过渡金属氧化物中多铁性及非常规铁电性的研究

发布时间：2018-06-22 21:48

本文选题：第二类多铁 + 磁电耦合　；参考：《南京大学》2017年博士论文

【摘要】：近十几年来,人们对于多功能小型器件的需求越来越强烈,磁电耦合材料因其在信息存储、磁电传感器等方面的潜在应用受到了广泛的关注。特别是多铁性材料中的第二类多铁,其独特的物理机制、较强的磁电耦合特性更是引起了人们浓厚的兴趣。然而,这类材料普遍存在铁电极化小和相变温度低的缺点,给应用带来了不小的阻碍。本论文以过渡金属氧化物中多铁性材料的性能提升和未来应用为驱动,一方面通过现代薄膜外延技术尝试对其性能进行改良和优化,另一方面深入探索并理解这类体系中的物理内涵。因此,本论文的安排如下:第一章对多铁性材料进行了概述,重点介绍了几种第二类多铁和非常规铁电机制及其研究现状,最后讨论了薄膜技术在多铁性材料中的应用。第二章主要讨论薄膜样品的制备方法、结构表征手段以及各种磁学电学物性的测量方法。在薄膜制备方面,主要介绍本论文所采用的脉冲激光沉积法。在结构表征方面,主要介绍X射线衍射技术的各种扫描模式和倒易空间图(RSM)相关的制作方法,以及电子显微镜(SEM和TEM)技术的应用。在磁电物性表征方面,简要介绍了本论文所采用的低温环境系统PPMS和MPMS,主要讨论薄膜电极的制作以及PUND和热释电这两种电学性质测量方法。第三章系统地研究了外延薄膜技术对第二类多铁性材料GdMn03的结构特征和多铁性质的调控。在这个工作中,我们利用PLD方法在(001)方向的SrTi03衬底上制备了高质量的(001)取向的GdMn03外延薄膜。详细的高分辨TEM分析发现其面内存在自组装的纳米尺度孪晶畴结构,它可以最大程度地释放薄膜面内较大的晶格失配。在磁和铁电性质的测量中,我们发现它展现出了非常独特的多铁行为,包括较高的铁电相变温度(Tc～75 K)、较大的铁电极化(Pa～4900 μC/m2)和相对较强的弱铁磁性(～105 K)。我们认为GdMnO3薄膜中自发铁电极化强度的增强来源于ab面内自旋螺旋序。另外,薄膜内部存在的自组装纳米尺度孪晶畴结构是导致其高温铁电相和弱铁磁性出现的主要诱因。第四章细致地研究了典型的第二类多铁性材料GdMn205多晶中室温铁电相对低温磁致铁电性的影响,并在热释电测量过程中提出了一种特殊的极化样品的方法。实验结果表明,足够强的室温铁电极化会诱发Mn-Mn短程磁关联——局域自旋序,它会对低温磁致铁电性产生重要的影响。在不同的极化模式下,诱发出的局域自旋序会改变Mn3+-Mn4+-Mn3+对称交换伸缩作用,使得与之相关的极化PMM最终与Gd3+-Mn4+-Gd3+对称交换伸缩诱导出的极化PGM平行或者反平行,从而带来低温部分铁电极化P=PGM ± PMM的不同行为。第五章研究了混合型非常规铁电体Ca3Ti207/SrTi03外延薄膜的结构特征及铁电性质,并发现了该体系中存在的超低矫顽场。混合型非常规铁电体中的铁电极化来源于两种或两种以上非铁电结构的集体扭曲,比如在Ca3Ti207家族中Ti-O八面体的旋转和倾斜。我们利用脉冲激光沉积的方法在(110)方向的SrTi03衬底上制备了高质量的(010)取向的Ca3Ti2O7外延薄膜,它在界面处的外延关系为[001]CTO//[001]STO和[100]CTO//[-110]STO,表现出了良好的外延性质。薄膜的面内铁电回线表明即便在低温下,它也有着～5 kV/cm的超低矫顽场,这几乎比单晶块体中的数值低了两个数量级。我们利用断面高分辨TEM(包括SAED)技术以及第一性原理计算等方法对Ca3Ti2O7薄膜内部的晶格畸变、晶格缺陷等因素进行了观测与分析,并最终认为外延薄膜与块体之间的巨大差异极有可能来源于薄膜中的晶格缺陷而不是衬底应变,表明了 Ca3Ti2O7的铁电性质对晶格缺陷的高敏感性。第六章是对本文的总结和展望。
[Abstract]:In the past decade, the demand for multi-functional small devices is becoming more and more intense. The potential applications of magnetoelectric coupling materials have attracted wide attention because of their potential applications in information storage and magnetoelectric sensors. Especially, the second kinds of multi iron in the multi ferroelectric materials have been caused by their unique physical machine system and strong magnetic and electrical coupling characteristics. However, this kind of material has the disadvantage of low ferroelectric polarization and low phase transition temperature, which has brought no small obstacles to the application. This paper is driven by the performance promotion and future application of the transition metal oxide. On the one hand, we try to improve and optimize its performance through the modern film epitaxy technology. On the one hand, the physical connotation of this kind of system is deeply explored and understood. Therefore, the arrangement of this paper is as follows: the first chapter is an overview of the multi iron material, focusing on several kinds of second kinds of multi iron and unconventional iron motor system and its research status. Finally, the application of thin film technology in the multi iron material is discussed. The second chapter is the main discussion. The preparation methods of membrane samples, the means of structural characterization, and the measurement of various magnetic properties of electrical properties. In the preparation of the film, the pulse laser deposition method used in this paper is mainly introduced. In the aspect of the structure characterization, the various scanning modes of X ray diffraction technology and the fabrication methods related to the inverted space map (RSM), and the electron are mainly introduced. The application of microscopes (SEM and TEM) technology. In the characterization of magnetoelectric properties, the low temperature environment system