Interface-induced room-temperature multiferroicity in BaTiO₃

Multiferroic materials possess two or more ferroic orders but have not been exploited in devices owing to the scarcity of room-temperature examples. Those that are ferromagnetic and ferroelectric have potential applications in multi-state data storage if the ferroic orders switch independently, or in electric-field controlled spintronics if the magnetoelectric coupling is strong. Future applications could also exploit toroidal moments and optical effects that arise from the simultaneous breaking of time-reversal and space-inversion symmetries. Here, we use soft X-ray resonant magnetic scattering and piezoresponse force microscopy to reveal that, at the interface with Fe or Co, ultrathin films of the archetypal ferroelectric BaTiO? simultaneously possess a magnetization and a polarization that are both spontaneous and hysteretic at room temperature. Ab initio calculations of realistic interface structures provide insight into the origin of the induced moments and bring support to this new approach for creating room-temperature multiferroics.     

弛豫多铁性材料研究进展

多铁性材料同时具有多种铁性(铁电性、铁磁性或铁弹性)的有序, 可实现电磁信号的相互控制, 成为近年来研究热点。在具有成分无序的复杂体系中, 长程铁性有序有可能被打破, 材料将表现出弛豫特性。我们将至少存在一种铁性弛豫特性的多铁性材料称之为弛豫多铁性材料。这类多铁性材料的极化强度(或磁化强度)在外加电场(或外加磁场)作用下响应更加灵敏, 其磁电耦合机制与长程有序的多铁性材料不同。本文结合国内外最新研究成果, 首先介绍了和弛豫铁性有序相关的物理概念, 重点阐述了多铁性材料在铁电和铁磁双弛豫态下的磁电耦合机制; 然后, 详细介绍了钙钛矿结构(包括PbB₁B₂O₃基和BiFeO₃基材料)和非钙钛矿结构(包括层状Bi结构和非正常铁电体)弛豫多铁性材料的研究进展; 最后, 对该领域亟待解决的问题进行了展望。     

Energy transduction ferroic materials

Ferroic materials and multiferroics, characterized by their ferroic orders, provide an efficient route for the coupling control of magnetic, mechanical, and electrical subsystems in energy transduction, which aims at converting one form of energy into another. A surge of interest in the ferroic coupling effect has stemmed from its potential use as a new versatile route for energy transduction. Here, the recent progress on the use of (multi)ferroic materials is reviewed, with special emphasis on the fundamental mechanisms that dictate the energy transduction process, including piezoelectricity, pyroelectricity, electrocaloric, magnetostriction, magnetocaloric, elastocaloric, magnetoelectricity, and emerging spin-charge conversion. Research on energy transduction ferroic materials paves the way for ubiquitous energy harvesting through magneto-mechano-electric-thermal coupling mechanisms. Finally, a summary and the future prospective directions of this field are discussed.     

Light‐Induced Reversible Control of Ferroelectric Polarization in BiFeO3

Manipulation of ferroic order parameters, namely (anti‐)ferromagnetic, ferroelectric, and ferroelastic, by light at room temperature is a fascinating topic in modern solid‐state physics due to potential cross‐fertilization in research fields that are largely decoupled. Here, full optical control, that is, reversible switching, of the ferroelectric/ferroelastic domains in BiFeO₃ thin films at room temperature by the mediation of the tip‐enhanced photovoltaic effect is demonstrated. The enhanced short‐circuit photocurrent density at the tip contact area generates a local electric field well exceeding the coercive field, enabling ferroelectric polarization switching. Interestingly, by tailoring the photocurrent direction, via either tuning the illumination geometry or simply rotating the light polarization, full control of the ferroelectric polarization is achieved. The finding offers a new insight into the interactions between light and ferroic orders, enabling fully optical control of all the ferroic orders at room temperature and providing guidance to design novel optoferroic devices for data storage and sensing.     

Metal Thio‐ and Selenophosphates as Multifunctional van der Waals Layered Materials

Since the discovery of Dirac physics in graphene, research in 2D materials has exploded with the aim of finding new materials and harnessing their unique and tunable electronic and optical properties. The follow‐on work on 2D dielectrics and semiconductors has led to the emergence and development of hexagonal boron nitride, black phosphorus, and transition metal disulfides. However, the spectrum of good insulating materials is still very narrow. Likewise, 2D materials exhibiting correlated phenomena such as superconductivity, magnetism, and ferroelectricity have yet to be developed or discovered. These properties will significantly enrich the spectrum of functional 2D materials, particularly in the case of high phase‐transition temperatures. They will also advance a fascinating fundamental frontier of size and proximity effects on correlated ground states. Here, a broad family of layered metal thio(seleno)phosphate materials that are moderate‐ to wide‐bandgap semiconductors with incipient ionic conductivity and a host of ferroic properties are reviewed. It is argued that this material class has the potential to merge the sought‐after properties of complex oxides with electronic functions of 2D and quasi‐2D electronic materials, as well as to create new avenues for both applied and fundamental materials research in structural and magnetic correlations.     

