| In recent years, multiferroics occasionally called ferroelectromagnets or magneto-electrics with a simultaneous existence of electric and magnetic orderings, have attracted much attention due to their great potential in practical applications and excellent abundance with fundamental physics. As an important lead-free ferroelectric/piezoelectric system, the past few decades have attracted increasing attention of bismuth layer structured ferroelectrics (BLSF) due to its comprehensive advantages in the areas of high temperature, high frequency and ferroelectric random access memory (FRAM). People search for a variety of methods to improve the bismuth layered material performance and preparation multiferroic materials that can exsist above room temperature. In this dissertation, the microstructure, ferroelectric, magnetic and dielectric properties of serveal metal oxide compounds including Bis-xPrxFe0l.5Co0.5Ti3O15 Bi5-xPrxFeo.5Nio.5Ti3Oi5 ceramics and Bi4.25Pro.75Feo.5Coo 5Ti3O15 prepared by different ways were systematically investigated, and the physical nature was also preliminary discussed. The main results of this dissertation are as follows:The polycrystalline Bis-xPrxFeo.sCoo.sTi3O15 (BPFCT-x:x= 0.25-0.80) ceramics were prepared by a improved solid state reaction method. X-ray diffraction structure analysis showed that the content of Pr has influence on the microstructure of sample, but all the samples are layered perovskite structure. The remanent polarization (2Pr) firstly increased and then decreased with the increase of Pr content (x), the same as magnetic and ferroelectric properties. The remanent polarization reached a maximum value of 6.43 juC/cm2, when x= 0.75.The remanent magnetization (2MT) increased to a maximum value of 0.097 emu/g when x= 0.75, and then decreased with the increase of Pr content (x). As the increase of Pr doping, the ferroelectric and ferromagnetic properties of sample at room temperature can be obviously improved, and when x = 0.75, multiferroic properties of the sample at room temperature is the best. The improvement of ferroelectric properties of sample is related to Pr doping. With the increase of Pr content (x), the defects concentration of the sample can be reduced, ferroelectric domain of movement can be improved, and the improvement of ferromagnetic property is possibly associated with lattice deformation which is affected by Pr.The polycrystalline Bi4.25Pr0.75F30.5Co0.5Ti3O15 ceramics were prepared by three different methods:(1) we mix all the oxide together, then prepared BPFCT-0.75; (2) we prepare for pure BFO, and then synthesis BTO→BPFCT-0.75; (3) we prepare for BPT, then synthesis BPT→№ BPFCT-0.75). X-ray diffraction spectrum show that all the samples are 4 layers perovskite structure, and there is no obvious impurity phase; Compared with the improved solid state reaction method, the remanent polarization (2Pr) reduce, as well as the remanent magnetization (2Mr). The defects concentration of the sample can be reduced, however the ferroelectric domain of movement can be improved. Polycrystalline ceramics prepared by the improved solid state reaction method can improve all the properties of BPFCT-0.75. But the temperature magnetic can see no obvious change of the Curie temperature. Studies show that Pr are tend to access the layer of magnetic ion.The first two chapters main research the influence on multiferroic materials of A-site doping, but in this chapter we research the influence on multiferroic materials of both A-site and B-site doping. Pr, Ni are doped in BFTO, and then we get BPFNT-x We mainly study the microstructure, ferroelectric and magnetic of samples. In this experiment, Ni replace the half content of Fe in BFTO; Pr replace Bi in BFTO, the content of Pr is 0.25,0.50,0.75,0.80. The remanent polarization (2Pr) firstly increase and then decrease with the increase of Pr (x) content, the same as magnetic and ferroelectric properties. The remanent polarization reach a maximum values of 3.86μC/cm2, when x= 0.75. The remanent magnetization (2Mr) increased to a maximum value of 0.045 emu/g, also when x= 0.75, and then decreased with the increase of Pr content (x). As the increase of Pr doping, the ferroelectric and ferromagnetic properties of sample at room temperature can be obviously improved, and when x= 0.75, multiferroic properties of the sample at room temperature is the best. The improvement of ferroelectric properties of sample is related to Pr doping. With the increase of Pr content (x), the defects concentration of the sample can be reduced, ferroelectric domain of movement can be improved, and the improvement of ferromagnetic property is possible associated with lattice deformation which is affected by Pr. The results show that A-site and B-site doping can improve the performance of more iron on the sample. At room temperature, the ferromagnetism of Bi5Mno.5Ni0.5Ti5O15is only 0.0021 emu/g; The leakage current is very large; Also, we can not get the Curie temperature through variable temperature magnetic, because the Curie temperature is too low. |
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