Multiferroic Properties And Magnetoelectric Coupling Effect In The Ceramics Of BiFeO3with ANbO3(A=K, Na, Li) Solid Solution

Multiferroic materials which possess more than one ferroic order such as the ferroelectric, ferromagnetic and ferroelastic orders have attracted much attention for the potential application in the area of information storage, sensors, etc. A large number of researches have been focused on the nature of the multiferroics as well as the coupling interaction between the ferroic orders in these systems.In this thesis, we have done the following work:(1) In order to research the multiferroic properties of BiFeO3and its solid solutions, we prepared the single-phase ceramics of (Na, K)NbO3solid solution, and investigate the ferroelectric, dielectric, and piezoelectric properties, respectively. We obtained the excellent ferroelectric and piezoelectric properties. In the ferroelectric hysteresis loops, the largest remnant polarization can reach16.9μC/cm2with a small coercive field of9.3kV/cm. And the piezoelectric constant d33can achieve112pC/N-1. In the temperature dependence of dielectric constant, we observed the obvious transition referring to the Curie temperature about402℃. Meanwhile, the physical properties show the clear grain size effect, the Curie temperature, dielectric constant, ferroelectric polarization, and piezoelectric constant were all strongly dependent on the grain sizes. The nature of the grain size effects on the electrical properties has also been discussed.(2) We prepared the dense ceramics of (1-x)BiFeO3-xNaNbO3(0.10≤x≤0.25) solid solution and investigated the microstructure, ferromagnetic, ferroelectric, and dielectric properties. The results show that the samples are single phase with hexagonal structure when x≤0.12. However, the structure transforms to orthorhombic structure when0.15≤x≤0.18, and the phases are the mixed-structure of hexagonal and orthorhombic. When x≥0.20, the structure becomes orthorhombic completely. In these samples, the ferromagnetic property was observed. The sample of x=0.15shows the best ferromagnetism, and the remnant magnetization and coercive field can reach0.15emu/g and4300Oe, respectively. However, the ferromagnetism disappears when x≥0.20, and the hysteresis loop shows the linear. It is observed that the ferroelectric property was enhanced in these solid solutions, compared to single phase BiFeO3ceramic. In the sample of x=0.15, the remnant polarization and coercive field are2.37μC/cm2and19.1kV/cm, respectively. From the dielectric measurement, the space charge polarization effect was observed, especially in the samples of x≤0.12, which was attributed to vacancy from the oxygen deficient and Bi volatilization. We analyzed and discussed the nature of the enhanced ferroelectric and ferromagnetic properties in this BiFeO3-NaNbO3solid solution roughly.(3) We prepared the ceramics of (1-x)BiFeO3-xANbO3(A=K, Li;0.05≤x≤0.20) solid solutions, respectively. Then, the magnetism, ferroelectricity, and dielectric property were investigated. The results show that there is no ferromagnetism was observed in these ceramics, and the hysteresis loops indicate the anti-ferromagnetism or paramagnetism. It is also observed that the ferroelectric property was enhanced in these solid solutions, compared to single phase BiFeO3ceramic. The dielectric measurement shows that there is space charge polarization effect in these ceramics. Meanwhile, we examined the XPS in the BiFeO3-NaNbO3and BiFeO3-KNbO3. And there is a mount of oxygen vacancy in BiFeO3-NaNbO3, resulting from the decreasing of elements' valence. In contrast, there is no change of elements'valence in BiFeO3-KNbO3. We discussed the origin and nature of the enhanced ferromagnetic and ferroelectric properties in BiFeO3-ANbO3(A=K, Na, Li) solid solutions.(4) Considering the outstanding electrical properties of (Na, K)NbO3, we prepared the ceramics of (1-x)BiFeO3-xNa0.5K0.5NbO3(0.06≤x≤0.18) solid solution, and investigated the microstructure, magnetism, ferroelectricity, and magnetoelectric coupling property. The results show that the samples are single phase with hexagonal structure when x≤0.10. However, the structure transforms to orthorhombic structure when x=0.15, and the phase is the mixed-structure of hexagonal and orthorhombic. However, when x≥0.18, the structure becomes orthorhombic, completely. This is consistent with the results of above BiFeO3solid solutions. It is shown that the samples are weak ferromagnetic when x≤0.15. The sample of x=0.10shows the best ferromagnetism, and the magnetization and coercive field can reach0.43emu/g and3530Oe, respectively. We also analyzed the nature of the magnetoelectric property. Besides, we observed the magnetodielectric or magnetocapacitance effect through the change of the capacitance under an external magnetic field in the ferromagnetic and non-ferromagnetic samples. We attempt to discover the intrinsic magnetoelectric coupling property in BiFeO3solid solutions.