| Multiferroic materials, which combine the properties of ferromagnetism, ferroelectricity, or ferroelasticity, have attracted more and more attention due to their possible application toward storage materials and intriguing fundamental physics. Besides, coupling between magnetic and electric order parameters can give rise to the magnetoelectric effect, in which the magnetization can be exhibited under an external electric field and vice versa. BiFeO3is one such material which shows a lot of potential for research. This material is antiferromagnetic and ferroelectric having an antiferromagnetic Neel temperature (TN) of643K and a ferroelectric Curie temperature (Tc) of1103K. It has been shown to possess a rhombohedrally distorted perovskite structure with space group R3c at room temperature. In addition, BiFeO3shows G-type antiferromagnetic order where the Fe magnetic moments are coupled ferromagnetically within the (111) planes and antiferromagnetically between adjacent planes. When the ferromagnetically ordered magnetic moments are aligned parallel to the (111) planes, the symmetry allows canting of the antiferromagnetic sublattices, which gives rise to macroscopic magnetization as a whole This spiral spin structure can be suppressed by doping. However, as for BiFeO3bulk ceramics, research work is still being hindered by the easy formation of second phases during synthesis and the low electrical resistivity of samples. Recent work mainly has focused on (a) binary or ternary solid solution of BiFeO3with other ABO3perovskite materials (such as BaTiO3, Bi0.5Na0.5TiO3) and (b) A/B sites co-doping.This paper based on BiFeO3multiferroic materials, and research on doping modification, as well as the formation of solid solution of with ferroelectric material (Bio5Na0.5TiO3of Bi4TisO12). We have done the following work:(1) We prepared the (1-x)BiFe03-xBio.5Nao.5Ti03ceramics and Ba doped0.7BiFe03-0.3Bio.5Nao.5Ti03, and investigated the microstructure, ferromagnetic, ferroelectric properties in detail. The results show that increasing Bi0.5Na0.5TiO3content induce a gradual phase transformation from rhombohedral to pseudocubic structure near x=0.4. Compared with pure BiFeO3, superior multiferroic properties are obtained for x=0.3with remnant polarization Pr=1.49μC/cm2and saturated magnetization Ms=0.51emu/g. Importantly, the paramagnetic to ferromagnetic transition is observed for the solutions, and the Curie temperature (Tc) can be tuned by varying the content of Bi0.5Na0.5TiO3.This observed ferromagnetic ordering is discussed in terms of the possible existence of the long-range superexchange interaction of Fe3+-Ti-O-Fe3+in the chemically ordered regions. At the same time, significant magnetic enhancement was observed for0.7BiFe03-0.3Bio.5Nao.5Ti03with Ba doping. Ferromagnetic hysteresis loops revealed the maximum remanent magnetization of0.55emu/g for0.7B1-xBxF-0.3BNT of x=0.2. The effect of introducing La is shown to increase the optical band gap for doped sample0.7B1-xBxF-0.3BNT.(2) We investigated the dynamic magnetic, dielectric and microwave absorbing properties in detail. The results show that all samples have the similar behaviors. With the increasing of the content x, the reflectivity of Bi0.5Na0.5TiO3doped samples decrease, that is to say, the microwave absorption properties decrease, and the bandwidths of lower than-lOdB was also decreased. At2-18GHz, x=0.1and0.2samples have relatively better absorption properties. But because of the great differences between complex dielectric constant and complex permeability, results in the bad microwave absorption properties, thus impedence matching need to further improve。(3) We prepared the Bi0.8Ba0.2Fe1-xNbx03ceramics and investigated the ferromagnetic, ferroelectric, dielectric constant versus temperature/dielectric constant versus frequency and room temperature magnetoelectric coupling properties. Substitution with Nb also improved the ferroelectric polarization. As a result, enhanced multiferroic properties of Bi0.8Ba0.2Fe0.975Nbo.o2503ceramics with remanent magnetization and polarization (Mr and Pr) of3.69emu/g and1.34μC/cm, respectively, were obtained. The reduction of low-frequency dispersion in permittivity and loss due to Nb substitution in was observed in its dielectric response curve. An anomaly in the dielectric constant was observed in the vicinity of the antiferromagnetic transition temperature. Nb Substitution was found to be helpful to reduce loss, especially at lower frequencies. In addition, an enhancement in remanent polarization after poling the samples with x=0.015and0.025in the dc magnetic field was evidence of magnetoelectric coupling at room temperature.(4) We prepared the Bi5Ti3FeO15ceramics and investigated the microstrcture, dielectric constant versus temperature, ac conductivity, ferroelectric, ferromagnetic, and optical performances. The results show that polycrystalline Bi5Ti3FeO15ceramics of layered perovskite phase, having particle size of2-5μm, Two obvious dielectric anomalies around1007and1090K were exhibited by this material, indicating that there are two phase transitions. While no peak was found in the tanδ-T curve. In addition, the conduction loss activation energies calculated at476-639K,652-966K, and980-1095K are0.156,0.262, and0.707eV, respectively. Polarization versus electric field hysteresis loops associated with2Pr of6.08μC/cm2and2EC of59kV/cm were obtained. The result of magnetic measurement indicated the weak ferromagnetic order of Bi5Ti3FeO15ceramics at room temperature, we also analyzed the origin of the obtained magnetism. An energy band gap of2.03eV was determined from the UV-vis diffuse absorption spectrum.(5) We prepared the La doped Bi9Ti3Fe5O27ceramics and investigated the materials’ microstrcture, dielectric constant versus temperature, ferroelectricity, ferromagnetism, and magnetoelectric coupling property. Results reveal that increasing the La3+ions content, the particles of the materials will be decreased, and the ferromagnetism will be enhanced. Meanwhile, the transition temperature from the ferroelectric structure to the paraelectric structure will shift toward lower temperatures. As for Bi6La3Ti3FesO27, the structural and HRTEM analysis indicates that the formation of BLTF with plate-like morphology with about2μm. in diameter and160-170nm in thickness result from the accelerating growth of (001) crystalline planes. La substitution has been shown to effectively induce the coexistence of ferroelectricity and ferromagnetism at room temperature, thus indicating a promising way for improving multiferroic properties of antiferromagnetic Bi9Ti3Fe5O27. Two dielectric relaxations were observed in the temperature ranges of500-590and600-650K in BLTF ceramics, and the higher temperature dielectric relaxation is related to the ferroelectric phase transition. Surprisingly, the magnetoelectric coupling between charge and spin ordering at room temperature was demonstrated by measuring the effect of magnetic poling on ferroelectric hysteresis loop and the change in dielectric constant with the external magnetic field. It was found that the magnetodielectric response increased with the increase of magnetic field and showed a frequency dependent nonlinear response. |
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