Dynamical magnetoelectric effects in multiferroic oxides

Multiferroics with coexistent ferroelectric and magnetic orders can provide an interesting laboratory to test unprecedented magnetoelectric (ME) responses and their possible applications. One such example is the dynamical and/or resonant coupling between magnetic and electric dipoles in a solid. As examples of such dynamical ME effects, (i)?the multiferroic domain wall dynamics and (ii)?the electric dipole active magnetic responses are discussed with an overview of recent experimental observations.     

Enhanced Resonance Magnetoelectric Coupling in (1‐1) Connectivity Composites

Bulk‐magnetoelectric (ME) composites consisting of various piezoelectric and piezomagnetic materials with (3‐0), (3‐1), (2‐2), and (2‐1) connectivity are proposed in a bid to realize strong ME coupling for next‐generation electronic‐device applications. Here, 1D (1‐1) connectivity ME composites consisting of a [011]‐oriented Pb(Mg,Nb)O₃‐PbTiO₃ (PMN‐PT) single‐crystal fiber laminated with laser‐treated amorphous FeBSi alloy (Metglas) and operating in L‐T mode (longitudinally magnetized and transversely poled) are reported, which exhibit an enhanced resonant ME coupling coefficient of ≈7000 V cm^?1 Oe^?1, which is nearly seven times higher than the best result published previously, and also a superhigh magnetic sensitivity of 1.35 × 10^?13 T (directly detected) at resonance at room temperature, representing a significant advance in bulk magnetoelectric materials. The theoretical analyses based on magnetic‐circuit and equivalent‐circuit methods show that the enhancement in ME coupling can be attributed to the reduction in resonance loss of laser‐treated Metglas alloy due to nanocrystallization and the strong magnetic‐flux‐concentration effect in (1‐1) configuration composites.     

Product properties of a two-phase magneto-electric composite: Synthesis and numerical modeling

Magneto-electric (ME) materials are of high interest for a variety of advanced applications like in data storage and sensor technology. Due to the low ME coupling in natural materials, composite structures become relevant which generate the effective ME coupling as a strain-mediated product property. In this framework, it seems to be possible to achieve effective ME coefficients that can be exploited technologically. The present contribution investigates the realization of particulate ME composites with a focus on their experimental and computational characterization. We will show that different states of pre-polarizations of the ferroelectric material have a decisive influence on the overall obtainable ME coefficient. Details on the synthesis of two-phase composite microstructures consisting of a barium titanate matrix and cobalt ferrite inclusions will be discussed. Subsequently we will employ computational homogenization in order to determine the effective properties of the experimental composite numerically. We investigate the influence of different states of pre-polarization on the resulting ME-coefficients. For the numerical incorporation of the pre-polarization we use a heuristic method.     

Magnetoelectric Radical Hydrocarbons

The molecular radicals, systems with unpaired electrons of open‐shell electronic structures, set the stage for a multidisciplinary science frontier relevant to the cooperative magnetic exchange interaction and magnetoelectric effect. Here ferroelectricity together with magnetic spin exchange coupling in molecular radical hydrocarbon solids is reported, representing a new class of magnetoelectrics. Electronic correlation through radical–radical interactions plays a decisive role in the coupling between magnetic and charge orders. A substantial photoconductance and visible‐light photovoltaic effect are found in radical hydrocarbons. The ability to simultaneously control and retrieve the changes in magnetic and electrical responses opens up a new breadth of applications, such as radical magnetoelectrics, magnets, and optoelectronics.     

