Unusual tunability of multiferroicity in GdMn_2O_5 by electric field poling far above multiferroic ordering point

The multiferroicity in the RMn_2O_5 family remains unclear, and less attention has been paid to its dependence on high-temperature(high-T) polarized configuration. Moreover, no consensus on the high-T space group symmetry has been reached so far. In view of this consideration, one may argue that the multiferroicity of RMn_2O_5 in the low-T range depends on the poling sequence starting far above the multiferroic ordering temperature. In this work, we investigate in detail the variation of magnetically induced electric polarization in GdMn_2O_5 and its dependence on electric field poling routine in the high-T range. It is revealed that the multiferroicity does exhibit qualitatively different behaviors if the high-T poling routine changes, indicating the close correlation with the possible high-T polarized state. These emergent phenomena may be qualitatively explained by the co-existence of two low-T polarization components, a scenario that was proposed earlier.One is the component associated with the Mn^(3+)–Mn^(4+)–Mn^(3+) exchange striction that seems to be tightly clamped by the high-T polarized state, and the other is the component associated with the Gd Mn^(3+)–Mn^(4+)–Mn^(3+) exchange striction that is free of the clamping. The present findings may offer a different scheme for the electric control of the multiferroicity in RMn_2O_5.

Intrinsic two-dimensional multiferroicity in CrNCl_(2) monolayer

Two-dimensional multiferroics,which simultaneously possess ferroelectricity and magnetism in a single phase,are well-known to possess great potential applications in nanoscale memories and spintronics.On the basis of first-principles calculations,a CrNCl_(2) monolayer is reported as an intrinsic multiferroic.The CrNCl_(2) has an antiferromagnetic ground state,with a N´eel temperature of about 88 K,and it exhibits an in-plane spontaneous polarization of 200 pC/m.The magnetic moments of CrNCl_(2) mainly come from the dxy orbital of the Cr cation,but the plane of the dxy orbital is perpendicular to the direction of the ferroelectric polarization,which hardly suppresses the occurrence of ferroelectricity.Therefore,the multiferroic exits in the CrNCl_(2).In addition,like CrNCl_(2),the CrNBr_(2) is an intrinsic multiferroic with antiferromagneticferroelectric ground state while CrNI_(2) is an intrinsic multiferroic with ferromagnetic-ferroelectric ground state.These findings enrich the multiferroics in the two-dimensional system and enable a wide range of applications in nanoscale devices.

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.

Ferrielectricity in DyMn_(2)O_(5):A golden touchstone for multiferroicity of RMn_(2)O_(5) family

The RMn_(2)O_(5) manganite compounds represent one class of multiferroic family with magnetic origins,which has been receiving continuous attention in the past decade.So far,our understanding of the magnetic origins for ferroelectricity in RMn_(2)O_(5) is associated with the nearly collinear antiferromagnetic structure of Mn ions,while the exchange striction induced ionic displacements are the consequence of the spin frustration competitions.While this scenario may be applied to almost all RMn_(2)O_(5) members,its limitation is either clear:the temperature-dependent behaviors of electric polarization and its responses to external stimuli are seriously materials dependent.These inconsistences raise substantial concern with the state-of-the-art physics of ferroelectricity in RMn_(2)O_(5).In this mini-review,we present our recent experimental results on the roles of the 4f moments from R ions which are intimately coupled with the 3d moments from Mn ions.DyMn_(2)O_(5) is a golden figure for illustrating these roles.It is demonstrated that the spin structure accommodates two nearly collinear sublattices which generate respectively two ferroelectric(FE)sublattices,enabling DyMn_(2)O_(5) an emergent ferrielectric(FIE)system rarely identified in magnetically induced FEs.The evidence is presented from several aspects,including FIE-like phenomena and magnetoelectric responses,proposed structural model,and experimental check by nonmagnetic substitutions of the 3d and 4f moments.Additional perspectives regarding possible challenges in understanding the multiferroicity of RMn_(2)O_(5) as a generalized scenario are discussed.

The twisted two-dimensional ferroelectrics

Since the beginning of research on two-dimensional(2D)materials,a few numbers of 2D ferroelectric materials have been predicted or experimentally confirmed,but 2D ferroelectrics as necessary functional materials are greatly important in developing future electronic devices.Recent breakthroughs in 2D ferroelectric materials are impressive,and the physical and structural properties of twisted 2D ferroelectrics,a new type of ferroelectric structure by rotating alternating monolayers to form an angle with each other,have attracted widespread interest and discussion.Here,we review the latest research on twisted 2D ferroelectrics,including Bernal-stacked bilayer graphene/BN,bilayer boron nitride,and transition metal dichalcogenides.Finally,we prospect the development of twisted 2D ferroelectrics and discuss the challenges and future of 2D ferroelectric materials.

