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.     

自支撑多铁性薄膜材料研究的机遇与挑战

多铁性材料兼具铁电、铁磁等两种或两种以上铁性有序,并且通过不同铁性之间的多物理场耦合实现新奇磁电效应,在信息存储、换能、传感等方面具有广阔的应用前景。当前,在基础及应用研究方面仍存在许多亟待解决的科学技术问题,譬如寻找及制备室温以上具有强磁电耦合效应的多铁性材料,以及解决与半导体的器件集成问题等,而近年来迅速发展的自支撑薄膜制备技术为此提供了新的机遇。与块体材料及束缚在刚性衬底上的外延薄膜相比,自支撑薄膜具有极为优异的晶格调控自由度,可获得前所未有的极端一维、二维应变(应变梯度),并能够实现人工异质结的构建与器件集成等,为多铁性材料的研究提供了新的材料体系和契机。文章围绕自支撑钙钛矿氧化物(多)铁性材料及人工异质结,总结了最近的重要研究进展,并尝试初步探讨该方向未来可能面临的机遇与挑战。     

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。     

磁电效应研究进展

多铁体(multiferroic)和磁电体(magnetoelectric material)最近几年已迅速成为物理界和材料界的研究热点,其潜在的应用价值必将随着未来的深入研究而得到进一步展现.围绕磁电效应这一多铁体和磁电体最重要的特性,本文介绍了近期理论和实验研究的多方面进展,其中重点评述了有关磁电效应产生机制,特别是一些新颖机制如界面磁电效应,表面磁电效应,电子型多铁性,螺旋自旋(spiral spin)铁电性,铁涡性(ferrotoroidic)和一些广义磁电效应如拓扑磁电效应等的相关研究.文章最后介绍了基于多铁体和磁电体的一些新型功能性器件,如四态存储器,多铁性内存,磁读电写硬盘等.     

BiFeO3光伏效应研究进展

BiFeO3是一个典型的多铁性材料,是凝聚态物理研究领域的热点之一,在信息存储、自旋电子学以及传感器等方面具有诱人的应用前景.近两年来,多铁材料的光电特性引起了人们的广泛关注,许多问题有待于深入研究和探索.本文综述了近年来多铁材料BiFeO3在光伏领域的研究进展,讨论了铁电极化场、电极界面肖特基势垒以及铁电畴结构对BiFeO3光伏特性的影响规律.     

钴氧化物中晶格与自旋的关联耦合效应研究

强关联电子体系具有多序参量耦合且极易受到外场高效调控的特性.钴氧化物(LaCoO₃)是一类典型的多铁性(兼具铁弹性和铁磁性)氧化物材料,受到了研究者们广泛和深入的研究.过去,针对钴氧化物的研究都集中于应力作用下的铁弹性相变和结构调控方面.近年来,研究人员新奇地发现钴氧化物薄膜在张应力作用下发生顺磁到铁磁相转变,但其根源一直存在较大争议.部分实验证据表明应力将会导致钴离子价态降低产生自旋态转变,而另一些研究者认为应力诱导的纳米畴结构会呈现高自旋态的长程有序排列,才是钴氧化物薄膜铁磁性的主要原因.本综述主要介绍近几年来钴氧化物薄膜和异质结中自旋与晶格之间关联耦合效应的系列进展.在保持钴离子价态不变时,通过薄膜厚度、晶格失配应力、晶体对称性、表面形貌、界面氧离子配位和氧八面体倾转等结构因素诱导钴氧化物薄膜的自旋态可逆转变,从而形成高度可调的宏观磁性.进而,研究者们利用原子级精度可控的薄膜生长技术构筑了单原胞层钴氧化物超晶格,通过高效的结构调控,实现了超薄二维磁性氧化物材料.这些系列进展不仅澄清了强关联电子体系中晶格与自旋等序参量之间的强耦合关系,而且为实现氧化物自旋电子...     

