Electric polarization reversal and memory in a multiferroic material induced by magnetic fields

Control of polarization in bulk ferroelectrics by mechanical dislocation imprint

Defects are essential to engineering the properties of functional materials ranging from semiconductors and superconductors to ferroics. Whereas point defects have been widely exploited, dislocations are commonly viewed as problematic for functional materials and not as a microstructural tool. We developed a method for mechanically imprinting dislocation networks that favorably skew the domain structure in bulk ferroelectrics and thereby tame the large switching polarization and make it available for functional harvesting. The resulting microstructure yields a strong mechanical restoring force to revert electric field-induced domain wall displacement on the macroscopic level and high pinning force on the local level. This induces a giant increase of the dielectric and electromechanical response at intermediate electric fields in barium titanate [electric field-dependent permittivity (ε₃₃) ≈ 5800 and large-signal piezoelectric coefficient (d ₃₃*) ≈ 1890 picometers/volt]. Dislocation-based anisotropy delivers a different suite of tools with which to tailor functional materials.

X-ray Natural Circular Dichroism Imaging of Multiferroic Crystals

The polarizing spectroscopy techniques in visible range optics have been used since the beginning of the 20th century to study the anisotropy of crystals based on birefringence and optical activity phenomena. On the other hand, the phenomenon of X-ray optical activity has been demonstrated only relatively recently. It is a selective probe for the element-specific properties of individual atoms in non-centrosymmetric materials. We report the X-ray Natural Circular Dichroism (XNCD) imaging technique which enables spatially resolved mapping of X-ray optical activity in non-centrosymmetric materials. As an example, we present the results of combining micro-focusing X-ray optics with circularly polarized hard X-rays to make a map of enantiomorphous twinning in a multiferroic SmFe3(BO3)4 crystal. Our results demonstrate the utility and potential of polarization-contrast imaging with XNCD as a sensitive technique for multiferroic crystals where the local enantiomorphous properties are especially important. In perspective, this brings a novel high-performance method for the characterization of structural changes associated with phase transitions and identification of the size and spatial distribution of twin domains.

For an ab initio calculation of the magnetic excitations: RelaxSE!

In this paper, we present a novel efficient and parallel implementation, RelaxSE, for the calculation of the low-lying excited states and energies of strongly correlated systems. RelaxSE is based on the fully uncontracted multi-reference method of Selected Active Space + Single excitations. This method has been specifically designed to be able to tackle systems with numerous open shells per atoms. It is, however, computationally challenging due to the rapid scaling of the number of determinants and their non-trivial ordering induced by the selection process. We propose a combined determinant-driven and integral-driven approach designed for hybrid OpenMP/MPI parallelization. The performances of RelaxSE are evaluated on a controlled test set and show linear scaling with respect to the number of determinants and a small overhead due to the parallelization. Systems with up to 1 × 10⁹ determinants are successfully computed.

Room-Temperature Magnetoelectric Coupling in Electronic Ferroelectric Film based on [(n-C3H7)4N][FeIIIFeII(dto)3] (dto = C2O2S2)

Great importance has been attached to magnetoelectric coupling in multiferroic thin films owing to their extremely practical use in a new generation of devices. Here, a film of [(n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃] (1; dto = C₂O₂S₂) was fabricated using a simple stamping process. As was revealed by our experimental results, in-plane ferroelectricity over a wide temperature range from 50 to 300 K was induced by electron hopping between Fe^II and Fe^III sites. This mechanism was further confirmed by the ferroelectric observation of the compound [(n-C₃H₇)₄N][Fe^IIIZn^II(dto)₃] (2; dto = C₂O₂S₂), in which Fe^II ions were replaced by nonmagnetic metal Zn^II ions, resulting in no obvious ferroelectric polarization. However, both ferroelectricity and magnetism are related to the magnetic Fe ions, implying a strong magnetoelectric coupling in 1. Through piezoresponse force microscopy (PFM), the observation of magnetoelectric coupling was achieved by manipulating ferroelectric domains under an in-plane magnetic field. The present work not only provides new insight into the design of molecular-based electronic ferroelectric/magnetoelectric materials but also paves the way for practical applications in a new generation of electronic devices.

Strain-Mediated Giant Magnetoelectric Coupling in a Crystalline Multiferroic Heterostructure

Multiferroic heterostructures based on the strain-mediated mechanism present ultralow heat dissipation and large magnetoelectric coupling coefficient, two conditions that require endless improvement for the design of fast nonvolatile random access memories with reduced power consumption. This work shows that a structure consisting of a [Pb(Mg_1/3Nb_2/3)O₃]_0.7-[PbTiO₃]_0.3 (001) substrate on which a crystalline FeGa(001)/MgO(001) bilayer is deposited exhibits a giant magnetoelectric coupling coefficient of order 15 × 10^-6 s m^-1 at room temperature. That result is a 2-fold increment over the previous highest value. The spatial orientation of the magnetization vector in the epitaxial FeGa film is switched 90° with the application of electric field. The symmetry of the magnetic anisotropy is studied by the angular dependence of the remanent magnetization, demonstrating that poling the sample generates a switchable uniaxial magnetoelastic anisotropy in the film that overcomes the native low 4-fold magnetocrystalline anisotropy energy. Magnetic force microscopy shows that the switch of the easy axis activates the displacement of domain walls and the domain structures remain stable after that point. This result highlights the interest in single-crystalline structures including materials with large magnetoelastic coupling and small magnetocrystalline anisotropy for low-energy-consuming spintronic applications.