Multiferroic monolayers VOX(X = Cl, Br, I): Tunable ferromagnetism via charge doping and ferroelastic switching

The fascinating properties arising from the interaction between different ferroic states of two-dimensional(2D) materials have inspired tremendous research interest in the past few years.Under the first-principles calculations,we predict the coexistence of antiferromagnetic and ferroelastic states in VOX(X=Cl,Br,I) monolayers.The results illustrate that the VOX monolayers exhibit indirect bandgap characteristics,i.e.,their gaps decrease with the halide elements changing from Cl to I.The ground states of all these VOX monolayers are antiferromagnetic(AFM) with the magnetic moments contributed by the V 3d electrons.Furthermore,the magnetic ground state changing from AFM to ferromagnetism(FM) can be realized by doping carriers.In addition,the moderate ferroelastic transition barrier and reversible switching signal ensure their high performances of nonvolatile memory devices.Our findings not only offer an ideal platform for investigating the multiferroic properties,but also provide candidate materials for potential applications in spintronics.

Three-dimensional general magneto-electro-elastic finite element model for multiphysics nonlinear analysis of layered composites

In this paper,by defining a general potential energy for the multiphase coupled multiferroics and applying the minimum energy principle,the coupled governing equations are derived.This system of equations is then discretized as a general three-dimensional(3D)finite element(FE)model based on the COMSOL software.After validating the formulation,it is then applied to the analysis and design of the common sandwich structure of multiferroics composites.Under the typical static loading,the effects of general lateral boundary conditions,material grading,nonlinearity,as well as polarization orientation on the composites are analyzed.For the magneto-electro-elastic(MEE)sandwich made of piezoelectric BaTiO_(3)and magnetostrictive CoFe_(2)O_(4)with different stacking sequences,various interesting features are observed which should be very helpful for the design of high-performance multiphase composites.

Prediction of a monolayer spin-spiral semiconductor:CoO with a honeycomb lattice

The recent successful fabrication of two-dimensional(2D)CoO with nanometer-thickness motivates us to investigate monolayer CoO due to possible magnetic properties induced by Co atoms.Here,we employ first-principles calculations to show that monolayer CoO is a 2D spin-spiral semiconductor with a honeycomb lattice.The calculated phonon dispersion reveals the monolayer's dynamical stability.Monolayer CoO exhibits a type-I spin-spiral magnetic ground state.The spinspiral state and the direct bandgap character are both robust under biaxial compressive strain(-5%)to tensile strain(5%).The bandgap varies only slightly under either compressive or tensile strain up to 5%.These results suggest a potential for applications in spintronic devices and offer a new platform to explore magnetism in the 2D limit.

Structural evolution-enabled BiFeO3 modulated by strontium doping with enhanced dielectric,optical and superparamagnetic properties by a modified sol-gel method

Multiferroic(BFO)nanoparticles doped with strontium with the general formula Bi1-xSrx FeO3(x=0,0.3,0.5,0.7)were synthesized using a modified sol-gel auto-combustion process.The structural,electrical,optical,and magnetic properties of the samples are discussed.The structural analysis,carried out using the x-ray powder diffraction technique,shows a structural transition from rhombohedral(R-3c)to cubic(Pm-3m)for the doping amount of strontium(Sr)equal to x=0.3.Morphological analysis of the prepared samples were carried out using scanning electron microscopy(SEM).Frequency-dependent dielectric constant and ac conductivity were studied.The doped samples,with improved dielectric properties,can be used to fabricate different optoelectronic devices.Strong dielectric dispersion and broad relaxation were exhibited by all the samples.Cole–Cole plots were employed as an effective tool to study the dispersion parameters,namely,the optical dielectric constant,static dielectric constant,relaxation time,and spreading factor.The activation energy was calculated from the relaxation peaks and Cole–Cole plots,which were found to be compatible with each other.The bandgap of the samples was calculated using diffuse reflectance spectral(DRS)analysis.Sharp and strong photoluminescence in the IR region was observed in the samples,similar to ZnO,which was reported for the first time.Room-temperature and low-temperature magnetization studies point towards the superparamagnetic nature of the samples,with an improvement in magnetic properties with doping.The antiferromagnetic behavior of bulk bismuth ferrite transforms to superparamagnetic in nature for both pure and Sr-substituted bismuth ferrite nanoparticles due to the close dimensions of crystallite size with magnetic domains leading to the break-down of the frustrated spin cycloidal moment.

