摘要
单相磁电多铁性材料同时具有磁性和铁电性,且二者可以通过磁电耦合效应相互影响,所以其在信息存储等诸多领域有着巨大的应用前景。因此,近十年来多铁性材料已经成为了凝聚态物理和材料物理等学科的研究热点,并且在研究过程中涌现了许多性能优异的多铁性材料。例如,铁酸铋就是目前已发现的多铁性材料中的佼佼者,它具有室温以上的反铁磁和铁电转变温度,室温环境下具有非常大剩余电极化强度。然而,铁酸铋的G型反铁磁磁基态使其受到磁场的调控很弱。实际上,目前发现的单相多铁性材料大多是反铁磁性的,同时具有铁磁性和铁电性的材料非常稀少。幸运的是,实验研究发现纳米化可以在保持结构不变的基础上增强反铁磁材料的磁性。所以把纳米技术引入到多铁性材料的研究中来,一方面可以增强很多反铁磁性的多铁性材料的磁性,进而获得更大的磁电耦合效应；另一方面,可能会出现一些新的物理现象,可以让人们对磁电耦合效应的理解和调控进入一个全新的层次。不过,多铁性材料多为多元的复杂氧化物,高质量的纳米晶体生长比较难以调控,相关的物性研究更是还处于初级阶段。
针对上述问题,本论文主要研究了水热法生长高质量的铁酸铋纳米晶体和具有多铁性的新型纳米材料,系统研究了它们在纳米化后的磁学特性,比如自旋团簇玻璃态的动力学行为和交换偏置效应。此外,论文最后还对非典型磁电材料的磁介电特性进行了深入的研究。论文内容共分为五章,每章的主要内容分别概括如下：
第一章综述了单相磁电多铁性材料的研究进展,其中重点介绍了铁酸铋的基本特性和相关研究,然后介绍了多铁性材料中的磁介电效应和与之相关的各个材料体系,最后介绍了多铁性纳米材料的相关研究,并根据以上几个方面给出了当前研究中存在的一些问题。
第二章提出了一种水热法合成铁酸铋纳米晶粒的途径,首次系统研究了铁酸铋纳米晶粒的自旋团簇玻璃态的动力学行为。相比于传统的固相反应法,水热法可以大大降低铁酸铋的成相温度,得到高质量、纯相的铁酸铋纳米晶粒。通过多种表征手段得到纳米晶粒的平均直径约为170nm,反铁磁转变温度为640K,铁电转变温度为1064K。通过测量BiFeO3内米晶粒在不同交流磁场频率和不同直流磁场大小下的交流磁化率随温度依赖关系,研究其磁动力学行为。利用不同的动力学方程对结果进行拟合,确认了铁酸铋纳米晶粒在低温下存在一个自旋团簇玻璃转变,这一转变的出现很可能与铁酸铋在纳米化后出现的大量未补偿自旋有关。
第三章系统研究了铁酸铋纳米晶粒的交换偏置效应。通过研究发现,铁酸铋纳米晶粒在2K到300K的温度范围内都存在交换偏置效应,且交换偏置场大小和温度之间具有非单调的依赖关系。其中,铁酸铋纳米晶粒在其自旋团簇玻璃转变温度以上的交换偏置效应非常特别,因为在其他自旋玻璃态的磁性纳米体系中的交换偏置效应会在此温度以上消失。针对这个问题,我们分别用随机场模型和2D-DAFF模型(two-dimensional diluted antiferromagnet in a field model)予以解释,并且同时给出了铁酸铋纳米晶粒中存在2D-DAFF的实验证据。
第四章利用水热法合成了一种新型室温多铁性材料Bi5Fe2O10o.5单晶纳米带。通过结构分析,发现Bi5Fe2O10.5内米带同构于高温超导体Bi2Sr2CaCu208+δ,并存在一个波矢为q*=(0,0.25,1)的公度的调制结构。因此,Bi5Fe2O10.5内米带在c轴方向上由结构上类似BiFeO3的钙钛矿层和绝缘性好的[Bi2O2]2+盐岩层交替排列而成,具有天然的磁电一介电超晶格结构。最重要的是,室温下在Bi5Fe2O10.5纳米带中发现了同时存在的磁性和铁电性,而巨大的磁介电效应表明其内部可能存在着磁有序和铁电有序的相互耦合。Bi5Fe2O10.5纳米带的设计与水热合成为探索新型的单相多铁性材料提供有益的帮助。
第五章系统研究了LaMn1-xFexO3(0.1≤x≤0.5)体系在外磁场和外电场作用下的介电响应。在x≤0.2的样品里观察到了显著的低场磁介电效应,并且随着Fe含量的增加,磁介电效应消失。同时,所有的样品都表现出了巨介电常数,且介电常数随外加偏置电场增大而减小。我们通过研究发现上述现象是由Maxwell-Wagner效应引起的。值得注意的是,在x=0.2的样品高频测量结果里发现了本征的磁介电效应,暗示了这一体系内可能存在内禀的磁电耦合效应。
Single-phase magnetoelectric multiferroic materials with broad application prospects in many fields, such as information storage and so on, have magnetism and ferroelectricity simultaneously, and these two orders can affect each other via magnetoelectric effect. Hence, multiferroic materials have become a hotspot for condensed-matter physics and materials physics during the past ten years, and many high-performanced multiferroics have been found in this upsurge. For example, BiFeO3is one of the best multiferroics, and its magnetic and ferroelectric transition temperatures are both above room temperature. Moreover, BiFeO3has a large remanent polarization at room temperature. But the G-type antiferromagnetism ground state makes it difficult for the magnetic control of the multiferroic in single phase bulk BiFeO3. In fact, most of the multiferroics are antiferromagnetic, and ferromagnetic ones are very rare so far. Fortunately, nanorization can be an effective way to enhance the magnetism in an antiferromagnetic material without a crystal structure alteration. Therefore, it will be useful for introducing nanotechnology to the multiferroic research. On one hand, it can enhance magnetism of the antiferromangetic multiferroics. On the other hand, nanotechnology could bring new physics, and allow people reach a whole new level for understanding and manipulating the mangetoeletric coupling effect. However, multiferroic materials are mostly multi-element complex oxides, so the high-quality nanocrystals are hard to controllably grow, and the correlative propery study is still in the initial stage.
