【作者】 郑朝丹；

【导师】 张端明；

【作者基本信息】 华中科技大学 ， 凝聚态物理， 2009， 博士

【摘要】 近年来，多铁材料引起了人们的极大兴趣。其中，BiFeO₃（BFO）由于在室温下同时具有铁电性和弱铁磁性，被认为在信息存储、集成电路、磁传感器以及自旋电子器件等方面有诱人的应用前景。本文采用优化的固相烧结工艺，制备出纯相的BFO陶瓷。在对Bi₂O₃-Fe₂O₃二元体系的相图和热重-差热分析（TG-DTA）的基础上，以固相烧结作为核心的工艺流程，确定了制备纯相、平整、光滑的BFO陶瓷大靶的优化工艺。另外，本文以制备出的陶瓷为靶材，采用射频磁控溅射工艺在Pt（111）／Ti／SiO₂／Si（001）衬底上沉积BFO薄膜，研究了退火条件对薄膜结构的影响，在大量实验的基础上，找到最佳的磁控溅射BFO薄膜的工艺流程。本论文系统研究了B位Ti掺杂对BFO陶瓷和薄膜性能的影响。随着Ti掺量的增加，BFO陶瓷的晶体结构逐渐向斜方变化，晶粒尺寸越来越小，铁电-顺电相变温度（T_c）逐渐降低。掺Ti后BFO材料的漏电流减小，剩余极化值增大。Ti掺杂后，BFO薄膜的微观形貌、漏电流、剩余极化和疲劳性都得到改善。Ti掺杂量为10％时薄膜样品的电性能最好，在200kV／cm电场下，漏电流密度仅为7．6×10^-8A／cm²；在500kV／cm的电场下，剩余极化2P_r为21．02μc／cm²；经过3×10⁹次极化反转后，剩余极化值仍然在初始值的90％以上。本论文首次系统研究了A位Dy掺杂对BFO陶瓷和薄膜性能的影响。当Dy含量为20％时，BFO陶瓷的晶体结构明显变化。随着Dy掺量增加，晶粒尺寸越来越小，铁电-顺电相变温度（T_c）逐渐降低。Dy掺量为10％时陶瓷的漏电流最小，剩余极化值最大。Dy掺杂量为10％时，薄膜样品的电性能最好，在200kV／cm电场下，漏电流密度仅为3．6×10^-8A／cm²，；在300kV／cm的电场下，剩余极化2P_r为13．33μc／cm²。结合前面的研究，本论文还对BFO陶瓷进行A、B位同时掺杂，研究了A、B位共掺的BFO陶瓷的性能。结果显示，A、B位共掺抑制了不纯相的生成，进一步提高了BFO陶瓷的铁电性能。最佳的掺杂方式和掺量为A位Dy和B位Ti同时掺杂，掺量均为10％。另外，本论文还通过射频磁控溅射，将A位掺杂BFO薄膜和B位掺杂BFO薄膜组成双层膜，研究了该双层膜的电性能，并对结果进行了理论分析。最后，本论文还提出了一个具有退极化场的铁电双层膜模型。给出了有退极化场和无退极化场时极化的空间分布和电滞回线的图象，研究了退极化场和界面耦合对二级相变铁电双层膜极化的影响。结果表明，铁电耦合时，退极化的作用使各层的极化趋于更均匀，界面耦合对极化的空间分布和电滞回线的形状都有很大的影响；反铁电耦合时，随着反铁电耦合的增强层内的极化增大，但退极化场导致层内极化的降低。当退极化场相对于层间耦合较强时，反铁电耦合的双层膜的层内极化取向一致。界面耦合和退极化场之间存在竞争关系。总而言之，本论文研究的主要结果和创新如下：1、获得了制备BFO陶瓷的固相烧结较佳工艺条件。采用快速升温、多次烘烤、单独排胶和带压烧结等方法制备出了纯相、致密、平整的BFO陶瓷。得到了最佳掺杂方式和掺杂量：A位Dy与B位Ti共掺，掺量各为10％。2、采用射频溅射工艺制备了BFO系列铁电单层和双层薄膜，探索了A位Dy掺杂、B位Ti掺杂薄膜的优化工艺，制备出结构均匀、铁电性较强及漏电流密度小的BFO薄膜。3、提出了具有退极化场的铁电双层膜模型，得到了退极化场的表达式。研究发现退极化场和界面耦合对二级相变铁电双层膜自发极化的空间分布和电滞回线有重要影响，铁电耦合时退极化场的作用使各层的极化趋于均匀，反铁电耦合时退极化场导致层内极化降低。更多还原