A short history of multiferroics

The realization that materials with coexisting magnetic and ferroelectric order open up efficient ways to control magnetism by electric fields unites scientists from different communities in the effort to explore the phenomenon of multiferroics. Following a tremendous development, the field has now gained some maturity. In this article, we give a succinct review of the history of this exciting class of materials and its evolution from “ferroelectromagnets” to “multiferroics” and beyond.

Analysis of crystal structure and magnetic properties of strontium substituted bismuth ferrite thin films

Thin films of Bi_(1-x)Sr_xFeO₃(X= 0.00, 0.05, 0.10, 0.20, and 0.30) are synthesized on ultra-cleaned glass substrates by simple spray pyrolysis method at 550 °C. The crystal structures of the thin films are investigated by x-ray diffraction (XRD). The Rietveld refinement data for all the thin films are confirmed a single-phase ABO₃type of rhombohedral structure belonging to theR3cspace group. The field emission scanning electron microscopy (FESEM) analysis reveals the size of grains from hexagonal to spherical shape with the addition of Sr doping. The vibrational modes are studied by Raman spectroscopy. The x-ray photoelectron spectroscopy (XPS) analysis unravels the influence of Sr²⁺in the creation of Fe²⁺defects and oxygen vacancies. The room temperature magnetic measurements of the thin films are carried out within a magnetic field range of ±10 kOe. Magnetic hysteresis loop confirms a significant increase in magnetization in Sr modified bismuth ferrite thin films. The composition i.e.X= 0.20 shows maximum enhancement of magnetic properties with saturation and Remanent magnetization values of 5.99 emu g^-1and 1.77 emu g^-1, respectively.

Colossal Magnetoelectric Effect in Core-Shell Magnetoelectric Nanoparticles

Magnetoelectric coefficient values of above 5 and 2 V cm^-1 Oe^-1 in 20 nm CoFe₂O₄-BaTiO₃ and NiFe₂O₄-BaTiO₃ core-shell magnetoelectric nanoparticles were demonstrated. These colossal values, compared to 0.1 V cm^-1 Oe^-1 commonly reported for the 0-3 system, are attributed to (i) the heterostructural lattice-matched interface between the magnetostrictive core and the piezoelectric shell, confirmed through transmission electron microscopy, and (ii) in situ scanning tunneling microscopy nanoprobe-based ME characterization. The nanoprobe technique allows measurements of the ME effect at a single-nanoparticle level which avoids the charge leakage problem of traditional powder form measurements. The difference in the frequency dependence of the ME value between the two material systems is owed to the Ni-ferrite cores becoming superparamagnetic in the near-dc frequency range. The availability of novel nanostructures with colossal ME values promises to unlock many new applications ranging from energy-efficient information processing to nanomedicine and brain-machine interfaces.

Constructing Asymmetrical Ni-Centered {NiN2O4} Octahedra in Layered Metal-Organic Structures for Near-Room-Temperature Single-Phase Magnetoelectricity

Layered metal-organic structures (LMOSs) as magnetoelectric (ME) multiferroics have been of great importance for realizing new functional devices in nanoelectronics. Until now, however, achieving such room-temperature and single-phase ME multiferroics in LMOSs have proven challenging due to low transition temperature, poor spontaneous polarization, and weak ME coupling effect. Here, we demonstrate the construction of a LMOS in which four Ni-centered {NiN₂O₄} octahedra form in layer with asymmetric distortions using the coordination bonds between diphenylalanine molecules and transition metal Ni(II). Near room-temperature (283 K) ferroelectricity and ferromagnetism are observed to be both spontaneous and hysteretic. Particularly, the multiferroic LMOS exhibits strong magnetic-field-dependent ME polarization with low-magnetic-field control. The change in ME polarization with increasing applied magnetic field μ₀H from 0 to 2 T decreases linearly from 0.041 to 0.011 μC/cm² at the strongest ME coupling temperature of 251 K. The magnetic domains can be manipulated directly by applied electric field at 283 K. The asymmetrical distortion of Ni-centered octahedron in layer spurs electric polarization and ME effect and reduces spin frustration in the octahedral geometry due to spin-charge-orbital coupling. Our results represent an important step toward the production of room-temperature single-phase organic ME multiferroics.

Multiferroic decorated Fe2O3 monolayer predicted from first principles

Two-dimensional (2D) multiferroics exhibit cross-control capacity between magnetic and electric responses in a reduced spatial domain, making them well suited for next-generation nanoscale devices; however, progress has been slow in developing materials with required characteristic properties. Here we identify by first-principles calculations robust 2D multiferroic behaviors in decorated Fe2O3 monolayers, showcasing Li@Fe2O3 as a prototypical case, where ferroelectricity and ferromagnetism stem from the same origin, namely Fe d-orbital splitting induced by the Jahn-Teller distortion and associated crystal field changes. These findings establish strong material phenomena and elucidate the underlying physics mechanism in a family of truly 2D multiferroics that are highly promising for advanced device applications.