多铁性异质结:1+1≠2

基于异质结界面而得以实现的复合型多铁性或磁电耦合性质本质上是一种呈展现象,即这种性质不是其组成成分单独所具有的特性,这体现了材料体系从简单到复杂过程中的某种跳跃式发展过程,即1+1≠2。文章主要介绍了近年来在多铁性异质结上的一些研究进展,包括界面磁电效应、多铁性隧道结以及基于异质结构的电控磁性等。这些进展表明,异质结研究极有可能是多铁性研究走向实用化的突破口。     

磁电效应研究进展

多铁体(multiferroic)和磁电体(magnetoelectric material)最近几年已迅速成为物理界和材料界的研究热点,其潜在的应用价值必将随着未来的深入研究而得到进一步展现。围绕磁电效应这一多铁体和磁电体最重要的特性,本文介绍了近期理论和实验研究的多方面进展,其中重点评述了有关磁电效应产生机制,特别是一些新颖机制如界面磁电效应,表面磁电效应,电子型多铁性,螺旋自旋(spiral spin)铁电性,铁涡性(ferrotoroidic)和一些广义磁电效应如拓扑磁电效应等的相关研究。文章最后介绍了基于多铁体和磁电体的一些新型功能性器件,如四态存储器,多铁性内存,磁读电写硬盘等。     

铋层状氧化物单晶薄膜多铁性研究进展

室温单相多铁材料非常稀缺,磁性元素掺杂的铋层状钙钛矿结构Aurivillius相氧化物是一类重要的单相室温多铁材料,但由于缺少单晶类样品,这一类多铁材料研究主要是围绕多晶类块体或者多晶薄膜展开,它们的磁、电等性能研究大都采用宏观探测方式,因此这类多铁材料的多铁性机理研究进行得非常困难.近年来在高质量单晶薄膜的基础上,研究了多种磁性元素掺杂和不同周期结构的铋层状氧化物多铁单晶薄膜.这些单晶薄膜在室温下大都具有层状面面内方向的铁电极化,以及比较小的室温磁化强度,低温区存在第二个磁性相变.通过X射线共振非弹性散射实验发现元素掺杂会改变金属和氧原子之间的氧八面体晶体场的劈裂,能够增强铁磁性.另一方面,通过极化中子反射实验发现薄膜主体的磁化强度远小于通常探测的宏观磁化强度,说明单晶薄膜中磁的来源及其磁电耦合机理和多晶块体很可能是不同的.铋层状单晶薄膜的多铁性对未来继续改善这类材料的多铁性能有很好的指导作用.     

磁电多铁性材料的宠儿:铁酸铋(BiFeO_3)研究进展的十年回顾

近十几年来,由于对新一代高性能(低能耗、高存储密度、高读写速度)电子功能器件的需求,多铁性材料特别吸引人们的关注。在这些多铁性化合物中,铁酸铋(BiFeO3,简写为BFO)具有高的铁电居里温度和高的反铁磁转变温度,是目前最有应用前景的多铁性材料之一。文章介绍了BFO的晶体结构、铁电极化结构以及反铁磁自旋结构,探讨了在它的基态和高应变状态下,极化与自旋是如何强耦合在一起的。在此基础上,进一步探讨了利用铁电/反铁磁BFO基体系来实现强磁电耦合效应(特别是在低维系统如异质结界面、畴壁或相界中)。文章还对BFO基纳米复合自组装结构中的磁电耦合做了简单介绍。通过对BFO这一多铁性模型体系的研究,可以帮助人们更好地认识铁性材料中衍生出的新奇量子现象,从而利用高等外延生长技术开发和设计新型人造超结构来实现材料的电性、磁性和弹性之间的耦合。     

Structural, spectroscopic, magnetic and electrical characterization of Ca-doped polycrystalline bismuth ferrite, Bi(1-x)Ca(x)FeO(3-x/2) (x ≤ 0.1)