基于共同物理基础的铁性智能材料研究概况

铁性智能材料是具有感知温度、力、电、磁等外界环境并产生驱动(位移等)效应的一类重要功能材料,主要包括形状记忆、压电和磁致伸缩三大类材料。长期以来,三类铁性智能材料的研究分布在几个不同的领域,没有形成共同的物理基础,从而导致只能运用有限的学术思路指导智能材料的研发。近年来实验探索研究成果表明:三类看似不同的铁性智能材料从序参量、畴结构到宏观性能层次具有高度的物理平行性,表明它们具有共同的物理基础,据此可望提供高性能铁性智能材料的物理新机制。     

Technology and metrology of new electronic materials and devices

Scaling of the metal oxide semiconductor (MOS) field-effect transistor has been the basis of the semiconductor industry for nearly 30 years. Traditional materials have been pushed to their limits, which means that entirely new materials (such as high-kappa gate dielectrics and metal gate electrodes), and new device structures are required. These materials and structures will probably allow MOS devices to remain competitive for at least another ten years. Beyond this timeframe, entirely new device structures (such as nanowire or molecular devices) and computational paradigms will almost certainly be needed to improve performance. The development of new nanoscale electronic devices and materials places increasingly stringent requirements on metrology.     

Dynamic conductivity of ferroelectric domain walls in BiFeO₃

Topological walls separating domains of continuous polarization, magnetization, and strain in ferroic materials hold promise of novel electronic properties, that are intrinsically localized on the nanoscale and that can be patterned on demand without change of material volume or elemental composition. We have revealed that ferroelectric domain walls in multiferroic BiFeO(3) are inherently dynamic electronic conductors, closely mimicking memristive behavior and contrary to the usual assumption of rigid conductivity. Applied electric field can cause a localized transition between insulating and conducting domain walls, tune domain wall conductance by over an order of magnitude, and create a quasicontinuous spectrum of metastable conductance states. Our measurements identified that subtle and microscopically reversible distortion of the polarization structure at the domain wall is at the origin of the dynamic conductivity. The latter is therefore likely to be a universal property of topological defects in ferroelectric semiconductors.     

轻质多孔材料与结构研究的最新进展(英文)

大力推进材料和装备的轻量化、减量化是实现节能减排、加快建设节约型社会的关键措施,是新世纪工程科技的发展方向。大至海洋平台、大飞机机身和动车组车体,再到日常生活中的车辆,乃至小电子散热器件等,轻量化和多功能化均成为其发展中重要一环。围绕相关特殊工况应用条件下的轻质材料与结构的设计和研究面临一系列挑战:质量轻、力学强度高、散热性能好、动力学性能和隔振、隔声性能可调等多功能要求,因此如何在现有的材料和结构基础上进一步减轻重量并获得更优良的综合性能是材料科学、固体力学、传热、声学、优化设计等诸多领域工作者面临的共同挑战。基于本课题组近5年来围绕"超轻多孔结构创新构型的多功能化基础研究"国家基础研究计划项目所开展的一系列工作,综述了国内超轻多孔材料与结构最新发展水平的研究成果,总结了具有特定或多功能化应用的这类新型轻质多孔材料多学科交叉研究的进展,包括材料制备,力学、热学和声学特性,以及无损检测及优化设计等。     

Identification of novel compositions of ferromagnetic shape-memory alloys using composition spreads

Exploration of new ferroic (ferroelectric, ferromagnetic or ferroelastic) materials continues to be a central theme in condensed matter physics and to drive advances in key areas of technology. Here, using thin-film composition spreads, we have mapped the functional phase diagram of the Ni-Mn-Ga system whose Heusler composition Ni(2)MnGa is a well known ferromagnetic shape-memory alloy. A characterization technique that allows detection of martensitic transitions by visual inspection was combined with quantitative magnetization mapping using scanning SQUID (superconducting quantum interference device) microscopy. We find that a large, previously unexplored region outside the Heusler composition contains reversible martensites that are also ferromagnetic. A clear relationship between magnetization and the martensitic transition temperature is observed, revealing a strong thermodynamical coupling between magnetism and martensitic instability across a large fraction of the phase diagram.     

Tunnel junctions with multiferroic barriers

Multiferroics are singular materials that can exhibit simultaneously electric and magnetic orders. Some are ferroelectric and ferromagnetic and provide the opportunity to encode information in electric polarization and magnetization to obtain four logic states. However, such materials are rare and schemes allowing a simple electrical readout of these states have not been demonstrated in the same device. Here, we show that films of La(0.1)Bi(0.9)MnO(3) (LBMO) are ferromagnetic and ferroelectric, and retain both ferroic properties down to a thickness of 2 nm. We have integrated such ultrathin multiferroic films as barriers in spin-filter-type tunnel junctions that exploit the magnetic and ferroelectric degrees of freedom of LBMO. Whereas ferromagnetism permits read operations reminiscent of magnetic random access memories (MRAM), the electrical switching evokes a ferroelectric RAM write operation. Significantly, our device does not require the destructive ferroelectric readout, and therefore represents an advance over the original four-state memory concept based on multiferroics.     