Enhancement of magnetoelectric coupling in Cr doped Mn2O3

The effect of Cr doping has been undertaken to investigate its effect on the structural, magnetic, dielectric and magnetoelectric properties of newly discovered multiferroics material α-Mn₂O₃. The Cr doping modifies the room temperature crystal symmetry i.e. transforms from orthorhombic to cubic symmetry. Similar to α-Mn₂O₃, two magnetic transitions have been observed in all Cr doped samples. The effect of Cr doping manifested on the low temperature transition. The lower magnetic transition shifted toward higher temperature (25 K for pristine to 40 K for Cr = 10%) whereas the high temperature transition decreases slightly with increasing Cr content. A clear frequency independent transition is observed in temperature dependent complex dielectric measurements for Mn_2-x Cr _x O₃ (0 ⩽ x ⩽ 0.10) samples around high temperature magnetic ordering ∼80 K which corroborate the magnetoelectric coupling in these samples. Interestingly, the magnetodielectric value enhanced significantly with Cr doping and a maximum increase of ∼21% is observed for 10% Cr doped sample at 5 K around 70 kOe magnetic field.

Solvent-Dependent Bending Ability of Salen-Derived Organic Crystals

The formation of plastic or brittle organic crystals of salen derivatives that depend on the solvents employed for crystallization is demonstrated. Large yellow crystals (ranging from mm to cm size) of ten different salen derivatives were obtained and investigated. Among them, (bis(2-hydroxyacetophenone)ethylenediimine) 2, which was recrystallized from dichloromethane, tetrahydrofuran or chloroform, exhibited plastic deformation behaviour when mechanical force was applied to the (001) face. In contrast, when 2 was recrystallized from benzene, brittle crystals were obtained. Face indexing confirmed that different crystal faces were obtained by depending on the solvent employed for recrystallization, which leads to either flexible (plastic) or brittle crystals. Photoluminescence with a band maximum at 510 nm and thermochromism related to tautomerism between OH and NH forms were also investigated, and indicate that 2 is a flexible organic single-crystal material with multifunctional properties.

Structure, Performance, and Application of BiFeO3 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, low-power 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.

Magnetic properties of the double perovskites Sm2Mn1+x Co1-x O6 (x = 0, 0.05, 0.12 and 0.26)

The magnetic properties of the double perovskites Sm₂Mn_1+x Co_1-x O₆ (x  =  0, 0.05, 0.12 and 0.26) were investigated. It was found that the Curie temperature, the lattice parameters and the net magnetic moments increased for increasing amounts of Co. An irreversible behavior was observed by measuring the magnetization after cooling the sample with and without applied magnetic fields (H). The temperature below which the irreversibility was observed is H dependent and the data were nicely fit to de Almeida-Thouless lines. The ac magnetic susceptibility was measured for frequencies f  in the range 0.03-10 kHz yielding [Formula: see text] for the shifting in the freezing temperature per decade of f . The spin-dynamics were found to follow a power-law with a product of the critical exponents [Formula: see text] of about 4.99. The overall results are understood within a framework where the variation in the bonding angle associated to the super-exchange interactions are taken into consideration.

Magneto-electric multiferroics: designing new materials from first-principles calculations

In parallel with the revival of interest for magneto-electric multiferroic materials in the beginning of the century, first-principles simulations have grown incredibly in efficiency during the last two decades. Density functional theory calculations, in particular, have so become a must-have tool for physicists and chemists in the multiferroic community. While these calculations were originally used to support and explain experimental behaviour, their interest has progressively moved to the design of novel magneto-electric multiferroic materials. In this article, we mainly focus on oxide perovskites, an important class of multifunctional material, and review some significant advances to which contributed first-principles calculations. We also briefly introduce the various theoretical developments that were at the core of all these advances.

van der Waals force layered multiferroic hybrid perovskite (CH3NH3)2CuCl4 single crystals

In inorganic-organic perovskites, the three-dimensional arrangement of the organic group results in more subtle balance of charge, spin and space, thereby providing an attractive route toward new multiferroics. Here we report the existing of multiple ferroic orderings in inorganic-organic layered perovskites with relative strong hydrogen bond ordering of the organic chains intra plane. In addition, the inter plane in perovskite is stacking via van der Waals force. However, such magnetoelectric coupling properties for this compound have not been reported since it is difficult to characterize the properties in single crystals since most of the hybrid perovskites are usually deliquescent and unstable when exposed to air. To deal with these problems, we synthesized a (CH₃NH₃)₂CuCl₄ single crystal by using a simple evaporation technique, and demonstrated ferroelectric, magnetic and magneto-electric properties of (CH₃NH₃)₂CuCl₄. The internal hydrogen bonding of easily tunable organic unit combined with 3d transition-metal layers in such hybrid perovskites make (CH₃NH₃)₂CuCl₄ a multiferroic crystal with magnetoelectrical coupling and offer an new way to engineer multifunctional multiferroic.

