【作者】 伍茂松；

【导师】 袁松柳；

【作者基本信息】 华中科技大学 ， 材料物理与化学， 2013， 博士

【摘要】 随着科学技术特别是现代信息技术的迅飞猛进发展,社会信息化的进程日新月异,人们对电子元器件的小型化/微型化以及实现多功能化的要求越加迫切,因此使得人们在对集多种物理效应于一体的多功能材料的研究上显示出更多的关注和兴趣。而多铁性材料就是这样一类能够同时呈现出两种或两种以上铁性(如铁电性、铁磁性、铁弹性以及铁旋性)的多功能磁电材料。更为值得关注的是,在多铁性材料有可能实现铁磁性和铁电性之间的相互调控,也就是说能够在外加电场下比不仅使材料具有铁电极化性质,还能够诱导激发磁化,同样在外加磁场也能够诱导材料的铁电极化性质。多铁性材料以及铁电性与铁磁性之间的耦合,包含着丰富的物理内容,它涉及到自旋与晶格或声子之间的耦合、铁磁与反铁磁耦合以及磁光耦合等,因此,在研究多铁材料中,可以通过外部物理场(磁场、温度场、电场以及电磁场)实施对多铁材料的调控,这就为铁电/铁磁性的互相耦合提供了依据。正是由于多铁材料具有以上丰富的物理研究内容,使得成为当前热门研究领域。本文以BiFeO3多铁材料为基础,研究了掺杂改性以及与铁电材料(Bi0.5Na0.5TiO3和Bi4Ti3O12)形成固溶体,主要研究内容如下：(1)我们采用硝酸盐-柠檬酸法制备了(1-x)BiFeO3-xBi0.5Na0.5TiO3系列固溶体陶瓷样品,以及Ba掺杂x=0.3的样品。详细研究了固溶体陶瓷样品微观结构、形貌、铁电性、磁性以及光学性能。研究结果显示,Bi0.5Na0.5TiO3含量逐渐增加引起晶体结构由菱方逐渐向膺立方相转变,转变临界点在x=0.4处。和纯相BiFeO3材料相比,x=0.3的样品具有较好的多铁性,获得的剩余极化强度和饱和磁化强度分别为Pr=1.49μC/cm2和Ms=0.51emu/g.更为重要的是,顺磁到铁磁相的转变在此固溶体材料中也被观察到,居里温度也能够通过变化Bi0.5Na0.5TiO3的含量进行调节。这里观察到的铁磁有序主要是由于在化学有序区域内可能存在Fe3+-O-Ti-O-Fe3+的长程超交换作用。同时,Ba掺杂0.7BiFeO3-0.3Bi0.5Na0.5TiO3固溶体,磁滞回线测量表明磁性能获得了极大的增强。在x=0.2固溶体中,磁滞回线测量获得了最大剩余磁化,数值为0.55emu/g。Ba的掺杂还提高了陶瓷粉末的禁带宽度。(2)我们详细研究了固溶体的动态磁性能、介电性能以及微波吸收性能。结果显示,所有样品的磁性能、介电性能随频率的变化趋势相同。随着含量x的增加,Bi0.5Na0.5TiO3掺杂固溶样品的反射损耗逐渐的减小,也就是说,微波吸收性能在减弱,小于-10dB的带宽也在不断的减小。在2-18GHz,x=O.1和0.2的样品相对来说具有较好的吸收特性。但是由于复介电常数和复磁导率存在巨大的差异性,导致了微波吸收性能较差。因此,有必要进一步提高阻抗匹配特性。(3)采用硝酸盐-柠檬酸法制备Bi0.8Ba0.2Fe1-xNbxO3系列陶瓷样品,研究了共掺杂材料的铁磁性、铁电性、介-频/介-N性质以及室温磁电耦合性质。Nb掺杂增强了材料的铁电和铁磁性质。结果显示,获得磁电增强的Bi0.8Ba0.2Fe0.975Nb0.025O3陶瓷的剩余极化强度和矫顽场(Mr和Pr)分别为3.69emu/g和1.34μC/cm2。从介电随频率测量中看到,Nb掺杂减小了低频频散和介电损耗。通过介温曲线测量发现,在反铁磁转变温度出现了介电反常。Nb的掺杂有助于减小介电损耗,尤其在低频范围内。同时,x=-0.015和0.025样品处于磁场磁化后,测量得到的剩余极化强度都获得了增强,这样就间接证明了室温磁电耦合的存在。(4)采用固相法制备了Bi5Ti3FeO15陶瓷样品,研究该材料的微观结构、介电性质随温度的变化关系、交流电导特性、铁电性、铁磁性以及光吸收特性。结果显示,Bi5Ti3FeO15陶瓷样品具有层状钙钛矿结构,颗粒尺寸在2-5μm范围内。介温测量显示在该材料中存在两个介电反常,分别在1007和1090K,预示着也有两种相变的存在,但是在tanδ-T曲线内并没有观察到峰的出现。同时,计算出在476-639K,652-966K和980-1095K内的激活能分别为0.156,0.262和0.707eV。通过电滞回线测量得到的剩余极化强度(2Pr)和矫顽场(2Ec)分别为6.08μC/cm2和59kV/cm。磁性测量表明Bi5Ti3FeO15陶瓷在室温下具有弱的铁磁性,我们对磁性的起源也作了分析。通过紫外-可见光吸收光谱测量,计算拟合得到的禁带宽度为2.03eV。(5)采用硝酸盐-柠檬酸法制备了La掺杂Bi9Ti3Fe5O27系列陶瓷样品,研究了材料的微观结构、介电性质随温度的变化关系、铁电性、铁磁性以及磁电耦合性质。研究表明随着La3+离子含量的增加,样品的颗粒尺寸逐渐减小,材料的磁性也得到逐步的增强。同时样品的铁电-顺电相变温度逐渐的移向低温区。对于Bi6La3Ti3Fe5O27样品,结构个高分辨电镜分析表明该合成直径为2μm厚度为160-170nm的片状结构材料,这是由于材料在(001)面加速生长造成的。La掺杂已经显示能够很好的实现室温铁电性／铁磁性的共存,因此也预示稀土掺杂是提高反铁磁Bi9Ti3Fe5O27多铁性质的一种较好方式。在Bi6La3Ti3Fe5O27陶瓷中,在500-590和600-650K温区内观察到两种介电反常现象,较高温度的反常对应着铁电-顺电相变。惊喜的是,通过测量磁化前后的电滞回线以及外加磁场下的介电常数发现样品在室温下存在着电荷与自旋间的磁电耦合效应。研究发现,磁介电响应随着外加磁场的增大而增大,而且对频率具有非线性依赖关系。更多还原

【Abstract】 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.更多还原