Magnetoelectric effects in multiferroic Y-type hexaferrites Ba(0.3)Sr(1.7)CoxMg(2-x)Fe(12)O(22)

Y-type hexaferrites with tunable conical magnetic structures are promising single-phase multiferroics that exhibit large magnetoelectric effects. We have investigated the influence of Co substitution on the magnetoelectric properties in the Y-type hexaferrites Ba(0.3)Sr(1.7)CoxMg(2-x)Fe(12)O(22)(x = 0.0, 0.4, 1.0, 1.6). The spin-induced electric polarization can be reversed by applying a low magnetic field for all the samples. The magnetoelectric phase diagrams of BaBa(0.3)Sr(1.7)CoxMg(2-x)Fe(12)O(22) are obtained based on the measurements of magnetic field dependence of dielectric constant at selected temperatures. It is found that the substitution of Co ions can preserve the ferroelectric phase up to a higher temperature, and thus is beneficial for achieving single-phase multiferroics at room temperature.

Magnetoelectric effects in multiferroic laminated plates with imperfect interfaces

Two-dimensional （2D） equations for multiferroic （MF） laminated plates with imperfect interfaces are established in this paper. The interface between two adjacent sublayers, which are not perfectly bonded together, is modeled as a general spring-type layer. The mechanical displacements, and the electric and magnetic potentials of the two adjacent layers are assumed to be discontinuous at the interface. As an example, the influences of imperfect interfaces on the magnetoelectric （ME） coupling effects in an MF sandwich plate are investigated with the established 2D governing equations. Numerical results show that the imperfect interfaces have a significant impact on the ME coupling effects in MF laminated structures.

Magnetoelectric memory effect in the Y-type hexaferrite BaSrZnMgFe_(12)O_(22)

We report on the magnetic and magnetoelectric properties of the Y-type hexaferrite BaSrZnMgFe12O22,which undergoes transitions from a collinear ferrimagnetic phase to a proper screw phase at 310 K and to a longitudinal conical phase at 45 K.Magnetic and electric measurements revealed that the magnetic structure with spiral spin order can be modified by applying a magnetic field,resulting in magnetically controllable electric polarization.It was observed that BaSrZnMgFe12O22 exhibits an anomalous magnetoelectric memory effect：the ferroelectric state can be partially recovered from the paraelectric phase with collinear spin structure by reducing magnetic field at 20 K.We ascribe this memory effect to the pinning of multiferroic domain walls,where spin chirality and structure are preserved even in the nonpolar collinear spin state.

Modulation of magnetic properties and enhanced magnetoelectric effects in MnW_(1-x)Mo_xO_4 compounds

In this letter, we investigate the magnetic and ferroelectric properties of polycrystalline MnW1-xMoxO4 （x = 0, 0.05, 0.10, 0.20） compounds. The substitution of nonmagnetic Mo6＋ ions for W6＋ ions modifies the magnetic transition tem- peratures of MnW1-xMoxO4 by changing the Mn-O-Mn bond. As a result, distinct ferroelectric properties and enhanced magnetoelectric effects are observed in Mo6＋-doped MnWO4 compounds. The effects of substitution of Mo6＋ ions on magnetic properties and magnetoelectric coupling are discussed.

Multiferroics and magnetoelectric effects in charge ordered compounds

The coexistence of magnetic ordering and ferroelectricity, ing on the origin of ferroelectricity, multiferroic materials known as multiferroics, has drawn a lot of research effort. Depend can be classified into different groups. In this paper, we review re cent progress in the field of multiferroics induced by different forms of charge ordering. In addition to a general description of charge order and electronic ferroelectricity, we focus on two specific systems： （1） charge order with frustration in RFe2O4 （R=Lu, Yb） system; （2） charge ordered perovskite manganites of the type （R1-xCax）MnO3 （R=La, Pr）. The charge ordering can be tuned by external electric fields, which results in pronounced magnetoelectric effects and strong dielectric tunability. Other materials and possible candidates with charge order induced multiferroics are also briefly summarized.

Multiferroic behaviour of epitaxial NiFe_2O_4-BaTiO_3 heterostructures

Multiferroic NiFe2O4 （NFO）-BaTiO3 （BTO） bilayered thin films are epitaxially grown on （001） Nb-doped SrTiO3 （STO） substrates by pulsed-laser deposition （PLD）. Different growth sequences of NFO and BTO on the substrate yield two kinds of epitaxial heterostructures with （001）-orientation, i.e. （001）-NFO/（001）-BTO/substrate and （001）- BTO/（001）-NFO/substrate. Microstructure studies from x-ray diffraction （XRD） and electron microscopies show differences between these two heterostructures, which result in different multiferroic behaviours. The heterostructured composite films exhibit good coexistence of both ferroelectric and ferromagnetic properties, in particular, obvious magnetoelectric （ME） effect on coupling response.