This dissertation focuses on the above challenges:the hydro thermal methods were used to grow high-quality BiFeO3nanocrystals and new type multiferroic nanobelts. The magnetic properties of them were systematically investigated, such as the dynamic properties of spin cluster glass and exchange bias effects. In addition, the magnetodielectric effect in an atypical magnetoelectric material was studied in-depth. The detailed experimental results and discussions are shown as follows:
In chapter1, we introduced the history and research progresses of multiferroic materials. The main contents include:the basic property study of BiFeO3, magnetodielectric effects and the related multiferroic systems, the research progresses of multiferroic nanomaterials. According to the general analyses, several open questions have been proposed at the end.
In chapter2, a hydrothermal method was developed for the fabrication of BiFeO3nanocrystals, and the dynamic properties of spin cluster glass in nanosized BiFeO3sample were systematically studied for the first time. Compared to conventional solid state method, the hydrothermal method can effectively reduce the formation temperature of BiFeO3, through which high-quality BiFeO3nanocrystals were produced. The BiFeO3nanocrystals were characterized with an average diameter of170nm, a Neel temperature of640K and a ferroelectric Curie temperature of1064K. We studied the dynamic properties of spin-glass-like state in BiFeO3nanocrystals by measuring the temperature dependence of ac susceptibility under different ac magnetic field frequencies or under different dc magnetic field values. By fitting the results with different dynamic laws, the spin cluster glass transition was confirmed, and it may be originated from a lot of the new uncompensated spins after the nanorization.
In chapter3, exchange bias effect in BiFeO3nanocrystals was fully investigated. It was found that exchange bias exist in BiFeO3nanocrystals for the whole temperature range we measured (from2K to300K), and exchange bias field changed non-monotonically with increasing temperature. The exchange bias above the spin cluster glass transition temperature is very special, as the exchange bias would vanish for other systems in the same condition. To explain this new phenomenon, a Malozemoff's random-field model and a2D-DAFF model were established, respectively. The evidence for the existence of2D-DAFF in the BiFeO3nanocrystals was also given.
In chapter4, a simple hydrothermal method was developed for the synthesis of single-crystal Bi5Fe2O10.5nanobelts. The products were characterized to be a layered structure with a commensurate modulation wave vector q*=(0,0.25,1) and isostructural with the high-temperature superconductor Bi2Sr2CaCu2O8+δ. The regular stacking of the BiFeO3-like perovskite blocks and the rock salt [Bi2O2]2+slabs along the c axis of the crystal makes the Bi5Fe2O10.5nanobelts have a natural magnetoelectric-dielectric superlattice structure. The most important result was the room-temperature multiferroic behavior in this new compound, and the giant magnetodielectric effect indicated the magnetoelectric coupling interactions. This finding may be useful for developing single phase multiferroics.
In chapter5, the dielectric properties of LaMn1-xFexO3(0.1≤x≤0.5) system under magnetic fields and electrical biases were studied. A low-field magnetodielectric effect is observed for x≤0.2samples, but it is suppressed by further Fe substitution. Besides the giant magnetodielectric effect, the dielectric constants can also be tuned by the dc electrical bias. These results have been explained by a contribution of the Maxwell-Wagner effect. It is notable that an intrinsic magnetodielectric effect in LaMno.8Fe0.2O3was observed, indicating the existence of a magnetoelectric coupling effect.
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