高温单相多铁性材料与强磁电耦合效应

高温多铁性与强磁电耦合效应是多铁性材料研究领域的两个核心内容。文章首先介绍了已知单相多铁性材料的分类,然后简要综述了一些具有高温多铁性与强磁电耦合效应的代表性材料体系,包括螺旋磁结构六角铁氧体、电荷有序化合物、金属—有机骨架材料等。     

伸缩-剪切模式自偏置铌酸锂基复合材料的磁电性能和高频谐振响应

选用多种切型铌酸锂(LiNbO_3)单晶,研究了铁基非晶合金(Metglas)/LiNbO_3叠层复合材料基于伸缩-剪切模式的磁电耦合性能,揭示了铌酸锂单晶压电系数与复合材料剪切磁电耦合系数的对应关系,在使用铌酸锂xzt/30~?切型时得到了最优化剪切磁电系数.通过SrFe12O19薄磁带提供偏置磁场,Metglas/LiNbO_3磁电复合材料可在没有外加直流磁场时实现剪切磁电响应,并在0.991 MHz和3.51 MHz频率时分别测出了谐振磁电系数,有望将铌酸锂基剪切磁电复合材料用于高频磁场探测.     

基于磁电耦合效应的基本电路元件和非易失性存储器

磁电耦合效应是指磁场控制电极化或者电场控制磁性的物理现象,它们为开发新型电子器件提供了额外的物理状态自由度,具有巨大的应用潜力.磁电耦合系数作为磁电耦合材料的重要参量,体现了材料磁化和电极化的耦合性能,其随外加物理场的变化可以表现出非线性回滞行为,具备作为非易失存储的物理状态特征.本文讨论了基于磁电耦合效应如何建立起电荷-磁通之间的直接关联,继而实现了第四种基本电路元件并构建了完整的电路元件关系图.在此基础上,研究了多铁性异质结中的非线性磁电耦合效应,并利用其独特的电荷-磁通关联特性,开发了基于磁电耦合系数的电写-磁读型非易失性信息存储、逻辑计算与类神经突触记忆等一系列新型信息功能器件.     

多铁材料HoMnO3中光学吸收和畸变驱动的第一性原理研究

六角钙钛矿结构锰氧化物HoMnO₃磁电效应的研究近年来已成为多铁性材料研究中极其重要的一个方面. 本文基于广义梯度近似下的密度泛函理论, 考虑电子自旋的非共线磁性结构, 计算研究了 d电子在位库仑作用和自旋-轨道耦合作用对HoMnO₃的电子结构、 轨道杂化和能态密度分布的影响. 结果显示, 当考虑在位库仑排斥势U作用时, 强烈的Ho 5d与O(3, 4) 2p以及Mn 3d与O(1, 2) 2p间的轨道杂化是驱动HoMnO₃发生铁电畸变的根源, 此时能隙和能带的分布为解释实验中发现的强烈的光学吸收峰提供了理论依据, 而自旋-轨道耦合使得Mn 3d-O(3, 4) 2p在 ab平面内的轨道交迭略有增强, 平面内部分能带简并消除, HoMnO₃材料呈现典型的间接性能隙绝缘体特征. 关键词： 磁电效应 铁电畸变 态密度 光学吸收     

多铁性:物理、材料及器件专题

正编者按作为凝聚态物理与材料物理的前沿分支之一,多铁性研究脱胎于磁电耦合的研究.固体中磁电耦合的概念最初由居里先生提出,至今已有一百多年.在漫长的历史长河中,磁电耦合领域的研究曾经在冷战时期短暂热闹过一阵,但随后是漫长的冷寂期.日内瓦大学的老先生Hans Schmid在磁电耦合领域坐了半辈子冷板凳,在1994年提出了多铁体(multiferroics)这个概念.九年之后,该领域研究才真正引起广泛关注.2003年以Bi Fe O3薄膜的大铁电极化和     

电荷调控下Ti3C2O2和V2CO2吸附CH4的第一性原理计算

本文采用第一性原理计算首先研究了Ti3C2O2和V2CO2与CH4气体分子之间的相互作用,发现Ti3C2O2和V2CO2对CH4的吸附较弱属于物理吸附,不适宜用作探测CH4。在此基础上研究了电荷调控下CH4气体分子与Ti3C2O2和V2CO2之间的相互作用。结果表明：随着体系电荷态的增加,Ti3C2O2和V2CO2对CH4气体分子的吸附作用逐渐增加变为化学吸附。当体系电荷态大于或等于-2时,CH4气体分子在Ti3C2O2和V2CO2表面可以被有效捕获。撤去电荷后,Ti3C2O2、V2CO2与CH4气体分子之间的吸附恢复至物理吸附,CH4气体分子易脱附。因此,通过调控Ti3C2O2和V2CO2的电荷态,可以简单地实现CH4的捕获与释放。Ti3C2O2和V2CO2有望成为CH4探测或捕获材料。     