Rodlike YMn2O5 Powders Derived from Hydrothermal Process Using Oxygen as Oxidant

A facile approach is proposed herein to fabricate YMn₂O₅ powders with the hydrothermal method with oxygen as an oxidant. The structure and morphology of the as-synthesized YMn₂O₅ powders were characterized by XRD, SEM, and high-resolution transmission electron microscopy (HRTEM). The results manifested that the main factors that affected the formation of the rod-like YMn₂O₅ structures were the stirring time, hydrothermal temperature, and hydrothermal time. The oxidation time in the air had a remarkable effect on the final product by oxidizing Mn²⁺ ions to Mn³⁺ ions and Mn⁴⁺ ions. The obtained YMn₂O₅ powder was single crystalline and possessed a nanorod morphology, where the growth direction was along the c axis. The possible formation mechanism involved a dissolution–crystallization mechanism. Under the 397 nm excitation, the Mn⁴⁺ ions exhibited an intense orange emission at 596 nm. The energy bandgap of YMn₂O₅ powders was 1.18 eV.

Manipulating On/Off Single-Molecule Magnet Behavior in a Dy(III)-Based Photochromic Complex

Exploitation of room temperature (RT) photochromism and photomagnetism to induce single-molecule magnet (SMM) behavior has potential applications toward optical switches and magnetic memories, and remains a tremendous challenge in the development of new bulk magnets. Herein, a series of chain complexes [Ln₃(H-HEDP)₃(H₂-HEDP)₃]·2H₃-TPT·H₄-HEDP·10H₂O (QDU-1; Ln = Dy (QDU-1(Dy)), Gd (QDU-1(Gd)), and Y (QDU-1(Y)); HEDP = hydroxyethylidene diphosphonate; TPT = 2,4,6-tri(4-pyridyl)-1,3,5-triazine) were synthesized by solvothermal reactions. All the compounds exhibited reversible photochromic and photomagnetic behaviors via UV light irradiation at RT, induced by the photogenerated radicals via a photoinduced electron transfer (PET) mechanism. More importantly, the PET process induced significant variations in magnetic interactions for the Dy(III) congener. Strong ferromagnetic coupling with remarkably slow magnetic relaxation without applied dc fields was observed between Dy^III ions and photogenerated O^• radicals, showing SMM behavior after RT illumination. For the first time, we observed the reversible RT photochromism and photomagnetism in the lanthanide-based materials. This work realized the radicals-actuated on/off SMM behavior via RT light irradiation, providing a new strategy for constructing the light-induced SMMs.

Structural, Dielectric, and Magnetic Properties of Multiferroic ( 1 - x ) La0.5Ca0.5MnO3-( x ) BaTi0.8Sn0.2O3 Laminated Composites

High-performance lead-free multiferroic composites are desired to replace the lead-based ceramics in multifunctional device applications. Laminated compounds were prepared from ferroelectric and ferromagnetic materials. In this work, we present the laminated ceramics compound by considering the ferromagnetic La_0.5Ca_0.5MnO₃ (LCMO) and the ferroelectric BaTi_0.8Sn_0.2O₃ (BTSO) in two different proportions. Compounds ( 1-x ) LCMO-( x ) BTSO with x = 1 and 0 (pure materials) were synthesized by the sol-gel method, and x = 0.7 and 0.5 (laminated) compounds were elaborated by welding appropriate mass ratios of each pure material by using the silver paste technique. Structural, dielectric, ferroelectric, microstructure, and magnetic characterizations were conducted on these samples. X-ray scattering results showed pure perovskite phases confirming the successful formation of both LCMO and BTSO. Scanning electron microscope (SEM) images evidenced the laminated structure and good quality of the interfaces. The laminated composite materials have demonstrated a multiferroic behavior characterized by the ferroelectric and the ferromagnetic hysteresis loops. Furthermore, the enhancement of the dielectric constant in the laminated composite samples is mainly attributed to the Maxwell-Wagner polarization.

Design and Manipulation of Ferroic Domains in Complex Oxide Heterostructures

The current burst of device concepts based on nanoscale domain-control in magnetically and electrically ordered systems motivates us to review the recent development in the design of domain engineered oxide heterostructures. The improved ability to design and control advanced ferroic domain architectures came hand in hand with major advances in investigation capacity of nanoscale ferroic states. The new avenues offered by prototypical multiferroic materials, in which electric and magnetic orders coexist, are expanding beyond the canonical low-energy-consuming electrical control of a net magnetization. Domain pattern inversion, for instance, holds promises of increased functionalities. In this review, we first describe the recent development in the creation of controlled ferroelectric and multiferroic domain architectures in thin films and multilayers. We then present techniques for probing the domain state with a particular focus on non-invasive tools allowing the determination of buried ferroic states. Finally, we discuss the switching events and their domain analysis, providing critical insight into the evolution of device concepts involving multiferroic thin films and heterostructures.