【Abstract】 Mutiferroics have been widely investigated recently. Among them, BiFeO₃ （BFO）shows coexistent ferroelectric order and magnetic order and considered to have importantpotential application in information storage, transducer, modulators, spin electronics anddriver system.In this thesis, pure BFO ceramics were prepared by optimized solid-state reactiontechnology. The effects of the dilute nitric acid percolate, sintering temperature andsintering time were addressed, based on the phase diagram of Bi₂O₃-Fe₂O₃ binary systemand TG-DTA analysis. Otherwise, we fabricated BFO thin films on Pt（111）/Ti/SiO₂/Si（001）substrates by RF-magnetron sputtering. And the effects of annealing conditions on thestructure of films were investigated.The doping effects of B-site titanium substitution of BFO series ceramics wereinvestigated. With the increase of Ti content, the average grain sizes of the BFO ceramicsand the ferroelectric transition temperature decreased. The leakage current density and theremnant polarization of the BFO ceramics are improved due to the decreased oxygenvacancies by Ti⁴⁺ doping. The doping effects of B-site titanium substitution of BFO seriesfilms were also investigated. With the increase of Ti content, the ferroelectric transitiontemperature decreased. The leakage current density and the remnant polarization of theBFO ceramics are improved. 10% TiO₂ excess shows the best leakage current density andremnant polarization.Based on it, the doping effects of A-site dysprosium substitution of BFO series filmswere also investigated. Best electric properties are obtained for the sample when thecontent of Dy is 10%, with the leakage current density being 3.6×10^-8A/cm² at200kV/cm, remnant polarization 2P_r being 13.33μc/cm² at 300kV/cm. Best electricproperties are obtained for the sample when the content of Dy is 10%. A site, Dy³⁺ and B site, Ti⁴⁺ co-doped multiferroic Bi_1-xDy_xFe_0.9Ti_0.1O₃ ceramics were successfullyprepared. The effect of ion doping on the microstructure, morphology, and the electric properties wasinvestigated. The result suggested that the co-doped BFO ceramic by Dy³⁺ and Ti⁴⁺ at A and B sitesshowed advantages in application over the pure BFO, doped BDFO and BFTO ceramics, respectively.The BFO ceramic with 10% Dy₂O₃ and TiO₂ excess shows the best ferroelectric properties.A ferroelectric bilayers model with considering depolarization field is proposed. The effect ofdepolarization field and interracial coupling on the polarization of second-order ferroelectric bilayersbased on the model investigated. The spatial profiles of spontaneous polarization and hysteresis loopswith and without depolarization field are obtained. Our results show that interracial coupling has greatinfluence on spatial profiles of spontaneous polarization and the shape of the hysteresis loop. Inaddition, the influence of the depolarization field is to make spontaneous polarization profile morehomogeneous in ferroelectric bilayers or superlattices and reduce the polarization in antiferroelectricbilayers or superlattices.The innovations of the thesis are as follows:1. Obtain the optimized solid-state reaction technology on preparation of pure ordoped BFO ceramics.2. Put forward the opitimized fabrication conditions to get BFO thin films withexcellent properties by rf-sputtering.3. propose a ferroelectric bilayers model with considering depolarization field. The effect ofdepolarization field and interracial coupling on the polarization of second-order ferroelectric bilayersbased on the model investigated.更多还原