含Bi层状钙钛矿型多铁材料中畴开关与磁电耦合机制的原位透射电镜研究

Magnetoelectric multiferroic materials and their devices are one of the highest priority research topics that have been investigated by the scientific ferroics community to develop the next generation of novel multifunctional materials over the past decade. These systems show the simultaneous existence of two or more ferroic orders, and cross-coupling between them. Based on the type of ordering and coupling, they have drawn increasing interest for a variety of device applications. Since single-phase materials exist rarely in nature with strong cross-coupling properties, intensive research activity is being pursued towards the discovery of new single-phase multiferroic materials and the design of new engineered materials with strong magneto-electric coupling. One of the most effective ways to obtain excellent multiferroic properties in single-phase matters is to incorporate magnetic species of perovskite like LaFeO3 into ferroelectric Bi4Ti3O12 matrix of layered perovskite, and thus multiferroic Bi4Ti3O12-nLaFeO3 (n = 0.5, 1.0, 1.5 and 2.0) materials are obtained. However, their crystal symmetry, domain structure and magneto-electric coupling mechanism are still unclear. In our previous study, both domain structures and domain walls mobility in Bi4Ti3O12-based ferroelectrics have been studied using transmission electron microscopy (TEM). It is clearly explained that why polarization fatigue is easy happen in Bi4Ti3O12 whereas rare-earth-element doped Bi4Ti3O12 is fatigue-free. In this project, we plane to study the crystal symmetry, the site of magnetic species and domain structure of multiferroic Bi4Ti3O12-nLaFeO3 single crystals using TEM, in-situ TEM study of ferroelectric domain switching under applied electric/magnetic filed. The magneto-electric coupling mechanism in Bi-layered perovskite will be understood by combining the TEM observations and their physical properties. This study will give significant guidance to the design of single-phase multiferroic materials which exhibit coexistence of ferroelectric and ferromagnetic order and with strong magneto-electric coupling at room temperature.

探寻在室温同时具有铁电和铁磁性且磁电耦合强的单相多铁材料是当今研究热点之一。在层状钙钛矿型Bi4Ti3O12铁电体中插入LaFeO3等磁性基团是获得单相多铁材料的有效途径，但这类多铁材料的晶体结构、电畴结构及磁电耦合的微观物理机制尚不明确。我们曾用透射电镜比较研究Bi4Ti3O12基铁电体的畴结构和畴壁可动性，揭示了Bi4Ti3O12中的畴开关易疲劳，而稀土掺杂Bi4Ti3O12耐疲劳的物理机制。本项申请拟用透射电镜研究Bi4Ti3O12-nLaFeO3 (n = 0.5、1.0、1.5和2.0)多铁单晶的晶体结构、磁性基团占位、电畴结构；接着在透射电镜中原位观察在外电场/磁场作用下电畴开关的动力学过程；最后，结合该材料体系的宏观磁电耦合特性，揭示磁电耦合的微观物理机制。为设计在室温同时具有优良铁电、铁磁和磁电耦合特性的单相多铁材料提供科学指导。

探寻在室温具有优异铁磁特性的单相多铁材料是当前研究热点。本项目用透射电镜研究了Bi4Ti3O12-nLaFeO3(n=0.5,1.0,1.5)多铁体的晶体结构、畴结构及畴开关行为，揭示了Bi4Ti3O12-nLaFeO3具有优异铁电特性的微观机制，为设计室温多铁材料提供了新思路。.用热压方法制备了Bi4Ti3O12-nLaFeO3室温多铁陶瓷。用X射线和电子衍射确定Bi4Ti3O12-nLaFeO3在室温下为正交结构，属于A21am空间群，表明它们的自发极化Ps沿着a轴。.用群论分析得出，Bi4Ti3O12-LaFeO3中结构对称性允许存在的畴结构类型共5种，它们是自发极化90°畴，180°畴，反相畴，反相畴与90°畴和180°畴的耦合。透射电镜研究表明，在Bi4Ti3O12-LaFeO3中观察到90°和180°畴，未观察到反相畴。90°畴厚度不均匀，畴壁无规则弯曲；180°畴呈带状，畴壁平直，大部分畴壁躺在(001)面。.用透射电镜原位研究了Bi4Ti3O12-LaFeO3中畴的极化翻转(开关)行为。结果表明，Bi4Ti3O12-LaFeO3中90°和180°畴在外电场下都能翻转。90°畴的翻转过程是通过新畴首先在原有90°畴壁、晶界和位错处成核，紧接着纵向生长和横向扩张。发现纵向生长速度明显比横向扩张快，前者速度近似是后者的5倍。180°畴的翻转则是通过原有180°畴的膨胀和收缩来完成，180°畴的翻转速度明显比90°畴的快。.对比研究了Bi5Ti3FeO15陶瓷的多铁性能和畴结构。Bi4Ti3O12-LaFeO3和Bi5Ti3FeO15的区别在于前者A位有部分La离子，而后者A位均为易挥发的Bi离子。从宏观上看，Bi4Ti3O12-LaFeO3和Bi5Ti3FeO15陶瓷的矫顽场相近，但前者的铁电性能明显优于后者；从微观上看，两者的畴结构类型和构型相似，但前者中电畴易开关而后者中难开关。考虑铁电氧化物中会存在氧空位缺陷，对畴壁有钉扎作用，可导致畴的翻转受阻；由于铁电体宏观上的剩余极化值正比于其中可翻转畴的体积，Bi4Ti3O12-LaFeO3中90°和180°畴在外电场下可高效翻转，说明其氧空位浓度低，畴壁钉扎效应弱，宏观上表现出剩余极化值大。根据本项目研究，可以理解Bi4Ti3O12-LaFeO3中A位有部分La离子，是其有优异铁电性能的微观起因。