The growth and improved magnetoelectric response of strain-modified Aurivillius SrBi4.25La0.75Ti4FeO18 thin films

In this study, we grew 5-layered SrBi4.25La0.75Ti4FeO18 (SBLFT) polycrystalline thin films (80-330 nm thick) via pulsed-laser deposition to study their ferroelectric and magnetoelectric response. Structural/microstructural analysis confirmed the formation of orthorhombic SBLFT with good crystallinity and randomly oriented Aurivillius phases. Detailed scanning transmission electron microscopy analysis of 120 nm film revealed a predominantly five-layered structure with the coexistence of four-layer stacking. Such stacking defects are found to be pertinent to the high structural flexibility of Bi-rich Aurivillius phases, alleviated by lattice strain. Raman spectral features at ambient temperatures depict the signature of the orthorhombic-tetragonal phase transition. SBLFT films have a strong ferroelectric nature (remanent polarization 2Pr of 35 μC cm-2) with a fatigue endurance up to 1010 cycles and strongly improved, switchable magnetization as opposed to its antiferromagnetic bulk counterpart. The scaling behavior of dynamic hysteresis reveals that ferroelectric domain reversal has good stability and low energy consumption. We observed the presence of SBLFT nanoregions (1-5 nm), distributed across the film, with Bi and Fe-rich compositions and oxygen vacancies that contribute to the weak ferromagnetic behavior mediated by the Dzyaloshinskii-Moriya interactions. Subtle changes in the structural strain and lattice distortions of thin films with varied thicknesses led to distinct ferroic properties. Stronger ferroelectric polarization of 80 nm and 120 nm films compared to that of thicker ones can be due to structural strain and the possible rearrangement of BO6 octahedra. The observation of the improved magnetoelectric coefficient of 50 mV cm-1 Oe-1 for 120 nm film, as compared to that of several Aurivillius oxides, indicates that the structural strain modification in SBLFT is beneficial for the fatigue-free magnetic field switching of ferroelectric polarization. The structural strain of the unit cell as well as the presence of Bi- and ferromagnetic Fe-rich nanoregions was found to be responsible for the improved multiferroic behaviour of the SBLFT films.

Pressure-dependent X-ray diffraction of the multiferroics RMn2O5

The room-temperature structural properties of the RMn₂O₅ multiferroics have been investigated under pressure, using powder X-ray scattering and density functional theory (DFT) calculations. It was possible to determine the lattice parameters and the main atomic positions as a function of pressure. Good agreement was observed between the X-ray and DFT results for most of the determined crystallographic data. From the DFT calculations, it was possible to infer the pressure evolution of the exchange interactions, and this analysis led to the conclusion that the onset of the q = (½, 0, ½) magnetic structure under pressure is related to the increase in the J₁ super-exchange terms (due to the reduction in the Mn-O distances) compared with the Mn-R exchange interactions. In addition, the 1D antiferromagnetic character of the compounds should be reinforced under pressure.

Optical torque induces magnetism at the molecular level

We report experimental observations of a mechanism that potentially supports and intensifies induced magnetization at optical frequencies without the intervention of spin-orbit or spin-spin interactions. Energy-resolved spectra of scattered light, recorded at moderate intensities (10⁸ W/cm²) and short timescales (<150 fs) in a series of non-magnetic molecular liquids, reveal the signature of torque dynamics driven jointly by the electric and magnetic field components of light at the molecular level. While past experiments have recorded radiant magnetization from magneto-electric interactions of this type, no evidence has been provided to date of the inelastic librational features expected in cross-polarized light scattering spectra due to the Lorentz force acting in combination with optical magnetic torque. Here, torque is shown to account for unpolarized rotational components in the magnetic scattering spectrum under conditions that produce only polarized vibrational features in electric dipole scattering, in excellent agreement with quantum theoretical predictions.

Single-phase multiferroics: new materials, phenomena, and physics

Multiferroics, where multiple ferroic orders coexist and are intimately coupled, promise novel applications in conceptually new devices on one hand, and on the other hand provide fascinating physics that is distinctly different from the physics of high-T_C superconductors and colossal magnetoresistance manganites. In this mini-review, we highlight the recent progress of single-phase multiferroics in the exploration of new materials, efficient roadmaps for functionality enhancement, new phenomena beyond magnetoelectric coupling, and underlying novel physics. In the meantime, a slightly more detailed description is given of several multiferroics with ferrimagnetic orders and double-layered perovskite structure and also of recently emerging 2D multiferroics. Some emergent phenomena such as topological vortex domain structure, non-reciprocal response, and hybrid mechanisms for multiferroicity engineering and magnetoelectric coupling in various types of multiferroics will be briefly reviewed.

Synthesized and Photocatalytic Mechanism of the NiO Supported YMnO3 Nanoparticles for Photocatalytic Degradation of the Methyl Orange Dye

The YMnO3, NiO, and NiO/YMnO3 nanocomposites are successfully prepared by a polyacrylamide gel method and a simple two phase recombination route. The phase structure, surface morphology, optical, magnetic and electrochemical properties and photocatalytic activity of as-prepared samples were measured by X-ray diffraction (XRD), Scanning electron microscope (SEM), UV-visible spectrophotometer, superconducting quantum interference device (SQUID), electrochemical workstation and 721 spectrophotometer. Phase structure analysis indicate that the YMnO3, NiO, and NiO/YMnO3 nanocomposites has good crystallization and no other impurities. The NiO supported YMnO3 composites improved the surface morphology, optical, magnetic and electrochemical properties and photocatalytic activity of YMnO3 nanoparticles significantly. Photocatalytic experiment analysis indicate that the NiO supported YMnO3 composites exhibits highest photocatalytic activity for degradation of the methyl red dye with dye concentration of 25 mg/L. The excellent photocatalytic activity for the NiO supported YMnO3 composites can be ascribed to the fast interfacial charge transfer, high effective charge separation and the ⋅OH radicals.