Switchable hidden spin polarization and negative Poisson's ratio in two-dimensional antiferroelectric wurtzite crystals

Two-dimensional(2D)antiferroelectric materials have raised great research interest over the last decade.Here,we reveal a type of 2D antiferroelectric(AFE)crystal where the AFE polarization direction can be switched by a certain degree in the 2D plane.Such 2D functional materials are realized by stacking the exfoliated wurtzite(wz)monolayers with“self-healable”nature,which host strongly coupled ferroelasticity/antiferroelectricity and benign stability.The AFE candidates,i.e.,Zn X and Cd X(X=S,Se,Te),are all semiconductors with direct bandgap atΓpoint,which harbors switchable antiferroelectricity and ferroelasticity with low transition barriers,hidden spin polarization,as well as giant in-plane negative Poisson's ratio(NPR),enabling the co-tunability of hidden spin characteristics and auxetic magnitudes via AFE switching.The 2D AFE wz crystals provide a platform to probe the interplay of 2D antiferroelectricity,ferroelasticity,NPR,and spin effects,shedding new light on the rich physics and device design in wz semiconductors.

Charge-mediated voltage modulation of magnetism in Hf_(0.5)Zr_(0.5)O_(2)/Co multiferroic heterojunction

We construct the Hall-bar device with the size of several hundred nanometers based on the HZO/Co multiferroic heterojunction. A remarkable voltage-controlled magnetism is observed in the device that possesses both ferroelectric property and perpendicular magnetic anisotropy(PMA). The nucleation field and coercivity can be modulated by voltage pulse while saturation field keeps stable. The non-volatile and reversible voltage-controlled magnetism is ascribable to interfacial charges caused by ferroelectric polarization. Meanwhile, the effective anisotropy energy density(Ku) can also be controlled by voltage pulse, a decrease of 83% and increase of 28% in Kuare realized under-3-V and 3-V pulses,respectively. Because the energy barrier is directly proportional to Ku under a given volume, a decreased or enhanced energy barrier can be controlled by voltage pulse. Thus, it is an effective method to realize low-power and high-stability magneto-resistive random-access memory(MRAM).

A-site ordered quadruple perovskite oxides AA＇_3B_4O_(12)

The A-site ordered perovskite oxides with chemical formula AA＇3B4O（12）display many intriguing physical properties due to the introduction of transition metals at both A and B sites. Here, research on the recently discovered intermetallic charge transfer occurring between A-site Cu and B-site Fe ions in La Cu3Fe4O（12） and its analogues is reviewed, along with work on the magnetoelectric multiferroicity observed in La Mn3Cr4O（12） with cubic perovskite structure. The Cu–Fe intermetallic charge transfer（LaCu3（3＋）Fe4（3＋）O（12）→ LaCu3（2＋）Fe4（3.75＋）O（12）） leads to a first-order isostructural phase transition accompanied by drastic variations in magnetism and electrical transport properties. The La Mn3Cr4O（12） is a novel spindriven multiferroic system with strong magnetoelectric coupling effects. The compound is the first example of cubic perovskite multiferroics to be found. It opens up a new arena for studying unexpected multiferroic mechanisms.

Structural changes concurrent with ferromagnetic transition

Ferromagnetic transition has generally been considered to involve only an ordering of magnetic moment with no change in the host crystal structure or symmetry, as evidenced by a wealth of crystal structure data from conventional X-ray diffractometry （XRD）. However, the existence of magnetostriction in all known ferromagnetic systems indicates that the magnetic moment is coupled to the crystal lattice; hence there is a possibility that magnetic ordering may cause a change in crystal structure. With the development of high-resolution synchrotron XRD, more and more magnetic transitions have been found to be accompanied by simultaneous structural changes. In this article, we review our recent progress in understand- ing the structural change at a ferromagnetic transition, including synchrotron XRD evidence of structural changes at the ferromagnetic transition, a phenomenological theory of crystal structure changes accompanying ferromagnetic transitions, new insight into magnetic morphotropic phase boundaries （MPB） and so on. Two intriguing implications of non-centric symmetry in the ferromagnetic phase and the first-order nature of ferromagnetic transition are also discussed here. In short, this review is intended to give a self-consistent and logical account of structural change occurring simultaneously with a ferromagnetic transition, which may provide new insight for developing highly magneto-responsive materials.

