bi0.89Ho0.08Sr0.03Fe0.97-XMn0.03ZnXO3(BHSFMZnXO)thin films and Bi0.89Ho0.08Sr0.03Fe0.95Mn0.03Zn0.02O3/Zn1-XNiXFe2O4(BHSFMZO/ZNXFO)films were prepared by sol-gel method.The effects of multielement co-doping and composites on the structure,RS effect,conduction mechanism,interface effect,ferroelectricity and ferromagnetism of the thin films were studied.The main results are as follows:(1)The structure,leakage current,ferroelectric and ferromagnetic properties of BHSrxFMO films were investigated.The results show that the structure of BHSr3FMO thin film belongs to the rhombohedral perovskite structure,and the ratio of R3c:H and R3m:R space group is close to 1:1,belonging to the morphotropic phase boundary(MPB).Under the electric field of 536 kV/cm,the 2Pr and 2Ec of the BHSr3FMO thin film are 81.9 μC/cm2 and 524 kV/cm,respectively.The polarization switching current(IS)of BHSr3FMO is 1.57 mA.(2)The structure,conduction mechanism,interface effect and multiferroic properties of Bi0.92-XHo0.08AXFe0.97Mn0.03O3/Zn0.5Ni0.5Fe2O4(BHAFMO/ZNFO)composite films were studied.The structure of BHCaFMO/ZNFO film is changed from rhombohedral(R3c:H and R3m:R space group)to triclinic(P1)structure.When the doping ion radius is close to Bi3+,the structure distorts obviously and the content of oxygen vacancies increases,which is beneficial to the improvement of magnetic properties in BHCaFMO/ZNFO thin film(Ms=67.2 emu/cm3,Hc = 5.4 Oe).When the doping ion radius is greater than Bi3+,the content of oxygen vacancies and the density of leakage current are decreased.And the MPB of the BHSrFMO/ZNFO film is beneficial to the improvement of ferroelectric properties(Pr～82.1 μC/cm2,Ec～566 kV/cm).The adjustment of the interface barrier and Schottky barrier by controlling the oxygen vacancies could not only change the conduction mechanism,but also adjust the ferroelectric domain switching to influence ferroelectric polarization.(3)The effects of Sr,Ho,Mn and Zn co-doping on the structure,defects,conduction mechanism,RS effect and multiferroic properties of the BFO films were investigated.The results show that Zn2+ doped BHSFMO films lead to the transformation of preferred orientation from(110)to(100).BHSFMZn0.01O and BHSFMZn0.020 films exhibit bulk effect under both the high resistance state(HRS)and low resistance state(LRS).The conduction mechanisms of BHSFMZn0.030 and BHSFMZn0.04O films are the Schottky emission and F-N tunneling at LRS.The BHSFMZn0.04O film shows the highest RS ratio(18.6)at 190 kV/cm.BHSFMZn0.02O films exhibit excellent ferroelectric polarization(Pr～132 μC/cm2 and IL～0.11 mA).Therefore,the RS effect and ferroelectric properties can be obtained by controlling the doping amount of Zn2+.(4)The effect of Ni doping on the mismatch strain,the conduction mechanism,RS effect,p-n heterojunction and multiferroic properties in the BHSFMZO/ZNXFO composite thin films were studied.The lattice mismatch strain of the BHSFMZO/ZNXFO composite film can be controlled by the amount of Ni doping in ZNXFO thin films,and the structure of the BHSFMZO/ZNXFO composite films can be significantly affected.The leakage current curves of the composite films show obvious diode rectifying properties,which indicates that p-n heterojunction is formed at the interface of BHSFMZO/ZFO,BHSFMZO/ZN0.1FO and BHSFMZO/ZN0.3FO composite films.There are different ferroelectric domain switching characteristics under different directions of electric field,which leads to the deformation of ferroelectric hysteresis loops.The BHSFMZO/ZNXFO films have excellent ferromagnetic properties. |
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