Thin film processing and multiferroic properties of Fe-BaTiO_3 hybrid composite

Multiferroic bi-layer Fe/BaTiO3 （BTO） thin films were successfully deposited on Pt（200）/MgO（100） substrates using ion beam sputter deposition （1BSD）, and the mutiferroic properties were studied at room temperature. X-ray diffraction （XRD） analyses showed that BTO films were c-axis oriented and epitaxially grown on platinum coated MgO substrates, and （110） epitaxial Fe films were subsequently grown on （001） BTO films. Fe/BTO bi-layer films showed good ferroelectric and ferromagnetic properties at room temperature and the multiferroic coupling was observed, which should be attributed to the hybridization of Fe and Ti occurring at the ferromagnetic-ferroelectric interface.

Al-doping-induced magnetocapacitance in the multiferroic AgCrS_2

In this paper, multiferroics and magnetocapacitive effect of triangular-lattice antiferromagnet Ag Al0.02Cr0.98S2 are investigated by magnetic, ferroelectric, pyroelectric current and dielectric measurement. We find that it is a multiferroic material and the magnetocapacitive effect reaches a factor of up to 90 in an external field of 7 T. The results imply the further possibility of synthesizing the magnetocapacitive materials by modifying the frustrated spin structure in terms of a few B-site doping nonmagnetic ions.

Multiferroicity in B-site ordered double perovskite Y_2MnCrO_6

Double perovskite manganite Y2MnCrO6 ceramic Novel multiferroic properties are displayed with respect is synthesized and its multiferroic properties are investigated. to other multiferroics, such as high ferroelectric phase transi- tion temperature, and the coexistence of ferrimagnetism and ferroelectricity. Moreover, the ferroelectric polarization of Y2MnCrO6 below the magnetic phase temperature can be effectively tuned by an external magnetic field, showing a re- markable magnetoelectric effect. These results open an effective avenue to explore magnetic multiferroics with spontaneous magnetization and ferroelectricity, as well as a high ferroelectric transition temperature.

Interface/surface magnetoelectric effects： New routes to the electric field control of magnetism

In this perspective paper, we discuss possible ways to control magnetism using electric-field. Special focus is given to interface/surface magnetoelectric effects, which will become important when the thickness of magnetic films drops to nanoscale. We show that significantly different mechanisms may lead to interface/surface magnetoelectric effects, providing great flexibility to apply such effects. As a result, we propose several protype devices utilizing these novel magnetoelectric effects, and strongly advocate experimental endeavors to realize such devices.

Room-temperature giant magnetotranstance effect in single-phase multiferroics

Single-phase multiferroic materials are usually considered useless because of the weak magnetoelectric effects,low operating temperature,and small electric polarization induced by magnetic orders.As a result,current studies on applications of the magnetoelectric effects are mainly focusing on multiferroic heterostructures and composites.Here we report a room-temperature giant effect in response to external magnetic fields in single-phase multiferroics.A low magnetic field of 1000 Oe applied on the spin-driven multiferroic hexaferrites BaSrCo_(2) Fe_(11)AlO_(22) and Ba_(0.9)Sr_(1.1)Co_(2) Fe_(11)AlO_(22)is able to cause a huge change in the linear magnetoelectric coefficient(α_(E)=dE/dH) by several orders,leading to a giant magnetotranstance(GMT) effect at room temperature.The GMT effect is comparable to the well-known giant magnetoresistance(GMR) effect in magnetic multilayers,and thus opens up a door toward practical applications for single-phase multiferroics.

MAGNETOELECTRIC RESPONSES IN MULTIFERROIC COMPOSITE THIN FILMS

Multiferroic composite thinfilms of ferroelectrics and magnets have attracted ever-increasing interest in most recent years.In this review,magnetoelectric(ME)responses as well as their underlying ME coupling mechanisms in such multiferroic composite thinfilms are discussed,oriented by their potential applications in novel ME devices.Among them,the direct ME response,i.e.,magnetic-field control of polarization,can be exploited for micro-sensor applications(sensing magneticfield,electric current,light,etc.),mainly determined by a strain-mediated coupling interaction.The converse ME response,i.e.,electric-field modulation of magnetism,offers great opportunities for new potential devices for spintronics and in data storage applications.A series of prototype ME devices based on both direct and converse ME responses have been presented.The review concludes with a remark on the future possibilities and scientific challenges in thisfield.