The crystal structure and physical properties of multiferroic polycrystalline Ca(2+)-doped BiFeO(3) samples have been investigated. The present experimental investigation suggests that Bi(1-x)Ca(x)FeO(3-x/2) (x ≤ 0.1) can be considered as a solid solution between BiFeO(3) and CaFeO(2.5). The oxidation state of Fe in these materials is + 3 and charge balance occurs through the creation of oxygen vacancies. For each composition, two structural phase transitions are revealed as anomalies in the variable-temperature in situ x-ray diffraction data which is consistent with the well-established high-temperature structural transformation in pure BiFeO(3). All compositions studied show antiferromagnetic behaviour along with a ferromagnetic component that increases with Ca(2+) doping. The resistivities of the Bi(1-x)Ca(x)FeO(3-x/2) samples at room temperature are of the order of 10(9) Ω cm and decrease with increasing Ca(2+) content. Arrhenius plots of the resistivity show two distinct linear regions with activation energies in the range of 0.4-0.7 and 0.03-0.16 eV. A correlation has been established between the critical temperatures associated with the structural phase transitions and the multiferroic properties. A composition of x = 0.085 is predicted to show maximum magneto-electric coupling.     

Effect of magnetic annealing on magnetization of BiFeO3 ceramics

The effect of magnetic annealing treatment on the magnetization of multiferroic BiFeO₃ was studied systematically. A series of pelletized nano-sized BiFeO₃ powders were annealed at high temperature under different magnetic fields. Typical ferromagnetic hysteresis loops were obtained at room temperature of the ceramics which were derived from ferromagnetic BiFeO₃ precursors. On the other hand, antiferromagnetic behaviors were observed in other samples synthesized from nonmagnetic precursors. The enhanced magnetic properties were ascribed to the magnetic anisotropy which was induced by the strong magnetic fields. This work indicates that the strong magnetic annealing method is an alternative approach to tuning the magnetic properties of high performance multiferroic materials with canted antiferromagnetic ordering.     

Controlling oxygen vacancies and properties of ZnO

Intrinsic defects in semiconductors play crucial roles on their electrical and optical properties. In this article, we report on a facile method to control concentration of oxygen vacancies inside ZnO nanostructures and related physical properties based on adjustment of thermal transformation conditions from ZnO₂ to ZnO, including annealing atmosphere and temperature. ZnO₂ spheres assembled with nanoparticles were formed through the reaction between zinc nitrate and hydrogen peroxide. Significantly, it was found that the adopted temperature and atmosphere have remarkable impact on the concentration of oxygen vacancies, which was revealed by the variations of featured Raman scattering peaks at 584 cm⁻¹. Furthermore, with the increase of oxygen vacancies inside ZnO, the optical band-gap was found to red-shift 350 meV and the room-temperature ferromagnetism became stronger up to 1.6 emu/mg. The defect formation and evolution were discussed according to the chemical equilibrium of decomposition reaction under special local heating environment. This work demonstrated that ZnO₂ decomposition is an effective process to control the defect states inside ZnO and related properties.     

Resistive switching in ceramic multiferroic Bi0.9Ca0.1FeO3

We report resistive switching effects in polycrystalline samples of the multiferroic Bi_0.9Ca_0.1FeO₃ with silver electrodes. Mössbauer spectroscopy shows that upon Ca-doping the Fe remains in a 3+ valence state, suggesting charge compensation through the creation of large amounts of oxygen vacancies. Electrical characterization shows that the oxide/metal resistance can be switched between high and low resistance states by applying voltage pulses. This process was shown to be forming free and a strong relaxation after switching was found. We rationalize our results by considering oxygen vacancies migration to and from the metal–oxide interface, resulting in variations of the Schottky potential barrier height that modulate the interface resistance.     

Influence of cycling electric polarization on multiferroic behaviors in heterostructural films composed of ferroelectric and ferromagnetic oxides

The CoFe₂O₄/Pb(Zr_0.52Ti_0.48)O₃ bilayer films were prepared by a sol–gel process, and the influence of cycling electric polarization on the multiferroic behaviors of the bilayer films was studied. The ferroelectric polarization hysteresis loops under various choices of magnetic bias were measured by an integrating current method. The results showed that after undergoing cycling electric polarization the ferroelectric polarization of the bilayer films enhanced and the suppression of ferroelectric polarization by external magnetic bias remarkably weakened. Based on the measurements of activation energy and leakage current, we confirmed that the oxygen vacancy migration in the bilayer films occurred during cycling electric polarization. Furthermore, we analyzed the mechanism of the influence of cycling electric polarization on the multiferroic behaviors of the bilayer films and attributed it to the oxygen vacancy migration, which could cause a part of ferroelectric domains to be unpinned from the oxygen vacancies and become more active under electric field and magnetic bias.     

