节点文献

多铁性材料外场调控介电性质研究

Study on the Dielectric Tunability with External Fields in Multiferroic Materials

【作者】 侯莹

【导师】 李晓光;

【作者基本信息】 中国科学技术大学 , 凝聚态物理, 2011, 博士

【摘要】 随着人们对信息存储和信息处理方面的要求逐步提高,存储器件的多功能化是必由之路。因此,对同时具有磁性和铁电性、有望实现磁电相互调控的多铁性材料的研究成为信息存储技术日趋提升的需要,具有重要的科学意义和应用前景。虽然人们已在相关领域开展了一些工作,但是在该体系中还存在一些亟待解决的问题,比如如何实现在室温附近的低场调控,并且很多材料的多铁性起源尚不十分明确。针对上述问题,本论文主要研究了若干多铁性材料的介电特性及其外场调控性质,探索了获得低场室温的介电可调性的途径,初步讨论了介电调控的起源及其与结构、磁、铁电和多铁性的关系。论文内容共分为六章,每章的主要内容分别概括如下:第一章主要综述了多铁性材料的研究进展,对每一类多铁材料的磁及铁电起源进行了概述,并详细介绍了“电子铁电体”LuFe2O4的结构和物理性能,讨论了电荷有序态对其磁电调控的重要意义。第二章我们研究了LuFe2O4体系的结构及光谱学特性,以及Mn掺杂的LuFe2O4体系的介电及多铁性质。发现在LuFe2O4的亚铁磁和铁电转变温度附近,均观察到了晶胞参数、Fe-Fe层间间距的异常变化。与此同时,拉曼和红外振动模的峰值频率随着温度的变化发生了相应的移动,并在磁和铁电转变处也出现了台阶状的突变。该结果证明了该体系中存在结构和磁性、铁电性之间的内在关联。另一方面,对LuFe2-xMnxO4 (0≤x≤1)体系的磁性、介电及其磁/电场调控的研究表明,Mn掺杂抑制了体系中Fe3+和Fe2+离子之间的电荷涨落和电子跃迁,使磁有序温度、介电常数和损耗降低。并且在这些样品中观测到了室温下的低磁/电场可调的介电特性。第三章研究了石榴石型铁氧体Lu3Fe5O12的多晶和薄膜样品的基本性能。发现不同退火条件对该Lu3Fe5O12多晶样品中氧含量有着重要的影响,进而引起磁性及介电性质的变化。结果表明,经过氧气退火处理的样品具有最低的介电损耗值。在此基础上,利用磁控射频溅射方法对Lu3Fe5O12外延薄膜样品的制备工艺进行了初步的探索,发现不同的沉积温度、气氛、退火条件等对该系列薄膜的磁性有着及其重要的影响。第四章介绍了可能的电荷有序多铁材料La1-xCaxMnO3电场调控的介电特性。通过研究不同组分(0.6≤x≤0.8)的La1-xCaxMnO3介电及阻抗特性,发现该体系中存在很大的介电调制效应,10 V/cm的电场下就有80%以上的介电常数变化,并且发现其调制效应与Maxwell-Wagner非本征效应密切相关,同时电荷有序态的稳定性也决定着介电调制的大小和可调频率范围。第五章研究了(1-x) La2NiMnO6-(x) La2/3Sr1/3MnO3复合体系的磁介电效应。发现该体系中存在着较大的室温磁介电效应,并且其磁介电数值随着x非单调变化,在x = 0.4时最大,该组分样品在1 kOe的磁场下可以获得20%的室温磁介电变化。该体系在较宽的温度和频率范围内都能观察到较强的磁介电效应。第六章介绍了光学浮熔区法生长Y/DyMnO3多铁性单晶的制备工艺、结构表征及其基本物性测量。

【Abstract】 Recently, with the development of the information storage industry, it is necessary to achieve multi-functionality in one device. Therefore, the mutual manipulation of the magnetic and electric properties in multiferroics, which shows both magnetic and ferroelectric properties, is of great interest. Although many efforts have been made in this system, there are lots of problems demanding prompt solution, such as how to achieve the room-temperature magnetic/electric tunability with low external fields. Moreover, the mechanism of magnetic and electric coupling in most of multiferroics is not so clear. So the further study on multiferroics becomes an important issue both for fundamental physics and technical applications.In this dissertation, the dielectric tunability with external fields is studied in some multiferroic materials. The possible routes to achieve the room-temperature tunability with low fields are proposed. In addition, the relationship among the structural, magnetic, ferroelectric properties and the origin of the tunability is discussed.In chapter one, the research history and relevant mechanisms of multiferroic materials are concisely reviewed. The multiferroic properties of“electronic ferroelectric”LuFe2O4 are particularly introduced, and the previous results show lots of interesting properties closely related to the charge ordering in this system.In chapter two, the phonon spectra and structural characteristics of the LuFe2O4 system are investigated using X-ray diffraction as well as Raman and infrared (IR) spectroscopic techniques. Two step-like anomalies of the wavenumbers of the peak in Raman and peak in IR spectra, as well as some weak anomalies of the lattice parameters, are found around the ferrimagnetic and ferroelectric transition temperatures. These results illustrate that the structural fluctuations could effectively influence the phonon modes, and a possible interplay among the structural, magnetic, and charge-ordering properties exists in this multiferroic system. On the other hand, the dielectric properties and tunability with external magnetic and electric fields for LuFe2-xMnxO4 (0≤x≤1) are systematically studied. It is found that the dielectric loss, the ferrimagnetic Curie temperature, as well as the conductivity reduces with increasing Mn doping. One of the most important results is that the room-temperature dielectric tunability with low magnetic and electric fields can be achieved in these samples. The analysis demonstrates that the electron transfer between Fe2+ and Fe3+is efficiently suppressed with Mn doping and thus results in the decreases of the leaky conductivity as well as the dielectric loss.In chapter three, the properties of garnet Lu3Fe5O12 bulks and films are investigated. The dielectric loss in bulks changes with different annealing processes is reduced effectively with decreasing oxygen deficiencies through annealing in O2. Furthermore, the epitaxial Lu3Fe5O12 films are grown on (111) Y3Al5O12 substrates by rf magnetron sputtering technique and subsequently annealed in O2 atmosphere. The magnetization loops and the ESR results provide that the magnetic properties can be tuned by atmospheres and temperatures during sputtering and annealing processes.In chapter four, the dielectric tunability with external electric fields in charge ordering La1-xCaxMnO3 is studied. The tunability is up to 80% under a bias field as low as the order of 10 V/cm. Through the dielectric and impedance analyses, the tunability is believed to be related to the Maxwell-Wagner effect associated with Schottky barriers and grain boundary. Moreover, the tunability strongly depends on the stability of the charge-ordered state of the compounds.In chapter five, the magnetodielectric effect in (1-x) La2NiMnO6-(x) La2/3Sr1/3MnO3 composites is studied. A large dielectric response with magnetic fields is found in wide temperature and frequency ranges. The results indicate that variation of dielectric constants with magnetic fields depends non-monotonously on the molar ratio of two compounds and reaches a maximum up to 20% for x = 0.4 at the frequency of 1 MHz and field of 1 kOe at room temperature.In chapter six, the crystal growth process for multiferroic Y/DyMnO3 by optical floating-zone single crystal furnace is introduced. The structural characteristics show the quality of these single crystals. The magnetic and diaelectric properties were also investigated briefly.

节点文献中: 

本文链接的文献网络图示:

本文的引文网络