Nd0.5Ca0.5Mn0.97Co0.03O3体系的超声研究

系统研究了Nd0.5Ca0.5Mn0.97Co0.03O3单相多晶样品在低温下的电磁性质和超声特性.超声测量结果表明,体系在低温下超声声速出现异常,超声声速从室温开始随着温度的降低逐渐减小,并在220K附近声速达到最小,其相对变化(ΔV/V)超过2%.之后,随着温度的进一步降低,声速急剧硬化,其相对变化达到14.5%.分析表明这是由于体系中电-声子相互作用导致的电荷有序态转变的结果,该电声子耦合来源于Mn3 的JahnTeller效应.同时,该体系在100K左右出现了金属绝缘体转变同时伴随着铁磁相变,分析表明在低温下Co离子与Mn离子之间存在着铁磁交换作用,因此微量的Co掺杂会在反铁磁区域内形成部分铁磁小团簇,当铁磁团簇连通后,体系由反铁磁电荷有序态(AFMCO)向铁磁金属态(FMM)转变.研究结果表明了样品在电荷有序相变温度TCO与金属绝缘体转变温度TMI之间(TMI相似文献   

多阶有序钙钛矿多铁性材料的高压制备与物性

钙钛矿是研究磁电多铁性最重要的材料体系之一.由于高的结构对称性,在以往的立方钙钛矿晶格中尚未发现多铁现象.另外,现有的单相多铁性材料很难兼容大电极化和强磁电耦合,严重制约多铁性材料的潜在应用.本文简单综述了利用高压高温条件制备的两个多阶有序钙钛矿氧化物的磁电多铁性质.在具有立方晶格的多阶钙钛矿LaMn_3Cr_4O_(12)中,观察到自旋诱导的铁电极化,表明该材料是第一个被发现的具有多铁性的立方钙钛矿体系.在另一个多阶有序钙钛矿BiMn_3Cr_4O_(12)中,随温度降低该材料依次经历了I类多铁相和II类多铁相.正因为这两类不同多铁相的同时出现,BiMn_3Cr_4O_(12)同时展示了大的电极化强度和强的磁电耦合效应,并且通过不同的电场调控可实现四重铁电极化态,为开发多功能自旋电子学器件与多态存储提供了先进的材料基础.     

多铁异质结构中逆磁电耦合效应的研究进展

电场调控磁性能够有效降低功耗,在未来低功耗多功能器件等方面具有巨大的潜在应用前景.铁磁/铁电多铁异质结构是实现电场调控磁性的有效途径,其中室温、磁电耦合效应大的应变媒介磁电耦合是最为活跃的研究领域之一.本文简要介绍在以Pb(Mg_(1/3)Nb_(2/3))_(0.7)Ti_(0.3)O_3为铁电材料的多铁异质结构中通过应变媒介磁电耦合效应对磁性、磁化翻转及磁性隧道结调控的研究进展.首先讨论了多铁异质结构中电场对磁性的调控;之后介绍了电场调控磁化翻转的研究进展及理论上实现的途径;然后简述了电场对磁性隧道结调控的相关结果;最后在此基础上,对多铁异质结构中电场调控磁性及磁性器件进行了总结和展望.     

2-2型PMNT/NFO复合磁电效应模拟

利用弹性力学模型,基于铁电相与铁磁相的本构方程,建立磁电复合材料的本构方程,推导2-2型非理想耦合的磁电双层、三层复合薄膜的纵向、横向磁电(ME)电压系数.研究铁磁相材料铁酸镍(NFO)和铁电相材料铌镁酸铅-钛酸铅(PMNT)复合的磁电效应,分析复合材料的磁电电压系数与PMNT体积分数、界面耦合参数、两相体积比及复合总层数的关系.结果表明两相材料的性能、体积分数以及耦合系数、复合层数都影响磁电电压系数.     