Structure, Performance, and Application of BiFeO_(3) Nanomaterials

Multiferroic nanomaterials have attracted great interest due to simultaneous two or more properties such as ferroelectricity,ferromagnetism,and ferroelasticity,which can promise a broad application in multifunctional,lowpower consumption,environmentally friendly devices.Bismuth ferrite(BiFeO3,BFO)exhibits both(anti)ferromagnetic and ferroelectric properties at room temperature.Thus,it has played an increasingly important role in multiferroic system.In this review,we systematically discussed the developments of BFO nanomaterials including morphology,structures,properties,and potential applications in multiferroic devices with novel functions.Even the opportunities and challenges were all analyzed and summarized.We hope this review can act as an updating and encourage more researchers to push on the development of BFO nanomaterials in the future.

Co-precipitation/Hydrothermal Synthesis of BiFeO_3 Powder

A coprecipitation/hydrothermal route was utilized to fabricate pure phase BiFeO3 powders using FeCl3·6H2O and Bi（NO3）3·5H2O as starting materials, ammonia as precipitant and NaOH as mineralizer. The synthesized powders were characterized by XRD, SEM and DSC-TG analysis. In the process, single-phase BiFeO3 powders could be obtained at a hydrothermal reaction temperature of 180 ℃, with NaOH of 0.15 mol/L, in contrast to 200 ℃ and 4 mol/L for conventional hydrothermal route. Meanwhile, the micro-morphology of synthesized BiFeO3 powders changed with different reaction temperatures and concentrations of NaOH. The N6el temperature, Curie temperature and decomposition temperature of the synthesized BiFeO3 powders were detected to be 301 ℃, 828 ℃ and 964 ℃, respectively. The hydrothermal reactions mechanism to fabricate BiFeO3 powders were discussed based on the in-situ transformation process.

Colossal magnetoresistance in manganites and related prototype devices

We review colossal magnetoresistance in single phase manganites, as related to the field sensitive spin-charge interactions and phase separation; the rectifying property and negative/positive magnetoresistance in manganite/Nb：SrTio3 p-n junctions in relation to the special interface electronic structure; magnetoelectric coupling in manganite/ferroelectric structures that takes advantage of strain, carrier density, and magnetic field sensitivity; tunneling magnetoresistance in tunnel junctions with dielectric, ferroelectric, and organic semiconductor spacers using the fully spin polarized nature of manganites; and the effect of particle size on magnetic properties in manganite nanoparticles.

Exchange coupling and helical spin order in the triangular lattice antiferromagnet CuCrO_2 using first principles

The magnetic and electronic properties of the geometrically frustrated triangular antiferromagnet CuCrO2 are investigated by first principles through density functional theory calculations within the generalized gradient approxi- mations （GGA）＋U scheme. The spin exchange interactions up to the third nearest neighbours in the ab plane as well as the coupling between adjacent layers are calculated to examine the magnetism and spin frustration. It is found that CuCrO2 has a natural two-dimensional characteristic of the magnetic interaction. Using Monte Carlo simulation, we obtain the Neel temperature to be 29.9 K, which accords well with the experimental value of 24 K. Based on non- collinear magnetic structure calculations, we verify that the incommensurate spiral-spin structure with （110） spiral plane is believable for the magnetic ground state, which is consistent with the experimental observations. Due to intra-layer geometric spin frustration, parallel helical-spin chains arise along the a, b, or a＋ b directions, each with a screw-rotation angle of about I20°. Our calculations of the density of states show that the spin frustration plays an important role in the change of d-p hybridization, while the spin-orbit coupling has a very limited influence on the electronic structure.

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.

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.

La-doped BiFeO_3:Synthesis and multiferroic property study

A new sol-foam-gel method was developed to fabricate La-doped BiFeO3 muttiferroic materials. It was demonstrated that a gradual increase in the content of La-doped into BiFeO3 results in its structure changing from rhombohedral to orthorhombic. A study of other property changes indicates that La-doping in BiFeO3 enhances its ferromagnetism and ferroelectricity. A temperature-dependent magnetization study suggests that the magnetic property of La-doped BiFeO3 samples varied from antiferromagnetic to ferromagnetic as the content of La-doped into BiFeO3 increased from 0 to 20%. Unique temperature-dependent zero field cooling （ZFC） and field cooling （FC） magnetization behaviors were observed in 15% La-doped BiFeO3 -- its ZFC temperature-dependent magnetization being ferromagnetic and its FC temperature- dependent magnetization being antiferromagnetic. A possible mechanism of such an interesting M-T behavior is discussed.