PPMS and MPMS used in this paper are briefly introduced. The fabrication of thin film electrodes and the two methods of measurement of PUND and pyroelectric properties are mainly discussed. The third chapter systematically studies the epitaxial film technology for the second kinds of multiferroic materials GdMn03 In this work, we use the PLD method to prepare a high quality (001) oriented GdMn03 epitaxial film on the (001) direction of the SrTi03 substrate. The detailed high resolution TEM analysis shows that the surface of its surface is in the self-assembled nanoscale twin domain structure, and it can release the film surface to the maximum. Large lattice mismatch. In the measurement of magnetic and ferroelectric properties, we find that it shows a very unique multi iron behavior, including high ferroelectric phase transition temperature (Tc ~ 75 K), large ferroelectric polarization (Pa to 4900 C/m2) and relatively strong ferromagnetism (~ 105 K). We think the enhanced source of the spontaneous iron electrode in GdMnO3 thin films In addition, the self assembled nanoscale twin domain structure in the film is the main cause of the high temperature ferroelectric phase and weak ferromagnetism. In the fourth chapter, the influence of the ferroelectric relative low temperature ferroelectric properties at room temperature in the second typical Polyferric material is carefully studied in the fourth chapter, and the pyroelectric measurement is made in the pyroelectric measurement. A special method of polarization samples is proposed in the process of quantity. The experimental results show that strong enough room temperature and iron electrode can induce Mn-Mn short range magnetic Association - local spin order, which will have an important influence on the ferroelectric properties of low temperature magnetically induced. In different polarization modes, the local spin order induced by different polarization modes will change the symmetry of Mn3+-Mn4+-Mn3+. In the fifth chapter, the structure characteristics and ferroelectric properties of the mixed type unconventional ferroelectric Ca3Ti207/ SrTi03 epitaxial film are studied in the fifth chapter. The ultra low coercive field in the system is found. The ferroelectric polarization in the hybrid unconventional ferroelectric is derived from the collective distortion of two or more than two kinds of non ferroelectric structures, such as the rotation and tilt of the Ti-O eight body in the Ca3Ti207 family. We have prepared high quality on the (110) SrTi03 substrate by the method of pulsed laser deposition. The (010) oriented epitaxial Ca3Ti2O7 epitaxial film, whose epitaxial relationship at the interface is [001]CTO//[001]STO and [100]CTO//[-110]STO, shows good epitaxy properties. The ferroelectric wire in the surface of the film shows that even at low temperature, it also has a 5 kV/cm ultra-low coercive field, which is almost two orders of magnitude lower than that in the single crystal block. We have observed and analyzed the lattice distortion and lattice defects in the Ca3Ti2O7 thin film by using the high resolution TEM (including SAED) and the first principle calculation, and finally believed that the huge difference between the epitaxial film and the bulk is likely to come from the lattice defects in the thin film rather than the substrate strain. It is clear that the ferroelectric properties of Ca3Ti2O7 are Gao Min sensitive to lattice defects. The sixth chapter is the summary and Prospect of this paper.
【学位授予单位】：南京大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O484

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