Nonreciprocal directional dichroism in multiferroics

Nonreciprocal directional dichroism in multiferroics,namely magnetoelectric coupling in the dynamic regime,is endowed with rich physics and promising applications,which are entangled with fundamental physical components,such as spin,orbital,lattice,charge,and topology.Such a linear nonreciprocal response behavior in the GHz-THz frequency range,represented by optical magnetoelectric effect and magnetochiral dichroism,occurs ubiquitously in material systems with the spontaneous breaking of space-time symmetry,and is subject to Onsager’s reciprocal theorem in the thermodynamic limit.Microscopically,these nonreciprocal responses are usually encoded by toroidization(chirality)and electromagnon(quasiparticle),thus establishing a comprehensive understanding of magnetoelectric coupling and irreversible dynamics.Herein,the basic mechanisms and emergent nonreciprocal directional dichroism in single-phase multiferroics are summarized.We expect that the present review will stimulate diverse possibilities toward nonreciprocal directional dichroism within and beyond multiferroics.

Multiferroic tunnel junctions

Magnetic tunnel junctions with ferroelectric barriers, often referred to as multiferroic tunnel junc- tions, have been proposed recently to display new functionalities and new device concepts. One of the notable predictions is that the combination of two charge polarizing states and the parallel and antiparallel magnetic states could make it a four resistance state device. We have recently studied the ferroelectric tunneling using a scanning probe technique and multiferroic tunnel junctions using ferromagnetic Lao.7Cao.3MnO3 and Lao.TSro.3MnO3 as the electrodes and ferroelectric （Ba, Sr）TiO3 as the barrier in trilayer planner junctions. We show that very thin （Ba, Sr）TiO3 films can sustain ferroelectricity up till room temperature. The multiferroic tunnel junctions show four resistance states as predicted and can operate at room temperatures.

A review on all-perovskite multiferroic tunnel junctions

Although the basic concept was proposed only about 10 years ago,multiferroic tunnel junctions(MFTJs)with a ferroelectric barrier sandwiched between two ferromagnetic electrodes have already drawn considerable interests,driven mainly by its potential applications in multi-level memories and electric field controlled spintronics.The purpose of this article is to review the recent progress of all-perovskite MFTJs.Starting from the key functional properties of the tunneling magnetoresistance,tunneling electroresistance,and tunneling electromagnetoresistance effects,we discuss the main origins of the tunneling electroresistance effect,recent progress in achieving multilevel resistance states in a single device,and the electrical control of spin polarization and transport through the ferroelectric polarization reversal of the tunneling barrier.

Studies on multiferroic materials in high magnetic fields

In this article, studies on the magnetoelectric effects of multiferroic materials in high magnetic fields, particularly pulsed magnetic fields, are discussed and results for some representative ma- terials are presented. In the discussions on representative materials, the relationship between the crystallographic symmetry and the linear magnetoelectric effect in Cr203 is introduced. Then dras- tic changes in polarization caused by magnetic transitions are discussed through a case study of manganites with a perovskite4ype structure. In addition, high field studies on the magnetoelectric effects in BiFeO3, which is an exceptional multiferroic material, are presented and discussed in the framework of the Landau-Ginzburg theory.

单相磁电多铁性体研究进展

文章从电磁耦合现象及磁电效应出发,简要介绍了单相磁电多铁性体(铁电磁体)的最新研究进展。

化学溶液沉积法制备自取向BiFeO_3多铁薄膜

通过化学溶液沉积法在氧化铟锡(ITO)/导电玻璃上生长了BiFeO3多晶薄膜.利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)、X射线光电子能谱分析仪(XPS)、磁性测量系统(MPMS)和铁电测试仪对样品的结构、形貌、元素价态、铁磁性和铁电性进行研究.结果表明,薄膜为自取向生长,具有良好的[101]生长取向和平整的表面.室温下,样品呈铁磁性,沿样品取向方向为易磁化轴方向.铁电测试结果表明,其饱和电极化强度达到51.3μC/cm2.

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。

Ni/BiFeO_3超晶格磁电效应的第一性原理计算

为了探究铁磁/铁电超晶格材料的磁电耦合效应,采用基于密度泛函理论的第一性原理计算方法,研究了Ni/BiFeO_3超晶格的电子结构和磁矩.结果表明,Ni/BiFeO_3超晶格界面处Ni-3d轨道与O-2p轨道发生了强烈的电子杂化作用,Ni和BiFeO_3界面主要通过以O原子为中间媒介的电荷转移进行相互影响.在外加电场作用下,由于铁电极化的存在,超晶格结构上下两个界面存在磁化差异,原子成键和电荷转移重新分布能够引起较强的磁电耦合效应. Ni/BiFeO_3超晶格界面磁电耦合系数约为3. 509×10-^(14)T·cm^2/V.