Enhancement Effects Of Magnetic,Dielectric And Ferroelectric Properties In Transition Metal Oxide Based Multiferroic Ceramics

Multiferroic materials have two or more kinds of ferroic ordering. Importantly, there are coupling effects among different ferroic properties. By using this effect, it can be realized that the magnetic field can be used to tune the polarization and vice versa. With more and more studies, preliminary understanding of the origination of the ferroelectric and ferromagnetic properties has been achieved. However, the physical mechanism of the magnetoelectric coupling is still unclear. Moreover, there are a lot of deficiencies in the discovered multiferroic materials, such as the small polarization, the low Curie temperature, and the weak magnetoelectric coupling effect. Therefore, it is necessary to explore the multiferroic property in other materials. In this paper, we focus on the enhancement effects of magnetic, dielectric and ferroelectric properties to perform the experimental studies in several multiferroic transition metal oxides. The main research contents and innovations are shown below:(1) The structure, magnetic and high temperature dielectric properties are studied in the SmCrO3 sample. The results show that the coupling of Cr3+ and Sm3+ has much influence on the magnetic behavior. Below the Neel temperature, there is canted antiferromagnetic structure between Cr3+ and Cr3+. The spin reorientation of Cr3+ occurs at ?50 K. Below this temperature, the antiparallel coupling of Cr3+ and Sm3+ leads to the decrease of total magnetization. Meanwhile, magnetic exchange bias behavior emerges. The high temperature dielectric relaxation suggests relatively high dielectric permittivity, and the origin of the giant dielectric effect is discussed based on the interal barrier layer capacitance (IBLC) model.(2) The doping effects of structure and magnetic properties are studied in LaxSm1-xCrO3(x=0-0.9) samples. The results show that there are field cooled and zero field cooled exchange bias behaviors, which depends on the coupling effect between Cr3+ and Sm3+. By introducing nonmagnetic La3+ into the sites of magnetic Sm+, the degree of decreased magnetization and coupling effect is weakened apparently, which further confirming that the coupling of Cr3+ and Sm3+ has much influence on the magnetic behavior.(3) The structure and high temperature dielectric properties are investigated in RFeO3 (R= La, Pr, Sm) samples. At the same sintering temperature, the partical size as well as the density increases from LaFeO3 to SmFeO3. The temperature dependence of dielectric permittivity exhibits relatively high dielectric property. The high frequency relaxation is shown to originate from the carrier hopping between Fe+ and Fe+, while the low frequency relaxation most likely arises from the conduction. Based on the complex impedance spectra, the high dielectric constant can be explained according to the IBLC model.(4) The structure and high temperature dielectric behavior are studied in polycrystalline Sm3Fe5O12 samples. The results show that the sample has high dielectric permittivity and low dielectric loss under low frequency at room temperature. Two types of dielectric relaxations are clearly observed. The high frequency relaxation is shown to originate from the carrier hopping between Fe2+ and Fe3+, while the low frequency relaxation most likely arises from the conduction. By comparing the dielectric constant and valence state of Fe in air and oxygen stmosphere sintered samples, it suggests that the high temperature relaxation is easily influenced by the concentration change of Fe2+ and Fe3+.(5) In the samples of Bi2(Fe1-xAlx)4O9 (x= 0-0.25), the structure, ferromagnetic, and ferroelectric properties are studied. Room temperature multiferroic property of Bi2Fe4O9 can be induced by Al3+ doping. Appropriate amount of Al3+ doping can lead to spin-glass-like behavior in the low temperature magnetic curves. The exchange bias behavior is investigated in the sample of Bi2Fe3AlO9.Below the spin-glass temperature, the spin-glass-like phase has much effect on the magnetic behavior. Above the spin-glass temperature, the exchange bias field varies nonmonotonically with temperature, which is interpreted using a random field model based on the thermal disturbance effect.(6) A series of Bi2Fe4O9 samples with different sintering temperature after magnetic field and nonmagnetic field presintersing are prepared. The crystal structure and room temperature magnetoelectric properties are studied. It is suggests that the magnetic field presenting technology can enhance the room temperature multiferroic property in samples.