Preparation And Physical Properties Of Co-doped BiFeO₃-based Multiferroic Ceramics And Thin Film

In recent years, Ferrite bismuth（BiFeO₃） have been obtained the widely attention because of the properties of temperature ferroelectric and antiferromagetic multiferroic, and have potential application in many fields such as spintronics,ferroelectric/magnetic data storage systems, quantum electromagnet, sensing device etc. Although BiFeO₃ is considered to be one of the most potential multiferroic materials, there is still remaining some problems like the high leakage current, small magnetization and polarization. Due to the leakage current is too large, it is difficult for BiFeO₃ to obtain saturation hysteresis loop and high residual polarization, which is related to the formation of oxygen defect, the fluctuation of Fe ion valence state and so on.In this paper, we prepare high purity BiFeO₃-base ceramics by improved traditional solid phase reaction method and room temperature high orientation of ferroelectric and ferromagnetic films by used pulsed laser deposition. We used the rare earth metals and 3d transition metal part to substitute A-site of Bi3+ and B-site of Fe3+, respectively, in order to study the influence of the combination, concentration,valence state of the co-doped ions on the microstructure, leakage current, ferroelectric,dielectric, and ferromagnetic of ceramics and thin film. Acquired the preparation of performance optimization parameters, which combined with observing and analyzing of domain structure. And we discussed the influence factors of the related mechanism in magnetic and electric properties of these materials, which providing the reference for development of the device. The main results are listed as followings:1. Modified preparation technology of ceramics on the basis of the traditional solid state reaction method. On the one hand, inhibited the formation of Bi2Fe4O9 impurity phase by adding excessive Bi2O3. On the other hand, removed Bi25FeO40 impurity phase effectively by adding a wet chemical reaction steps to obtain the preparation technology of purity phase BiFeO₃ ceramic.2. Co-doped rare earth metals and 3d transition metal on A-site and B-site of BiFeO₃, respectively. And we attempted to dope three combination of ions: Ca/Cr,Nd/Cr, La/Cr. The results show that the impurity phase in ceramic which doped La/Cr is reduced significantly, and the ferromagnetic properties of leakage has been improved obviously.3. The microstructure and properties is complex due to more co-doping ion species. We started the single La doping BiFeO₃ that we doped La3+ on A-site of BFO to prepare Bi1-x LaxFeO3（10 at%≤ x ≤20 at%） pure ceramics. Then, we synthesized Bi1-x LaxFe0.9Cr0.1O3（10 at%≤ x ≤20 at%）ceramics which obtained through co-doping La/Cr on A-site and B-site of BiFeO₃ by fine-tuning the technology. The phase structure of the sample is changed from rhombohedral（R3c） to triclinic（P1）. There Fe ions consist of Fe2+ and Fe3+ ions, and the proportion of Fe3+ is increased with the increase of La content, which means that the oxygen vacancy concentration is reduced and leakage performance is improved obviously. The magnetic of the sample is improved significantly because lattice distortion and structural phase transition destroyed BiFeO₃ spiral linear magnetic structure, and the ferromagnetic superexchange interaction with oxygen ions as the medium produced between B-site doped Cr3+ and Fe3+ ions.4. La/Cr-codoped BFO thin films were grown on Pt（111）/Ti/SiO2/Si substrates using pulsed laser deposition. By changing the oxygen sputtering pressure, laser power, laser frequency, substrate temperature and deposition time, obtained the growth conditions of pure phase codoped thin film. The film made highly oriented growth along（012） direction. the leakage current of films is reduced and the magnetization is enhanced through the combined action of La/Cr co-doping for the promotion of the lattice distortion, the inhibition of oxygen vacancy, the damage to spiral spin structure, excitation of new ferromagnetic superexchange mechanism and so on. The film has the smallest leakage current, the highest residual polarization,dielectric constant and saturation magnetization when x=20 at%.