Effect of holmium substitution for the improvement of multiferroic properties of BiFeO3

This paper reports on multiferroic properties of Ho substituted BiFeO₃ (Bi_1−xHo_xFeO₃) ceramics. It is observed that for x=0.15, a prominent ferroelectric loop is seen at 300 K even if the system remains in rhombohedral (R3c) phase without appearance of any observable impurity phases. A well shaped M-H loop is observed at 10 K for x=0.15. However it showed ferromagnetism, confirming the contribution of Ho³⁺ towards enhancement of ferromagnetic properties of BiFeO₃ at 300 K. Suppression of impurity phases of pure BiFeO₃ bulk ceramic favors the reduction of mobile oxygen vacancies and reduces leakage current, due to which ferroelectric properties of BiFeO₃ is enhanced. We argue that Ho substitution at Bi site is likely to suppress the spiral spin modulation and at the same time increase the canting angle, which favors enhanced multiferroic properties. XRD, SEM, magnetization, polarization and chemical bonding analysis measurements were carried out to explain the multiferroic behavior.     

Possibility of room-temperature multiferroism in Mg-doped ZnO

Room-temperature multiferroic properties in Mg-doped ZnO samples are reported wherein Mg replaces Zn in the ZnO matrix and retains hexagonal wurtzite structure. The saturation magnetisation is increased from ～2×10^?4 emu/g to 3×10^?4 emu/g for the dilute doping of 2 % Mg in pure ZnO and the ferroelectricity is also increased. Higher concentration of Mg does not lead to a significant enhancement in the magnetisation but improves the ferroelectric properties. An X-ray absorption spectroscopic study shows an enhancement in O vacancies with dilute doping of Mg. The origin of the multiferroic behaviour is understood based on their crystal and electronic structures.     

Dual electronic tunability of the dispersion characteristics of electromagnetic-spin waves in high-anisotropic layered multiferroic structures

Electric and magnetic tuning of phase, frequency, and delay time of electromagnetic-spin waves in thin-film layered multiferroic structure (metal-dielectric gap-ferroelectric material-hexaferrite-dielectric substrate) is numerically simulated. The results are used to demonstrate prospects for application of layered multiferroic hexaferrite-ferroelectric structures in the construction of basically new devices for generation and processing of microwave signals under dual (electric and magnetic) tuning of the working characteristics in the subterahertz frequency range.     

Oxygen vacancy dominant strong visible photoluminescence from BiFeO3 nanotubes

In this Letter, we report oxygen vacancy dominant strong visible photoluminescence (PL) from multiferroic BiFeO₃(BFO) nanotubes (NTs) prepared by sol–gel template method. Abundant oxygen vacancies present in BFO NTs provide alternate paths for the photo‐induced carrier generation and recombination thus affecting the PL and photoabsorption characteristics. This study not only assists in understanding the optoelectronic characteristics of BFO NTs at nanoscale but also suggests BFO nanostructures as potential candidates for future photonic and sensing applications. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

氧空位对Eu2+, Dy3+掺杂的Ca5MgSi3O12发光及余辉性能的影响

利用高温固相法合成了稀土离子Eu²⁺, Dy³⁺掺杂的Ca₅MgSi₃O₁₂长余辉发光材料. 利用光谱学证明了在材料内部存在与氧空位有关的缺陷发光. 通过对比不同条件下合成样品的发光及余辉性能, 发现氧空位对材料的发光及余辉均起到促进作用. 同时发现氧空位发光可以向发光中心传递能量. 利用热释光曲线系统的分析了氧空位对余辉性能的影响. Ca₅MgSi₃O₁₂:Eu²⁺,Dy³⁺是一种潜在的长余辉发光材料. 关键词： 长余辉 氧空位 能量传递 热释光