钙钛矿超晶格的演生磁电物性

钙钛矿过渡金属氧化物中存在诸多自由度（如晶格、电荷、自旋和轨道）,这些自由度 之间的相互耦合以及相互竞争会诱导出很多奇异物性,如高温超导、庞磁电阻效应、多铁性等, 这些物性在量子器件的发展过程中扮演了重要的角色。在此基础上,若再将不同特性的氧化物材 料耦合在一起形成超晶格,通过界面处的晶格重组与电子重组,体系可呈现出更加丰富的物理和 更多可调控的性能。本综述主要关注钙钛矿超晶格中的磁电物性。首先介绍了超晶格中磁性调控 的几种物理机制,然后对超晶格中的杂化非本征铁电性以及电子铁电性进行了重点讨论,最后围 绕超晶格中的磁电耦合效应进行了讨论和总结。     

激发态电荷转移有机体的多铁性研究

随着人们对多铁性的深入了解,越来越多不同类型的有机多铁材料被合成出来.激发态电荷转移有机体的电荷转移网络是由一个提供电子的分子(给体donor,D~+)和一个接受电子的分子(受体acceptor,A~-)有序排列后构成的.D~+A~-长程有序排列,其激发态(激子)具有较长寿命和±1/2自旋,这是产生室温铁电性和铁磁性的根本原因.激发态容易受外场刺激,因此光照、磁场、电场、应力等能够很好地调控这类材料的铁电极化、磁矩和相应的磁电耦合系数.激发态电荷转移有机体不仅大大丰富了室温多铁材料体系,而且可以为开发新型多功能电子器件提供材料基础和技术储备.     

Advances in ab-initio theory of multiferroics

Within the broad class of multiferroics (compounds showing a coexistence of magnetism and ferroelectricity), we focus on the subclass of ??improper electronic ferroelectrics??, i.e. correlated materials where electronic degrees of freedom (such as spin, charge or orbital) drive ferroelectricity. In particular, in spin-induced ferroelectrics, there is not only a coexistence of the two intriguing magnetic and dipolar orders; rather, there is such an intimate link that one drives the other, suggesting a giant magnetoelectric coupling. Via first-principles approaches based on density functional theory, we review the microscopic mechanisms at the basis of multiferroicity in several compounds, ranging from transition metal oxides to organic multiferroics to organic-inorganic hybrids.     

Electric polarization, magnetoelectric effect, and orbital state of a layered iron oxide with frustrated geometry

A layered iron oxide RFe2O4 (R denotes rare-earth-metal elements) is an exotic dielectric material with charge-order (CO) driven electric polarization and magnetoelectric effect caused by spin-charge coupling. In this paper, a theory of electronic structure and dielectric property in RFe2O4 is presented. Charge frustration in paired-triangular lattices allows a charge imbalance without inversion symmetry. Spin frustration induces reinforcement of this polar CO by a magnetic ordering. We also analyze an orbital model for the Fe ion which does not show a conventional long-range order.     

5d过渡金属氧化物中的奇异量子物性研究

相比于3d和4d过渡金属元素, 5d过渡金属元素既具有很强的自旋轨道耦合相互作用, 同时它们的电子关联作用也不可忽略. 因而5d过渡金属氧化物体系具有许多奇异的量子特性. 这篇综述主要介绍我们在5d过渡金属氧化物中的一些理论进展. 首先介绍烧绿石结构铱氧化物(A₂Ir₂O₇, A=Y或稀土元素)中的Weyl拓扑半金属性. 我们确定出A₂Ir₂O₇这一类具有阻挫结构材料的磁基态, 并预言其是Weyl半金属; 其Weyl 点受到拓扑保护而稳定, 而且它的表面态在费米能级形成特别的费米弧. 其次预言尖晶石结构锇氧化物(AOs₂O₄, A=Ca, Sr)是具有奇异磁电响应的Axion绝缘体; 然后分析了电子关联、自旋轨道耦合对钙钛矿结构的锇氧化物(NaOsO₃)的影响, 并成功定出它的基态磁构型, 最终确定其为Slater绝缘体. 最后介绍了LiOsO₃中铁电金属性的成因.     