Magnetic performance and defect characterization studies of core-shell architectured MgFe2O4@BaTiO3 multiferroic nanostructures

Multiferroics that permit manipulation of the magnetization vector exclusively by electric fields have spawned extensive interest for memory and logic device applications. In line with this understanding, we herein report the encapsulation of non-ferroelectric magnesium ferrite (MgFe2O4) nanoparticles in a ferroelectric shell of BaTiO3 to produce a system with engineered dielectric, magnetic, magneto-electric and ferroelectric properties. The interface effect on the strain transfer was observed to strongly influence the magneto-electric coupling and the electric and magnetic properties of the system. The model polyhedral image of MgFe2O4@BaTiO3 has helped to get an insight into the core-shell structure. The multiferroicity induced by the excellent coupling between the ferroelectric and magnetostrictive phases at the core-shell interface unlocks wide prospects for device downscaling and information storage applications. The influence of magnetostrictive stress on the magneto-electric coupling effects and domain dynamics was further studied using transmission electron microscopy (TEM) and atomic force microscopy images. Interestingly, the realization of a superparamagnetic multiferroic system has been a breakthrough and facilitates ultra high density magnetic data storage technologies. Evidence for spontaneous polarization and the ferroelectric trait exhibited by the multiferroic samples was revealed from the P-E hysteresis loop. The investigation of defect evolution in the system was carried out using positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening spectroscopy (CDBS) of annihilation radiation and the studies revealed thermal diffusion of positrons into the interfacial regions within the core-shell structure and the "formation and pick-off annihilation of orthopositronium atoms". It is concluded that interface engineering is a strong means for manipulation of the magnetic, dielectric and magneto-electric properties in multiferroic heterostructures for high density electrical energy and magnetic data storage.

Magnetization-polarization cross-control near room temperature in hexaferrite single crystals

Mutual control of the electricity and magnetism in terms of magnetic (H) and electric (E) fields, the magnetoelectric (ME) effect, offers versatile low power consumption alternatives to current data storage, logic gate, and spintronic devices. Despite its importance, E-field control over magnetization (M) with significant magnitude was observed only at low temperatures. Here we have successfully stabilized a simultaneously ferrimagnetic and ferroelectric phase in a Y-type hexaferrite single crystal up to 450 K, and demonstrated the reversal of large non-volatile M by E field close to room temperature. Manipulation of the magnetic domains by E field is directly visualized at room temperature by using magnetic force microscopy. The present achievement provides an important step towards the application of ME multiferroics.

Mutual control of the electric polarization and magnetization promises for low power consumption spintronic devices but remains challenging. Here the authors show reversal of non-volatile magnetization by electric field as well as the polarization switching by magnetic field in a single-component material, close to room temperature.

Novel Dynamical Magnetoelectric Effects in Multiferroic BiFeO_{3}

An atomistic effective Hamiltonian scheme is employed within molecular dynamics simulations to investigate how the electrical polarization and magnetization of the multiferroic BiFeO_{3} respond to time-dependent ac magnetic fields of various frequencies, as well as to reveal the frequency dependency of the dynamical (quadratic) magnetoelectric coefficient. We found the occurrence of vibrations having phonon frequencies in both the time dependency of the electrical polarization and magnetization (for any applied ac frequency), therefore making such vibrations of electromagnonic nature, when the homogeneous strain of the system is frozen (case 1). Moreover, the quadratic magnetoelectric coupling constant is monotonic and almost dispersionless in the sub-THz range in this case 1. In contrast, when the homogeneous strain can fully relax (case 2), two additional low-frequency and strain-mediated oscillations emerge in the time-dependent behavior of the polarization and magnetization, which result in resonances in the quadratic magnetoelectric coefficient. Such additional oscillations consist of a mixing between acoustic phonons, optical phonons, and magnons, and reflect the existence of a new quasiparticle that can be coined an "electroacoustic magnon." This latter finding can prompt experimentalists to shape their samples to take advantage of, and tune, the magnetostrictive-induced mechanical resonance frequency, in order to achieve large dynamical magnetoelectric couplings.

Distorted Monolayer ReS2 with Low-Magnetic-Field Controlled Magnetoelectricity

Two dimensional (2D) materials possessing ferroelectric/ferromagnetic orders and especially low-magnetic-field controlled magnetoelectricity have great promise in spintronics and multistate data storage. However, ferroelectric and magnetoelectric (ME) dipoles in the atom-thick 2D materials are difficult to be realized due to structural inversion symmetry, thermal actuation, and depolarized field. To overcome these difficulties, the monolayer structure must possess an in-plane inversion asymmetry in order to provide out-of-plane ferroelectric polarization. Herein, crystal chemistry is adopted to engineer specific atomic displacement in monolayer ReS₂ to change the crystal symmetry to induce out-of-plane ferroelectric polarization at room temperature. The cationic Re vacancy in the atom-displaced ReS₂ monolayer causes spin polarization of two immediate neighbor sulfur atoms to generate magnetic ordering, and the ferroelectric distortion near the Re vacancy locally tunes the ferromagnetic order thereby triggering low-magnetic-field controlled ME polarization at about 28 K. As a result, 2D ME coupling multiferroics is achieved. Our results not only reveal a design methodology to attain coexistence of ferroelectric and ferromagnetic orders in 2D materials but also provide insights into magnetoelectricity in 2D materials.