Microwave ferrites, part 2: passive components and electrical tuning

Low-loss ferrimagnets are the basis for passive microwave components operating in a wide range of frequencies. The magnetic resonances of passive components can be tuned using static magnetic fields over a wide frequency range, where higher operation frequencies require higher magnetic bias unless hexaferrites with large crystalline anisotropy are used. However, electrical tuning of the operation frequency, which can be achieved if the magnetic property of the material is sensitive to the field through magnetoelectric (ME) coupling, is more attractive than magnetic tuning. In the so-called multiferroic materials such as TbMnO₃, TbMn₂O₅, BiFeO₃, Cr₂O₃, and BiMnO₃, which possess simultaneously both the ferroelectric and ferromagnetic properties, ME coupling is very small to be practical. The ME effect, however, can be significantly enhanced in the case of bilayer/multilayer structures with one constituent highly piezoelectric, such as Pb(Zr_1 ? x Ti _x )O₃ (PZT) and 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (PMN-PT), and the other highly ferromagnetic, opening up the possibility for a whole host of tunable microwave passive components. In such structures, the strain induced by the electric field applied across the piezoelectric material is transferred mechanically to the magnetic material, which then experiences a change in its magnetic permeability through magnetostriction. Additionally, electrical tuning coupled with high dielectric permittivity and magnetic susceptibility could lead to miniature microwave components and/or make operation at very high frequencies possible without the need for increased size and weight common in conventional approaches. In Part 1 of this review, fundamentals of ferrite materials, interconnecting chemical, structural, and magnetic properties with the treatment of various types of ferrites used in microwave systems are discussed. Part 2 discusses the basis for coupling between electrical and magnetic properties for highly attractive electrical tuning of passive components by combining piezoelectric materials with ferrites and various device applications of ferrites.     

三聚氰胺甲醛树脂纤维素共混模塑料的热稳定性

按n(F)/n(M)(甲醛与三聚氰胺物质的量比)=2.0~4.0合成系列三聚氰胺甲醛树脂,制备了纤维素共混模塑料。通过热重(TG)、动态热机械分析(DMA)、弯曲和冲击试验研究了模塑料从室温到600℃的性能变化,通过分析其热行为,确认了模塑料中的主要化学结构。当n(F)/n(M)=3.0~3.5时,树脂交联结构完善,能获得最高的冲击强度和Tg,分别是1.28 kJ/m2和118.8℃。模塑料中的树脂在150℃以上会发生后固化反应,在180℃以上开始断链和降解。α-纤维素通过N-羟甲基与树脂分子发生交联反应,使纤维素的热分解速率下降,模塑料刚性增加。     

Exploiting Ionic Coupling in Electronic Devices: Electrolyte‐Gated Organic Field‐Effect Transistors

Currently there is great interest in using organic semiconductors to develop novel flexible electronic applications. An emerging strategy in organic semiconductor materials research involves development of composite or layered materials in which electronic and ionic conductivity is combined to create enhanced functionality in devices. For example, we and other groups have employed ionic motion to modulate electronic transport in organic field‐effect transistors using solid electrolytes. Not only do these transistors operate at low voltages as a result of greatly enhanced capacitive coupling, but they also display intriguing transport phenomena such as negative differential transconductance. Here, we discuss differences in operation between traditional (e.g., SiO₂) and electrolyte‐based dielectrics, suggest further improvements to currently used electrolyte materials, and propose several possibilities for exploiting electrolytes in future applications with both organic and inorganic semiconductors.     

Unexpected controllable pair-structure in ferroelectric nanodomains

The imminent inability of silicon-based memory devices to satisfy Moore's Law is approaching rapidly. Controllable nanodomains of ferroic systems are anticipated to enable future high-density nonvolatile memory and novel electronic devices. We find via piezoresponse force microscopy (PFM) studies on lead zirconate titanate (PZT) films an unexpected nanostructuring of ferroelectric-ferroelastic domains. These consist of c-nanodomains within a-nanodomains in proximity to a-nanodomains within c-domains. These structures are created and annihilated as pairs, controllably. We treat these as a new kind of vertex-antivertex pair and consider them in terms of the Srolovitz-Scott 4-state Potts model, which results in pairwise domain vertex instabilities that resemble the vortex-antivortex mechanism in ferromagnetism, as well as dislocation pairs (or disclination pairs) that are well-known in nematic liquid crystals. Finally, we show that these nanopairs can be scaled up to form arrays that are engineered at will, paving the way toward facilitating them to real technologies.