复合多铁链的磁电耦合行为与外场调控

对含有界面磁电耦合的有限长铁电-铁磁多铁链体系进行了研究,基于矢量离散化思想,构建了描述其磁电性质的微观海森伯模型.利用传递矩阵方法获得了磁化强度、电极化强度、磁电化率等关键热力学量的解析表达式,重点探讨了界面磁电耦合、外场以及单离子各向异性对体系磁电耦合行为的影响和调控.研究结果表明,界面磁电耦合对体系的磁化强度和电极化强度均起促进作用.电场驱动下的电致磁电化率具有更强的磁电关联效应,预示着外电场能够有效地调控体系的磁性行为.而在磁致磁电化率中观察到的低温峰主要源于外磁场的诱导.此外,在高电场作用下体系比热容还呈现出有趣的三峰结构,这种三峰结构是自旋态的热激发以及电偶极矩的电场和温度共同激发导致的.     

Computational design of axion insulators based on 5d spinel compounds

Based on density functional calculation using the local density approximation+U method, we predict that osmium compounds such as CaOs(2)O(4) and SrOs(2)O(4) can be stabilized in the geometrically frustrated spinel crystal structure. They show ferromagnetic order in a reasonable range of the on-site Coulomb correlation U and exotic electronic properties, in particular, a large magnetoelectric coupling characteristic of axion electrodynamics. Depending on U, other electronic phases including a 3D Weyl semimetal and Mott insulator are also shown to occur.     

Emergent ferroelectricity in disordered tri-color multilayer structure comprised of ferromagnetic manganites

Multiferroic materials,showing the coexistence and coupling of ferroelectric and magnetic orders,are of great technological and fundamental importance.However,the limitation of single phase multiferroics with robust magnetization and polarization hinders the magnetoelectric effect from being applied practically.Magnetic frustration,which can induce ferroelectricity,gives rise to multiferroic behavior.In this paper,we attempt to construct an artificial magnetically frustrated structure comprised of manganites to induce ferroelectricity.A disordered stacking of manganites is expected to result in frustration at interfaces.We report here that a tri-color multilayer structure comprised of non-ferroelectric La_(0.9)Ca_(0.1)MnO_3(A)/Pr_(0.85)Ca_(0.15)MnO_3(B)/Pr_(0.85)Sr_(0.15)MnO_3(C) layers with the disordered arrangement of ABC-ACBCAB-CBA-BAC-BCA is prepared to form magnetoelectric multiferroics.The multilayer film exhibits evidence of ferroelectricity at room temperature,thus presenting a candidate for multiferroics.     

Multiferroic materials and magnetoelectric physics: symmetry,entanglement, excitation,and topology

Multiferroics are those materials with more than one ferroic order, and magnetoelectricity refers to the mutual coupling between magnetism (spins and/or magnetic field) and electricity (electric dipoles and/or electric field). In spite of the long research history in the whole twentieth century, the discipline of multiferroicity has never been so highly active as that in the first decade of the twenty-first century, and it has become one of the hottest disciplines of condensed matter physics and materials science. A series of milestones and steady progress in the past decade have enabled our understanding of multiferroic physics substantially comprehensive and profound, which is further pushing forward the research frontier of this exciting area. The availability of more multiferroic materials and improved magnetoelectric performance are approaching to make the applications within reach. While seminal review articles covering the major progress before 2010 are available, an updated review addressing the new achievements since that time becomes imperative. In this review, following a concise outline of the basic knowledge of multiferroicity and magnetoelectricity, we summarize the important research activities on multiferroics, especially magnetoelectricity and related physics in the last six years. We consider not only single-phase multiferroics but also multiferroic heterostructures. We address the physical mechanisms regarding magnetoelectric coupling so that the backbone of this divergent discipline can be highlighted. A series of issues on lattice symmetry, magnetic ordering, ferroelectricity generation, electromagnon excitations, multiferroic domain structure and domain wall dynamics, and interfacial coupling in multiferroic heterostructures, will be revisited in an updated framework of physics. In addition, several emergent phenomena and related physics, including magnetic skyrmions and generic topological structures associated with magnetoelectricity will be discussed. The review is ended with a set of prospectives and forward-looking conclusions, which may inevitably reflect the authors' biased opinions but are certainly critical.     

Direct observation of magnetodielectric effect in type-I multiferroic PbFe0.5Ti0.25W0.25O3

In type-I multiferroics, where ferroelectricity and magnetism arise from different origins, the explicit measurement of magnetoelectric effect is commonly prohibited due to the weak magnetoelectric coupling. From our investigation of magnetic and dielectric properties in a perovskite PbFe_0.5Ti_0.25W_0.25O₃, we have directly demonstrated magnetic-field-driven change of dielectric constant in a highly non-linear fashion. Our result offers a potential utilization of magnetoelectric functionality in type-I multiferroics.