A First-Principles Study on the Multiferroic Property of Two-Dimensional BaTiO₃ (001) Ultrathin Film with Surface Ba Vacancy

In this work, the multiferroic property of Ba-deficient BaTiO₃ (001) ultrathin film is studied employing the first-principles approach. The BaTiO₃ (001) ultrathin film is more energetically stable and behaves as a semiconductor relative to the (111) and (101) configurations, confirmed from the surface grand potential and electronic density of states. The electronic structures show that the O vacancy can switch the (001) film from a semi-conductor into a metal, while the Ba defect has a slight influence on the band gap, at a concentration of ~2.13%. In Ba-deficient (001) film, the spontaneous polarization pattern is changed and a spontaneous polarization parallel to the surface is observed. Furthermore, a magnetic moment is induced, and it is found to be originated from the O atoms in the supercell. Our results suggest that a strong magnetoelectric coupling occurs because the magnetic moment exhibits a 43.66% drop when the spontaneous polarization increases from 12.84 µC/cm² to 23.99 µC/cm² in the deficient BaTiO₃ with m = 2 under the bi-axial compress stress field.

Recent developments in single-molecule toroics

An update overview of emerging single-molecule toroics (SMTs) is expounded to elucidate the strategy to design SMTs and ultimately inspire the seeking of SMTs with enhanced toroidal moment.

Weak ferromagnetism derived from spin canting in an amido-bridged homochiral Mn(iii) 1-D coordination polymer

An amido-bridged homochiral one-dimensional (1D) Mn(iii) coordination polymer behaves as a weak ferromagnet arising from spin canting with a coercive field of 3.0 kOe.

Anisotropic Magnetoelectric Coupling and Cotton-Mouton Effects in the Organic Magnetic Charge-Transfer Complex Pyrene-F4TCNQ

Magnetoelectric coupling is of high current interest because of its potential applications in multiferroic memory devices. Although magnetoelectric coupling has been widely investigated in inorganic materials, such observations in organic materials are extremely rare. Here, we report our discovery that organic charge-transfer (CT) complex pyrene-2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (pyrene-F₄TCNQ) can display anisotropic magnetoelectric coupling. Investigation of the crystal structure of pyrene-F₄TCNQ complex demonstrates that the magnetoelectric coupling coefficient along the π-π interaction direction is much larger than the value along other directions. Furthermore, magnetoelectric coupling and magnetization can be tuned by changing the fluorine content in complexes. Besides, the Cotton-Mouton effect in pyrene-F₄TCNQ is observed, enabling the control of optomagnetic devices. These results can pave the way for a new method for the future development of organic CT complexes and their applications in perpendicular memory devices and energy-transfer-related multiferroics.

Nanoscale LuFeO3: shape dependent ortho/hexa-phase constitution and nanogenerator application

In multiferroic LuFeO₃ the hexagonal (-h) phase is an intermediate metastable phase encountered during the amorphous to orthorhombic (-o) transformation and is ferroelectric in nature. Thus far it has only been stabilized in a substrate-supported few layered ultrathin film form. Herein we show that the surface-induced strain field intrinsically present in nano-systems can self-stabilize this phase and the hexagonal to orthorhombic phase constitution ratio depends on the shape of the nanomaterial. Thus, nanoparticles (nanofibres) strain-stabilize the o : h ratio of about 75 : 25 (23 : 77). The inclusion of nano-LuFeO₃ into PDMS renders impressive nanogenerator performance, consistent with the ferroelectric phase content.

Comprehensive inelastic neutron scattering study of the multiferroic M n 1 - x C o x W O 4

Using high-resolution inelastic neutron scattering, we examine the spin dynamics ofM n 1 - x C o x W O 4 in the collinear AF1, the a c - b spiral AF2, and the a c cycloidal AF5 phases. The spin wave excitations are well described by a Heisenberg model with competing long-range exchange interactions (J i up to 12th nearest neighbors) and the single-ion anisotropy K induced by the spin-orbit interaction. While the exchange constants are relatively unchanged, the dominant effect of doping is to change the single-ion anisotropy from easy axis ( K > 0 ) in the collinear AF1 phase to easy plane ( K < 0 ) in the two multiferroic phases.

Magnetic and Structural Studies of G-Phase Compound Mn6Ni16Si7

Transition metal compounds with complex crystal structures tend to demonstrate interesting magnetic coupling resulting in unusual magnetic properties. In this work, the structural and magnetic characterization of a single crystal of the Ni-Mn-Si based G-phase compound, Mn₆Ni₁₆Si₇, grown by the Czochralski method, is reported. In this structure, isolated octahedral Mn₆ clusters form an fcc lattice. As each octahedron consists of eight edge-sharing equilateral triangles, the possibility for geometric frustration exists. Magnetization and specific heat measurements showed two magnetic phase transitions at 197 and 50 K, respectively. At 100 K neutron diffraction on powder samples shows a magnetic structure with k = (001) in which only four of the six Mn spins per cluster order along ⟨100⟩ directions giving a two-dimensional magnetic structure consistent with intracluster frustration. Below the 50 K phase transition, the Mn spin-cants away from ⟨100⟩ directions, and a weak moment develops on the two remaining Mn octahedral sites.

Spin-reorientation-induced magnetodielectric coupling effects in two layered perovskite magnets

Spin-reorientation-induced magnetodielectric coupling effects were discovered in two layered perovskite magnets, [C6H5CH2CH2NH3]2[MCl4] (M = Mn2+ and Cu2+), via isothermal magnetodielectric measurements on single-crystal samples. Specifically, peak-like dielectric anomalies and spin-flop transitions appeared simultaneously at around ±34 kOe for the canted antiferromagnet (M = Mn2+) at below 44.3 K, while a low-field (1 kOe) controlled magnetodielectric effect was observed in the "soft" ferromagnet (M = Cu2+) at below 9.5 K. These isothermal magnetodielectric effects are highly reproducible and synchronous with the field-induced magnetization at different temperatures, well confirming the essential role of spin reorientation on inducing magnetodielectric coupling effects. These findings strongly imply that the layered perovskite magnets are new promising organic-inorganic hybrid systems to host magnetodielectric coupling effects.

Magnetoelectric and magnetodielectric coupling and microwave resonator characteristics of Ba0.5Sr0.5Nb2O6/CoCr0.4Fe1.6O4 multiferroic composite

A multiferroic composite consisting of single phases of 30 vol.% magnetostrictive ferrite and 70 vol.% relaxor ferroelectric has been synthesized. The ferrite exhibits a diffuse dielectric phase transition (DPT)with the transition temperature varying from 450 K to 600 K and an activation energy of 0.29 eV. Magnetically, it has a soft behavior with 70 emug^-1 saturation magnetization and a Curie transition at ~620 K. The relaxor ferroelectric phase on the other hand exhibits two clear DPTs at 390 K-400 K and 150 K-300 K. The composite of these two shows a soft ferromagnetic behavior reminiscent of the ferrite along with 3 DPTs. There is strong coupling between the two orders - magnetostrictive and piezoelectric in the composite. The capacitance decreases by 45% in the presence of magnetic field corresponding to a sensitivity of 0.9% kOe^-1, an extremely large value. The magnetoelectric coupling constant is found to be 20.6 mVcm^-1Oe^-1, a large value for a bulk composite. Microwave band stop filters of different thicknesses made from the composite have resonant frequencies which upshift in the presence of magnetic field indicating a multiferroic behavior with possibility for electric field tuning of resonant frequency.

Coupling two order parameters in a quantum gas

Controlling matter to simultaneously support coupled properties is of fundamental and technological importance¹ (for example, in multiferroics^2-5 or high-temperature superconductors^6-9). However, determining the microscopic mechanisms responsible for the simultaneous presence of different orders is difficult, making it hard to predict material phenomenology^10,11 or modify properties^12-16. Here, using a quantum gas to engineer an adjustable interaction at the microscopic level, we demonstrate scenarios of competition, coexistence and mutual enhancement of two orders. For the enhancement scenario, the presence of one order lowers the critical point of the other. Our system is realized by a Bose-Einstein condensate that can undergo self-organization phase transitions in two optical resonators¹⁷, resulting in two distinct crystalline density orders. We characterize the coupling between these orders by measuring the composite order parameter and the elementary excitations and explain our results with a mean-field free-energy model derived from a microscopic Hamiltonian. Our system is ideally suited to explore quantum tricritical points¹⁸ and can be extended to study the interplay of spin and density orders¹⁹ as a function of temperature²⁰.

Theoretical spin-wave dispersions in the antiferromagnetic phase AF1 of MnWO4 based on the polar atomistic model in P2

The spin wave dispersions of the low temperature antiferromagnetic phase (AF1) MnWO₄ have been numerically calculated based on the recently reported non-collinear spin configuration with two different canting angles. A Heisenberg model with competing magnetic exchange couplings and single-ion anisotropy terms could properly describe the spin wave excitations, including the newly observed low-lying energy excitation mode [Formula: see text] meV appearing at the magnetic zone centre. The spin wave dispersion and intensities are highly sensitive to two differently aligned spin-canting sublattices in the AF1 model. Thus this study reinsures the otherwise hardly provable hidden polar character in MnWO₄.

Multiferroic nanocomposite fabrication via liquid phase using anodic alumina template

We report a novel and inexpensive fabrication process of multiferroic nanocomposite via liquid phase using an anodic alumina template. The sol-gel spin-coating technique was used to coat the template with ferrimagnetic CoFe₂O₄. By dissolving the template with NaOH aqueous solution, a unique nanotube array structure of CoFe₂O₄ was obtained. The CoFe₂O₄ nanotube arrays were filled with, and sandwiched in, ferroelectric BaTiO₃ layers by a sol-gel spin-coating method to obtain the composite. Its multiferroicity was confirmed by measuring the magnetic and dielectric hysteresis loops.