Multiferroic Materials Research Articles

Composite quadrupole order in ferroic and multiferroic materials

The formalism of composite and intertwined orders has been remarkably successful in discussing the complex phase diagrams of strongly correlated materials and high-$T_c$ superconductors. Here, we propose that composite orders are also realized in ferroelectric and ferromagnetic materials when lattice anisotropy is taken into account. This composite order emerges above the ferroic phase transition, and its type is determined by the easy axis of magnetization or polarization, respectively. In multiferroic materials, where polarization and magnetization are coupled, composites of both orders are possible. This formalism of composite orders naturally accounts for magnetoelectric monopole, toroidal, and quadrupole orders. More broadly, composite orders may explain precursor phenomena in incipient (multi)ferroic materials, arising at temperatures above the ferroic phase transition and potentially contributing to the characterization of currently hidden orders.

Structural, Electronic, and Magnetic Properties of Gd-Substituted Bi2Fe4O9 Multiferroic: A Combined Experimental and DFT Approach

Abstract We present the structural, electronic, and magnetic properties of Gd3+ substituted Bi2Fe4O9 (BFO) via experimental analysis as well as density functional theory (DFT). Rietveld refined X-ray diffraction data shows phase purity of the samples having orthorhombic phase with space group: ‘Pbam’. Gd3+ ions substitution at Bi3+-site is confirmed by the shift in peaks ((002) and (220)) at higher 2θ angles as well as the reduction in lattice parameters. The PBE+U calculations predict a band gap of 1.76 eV (BFO) and 1.6 eV (Gd substituted BFO) which is in close agreement with the experimental values. This reduction in band gap due to Gd3+ substitution enhances conduction in substituted samples. The calculated density of states illustrates considerable hybridization between Fe-3d and O-2p states with substantial overlap among the Bi-6p and O-2p states. Incorporating Gd3+ ions further introduces additional exchange interactions between Gd-Fet and Gd-Feo, thus leading to enhanced magnetization as well as an increase in antiferromagnetic transition temperature (TN). This characteristic feature is supported by temperature-dependent magnetic susceptibility (χ) and dχ/dT plots. Hence, our experimental and theoretical findings suggest that BFO and its substituted samples are potential multiferroic materials for various device applications.

Correction: Synthesis, structural, improved magnetic and electrical properties of Sr2+ doped BiFeO3 multiferroic materials

Correction: Synthesis, structural, improved magnetic and electrical properties of Sr2+ doped BiFeO3 multiferroic materials

Ferromagnetic ferroelectricity due to the Kugel-Khomskii mechanism of orbital ordering assisted by atomic Hund's second rule effects

The exchange interactions in insulators depend on the orbital state of magnetic ions, obeying certain phenomenological principles, known as Goodenough-Kanamori-Anderson rules. Particularly, the ferro order of alike orbitals tends to stabilize antiferromagnetic interactions, while the antiferro order of unlike orbitals favors ferromagnetic interactions. The Kugel-Khomskii theory provides a universal view on such coupling between spin and orbital degrees of freedom, based on the superexchange processes: namely, for a given magnetic order, the occupied orbitals tend to arrange in a way to further minimize the exchange energy. Then, if two magnetic sites are connected by the spatial inversion, the antiferro orbital order should lead to the ferromagnetic coupling break the inversion symmetry. This constitutes the basic idea of our work, which provides a pathway for designing ferromagnetic ferroelectrics: the rare but fundamentally and practically important multiferroic materials. After illustrating the basic idea on toy-model examples, we propose that such behavior can be indeed realized in the van der Waals ferromagnet VI3, employing for this analysis the realistic model derived from first-principles calculations for magnetic 3d bands. We argue that the intra-atomic interactions responsible for Hund's second rule, acting against the crystal field, tend to restore the orbital degeneracy of the ionic d2 state in VI3 and, thus, provide a necessary flexibility for activating the Kugel-Khomskii mechanism of the orbital ordering. In the honeycomb lattice, this orbital ordering breaks the inversion symmetry, stabilizing the ferromagnetic-ferroelectric ground state. The symmetry breaking leads to the canting of magnetization, which can be further controlled by the magnetic field, producing a huge change of electric polarization. Published by the American Physical Society 2024

Strain-Induced Tunable Enhancement of Piezoelectricity in a Novel Molecular Multiferroic Material.

Multiferroics are appealing because of application potentials in data storage devices, sensors, transducers, and energy harvesters. Molecular multiferroics emerge as a promising alternative to inorganic multiferroics due to flexibility, light weight, low toxicity, solution processing, structural diversity, and chemical tunability. While researches have predominantly focused on perovskite structures, studies on molecular ionic multiferroics remain relatively limited. It is urgent to creatively build a novel platform for studying and developing the coupling and interaction between the stress, electricity, and magnetism. Knowing this, the work focuses on a novel organic-inorganic hybrid multiferroic N-ethyl-N-(fluoromethyl)-N-methylethylammonium tetrabromoferrate (III) showing coexisting magnetic and electric orderings. It undergoes antiferromagnetic, ferroelectric, and ferroelastic transitions. Notably, under a strain of 2.0%, the piezoelectric response increases tenfold, and the coercive field of ferroelectric polarization is reduced by half. The strain-induced enhancement of piezoelectricity is rarely reported in molecular multiferroics. Density functional theory is employed to predict that the mechanism of the large piezoelectric response under strain engineering is related to the cation rotation and phase switching between the stable phase and an energetically competitive metastable phase. This study creates a new paradigm to develop molecular multiferroics and future microelectronic devices for energy conversion.

Theoretical Study on BiCoO3: Pressure‐Tuned Transformation of Structure, Magnetism, Charge, and Spin State

AbstractThe atomic structures, electronic structures, magnetic properties, and spin states of BiCoO3 multiferroic material under hydrostatic pressure are comprehensively investigated by using first‐principles calculation based on density functional theory. Firstly, we found the pressure‐tuned structural symmetry transition from tetragonal P4 mm (No. 99) to cubic Pmm (No. 221) space group. The structural transformation prompts the Co─O polyhedral coordination transfer from a CoO5 pyramidal to a CoO6 distorted octahedron. Secondly, the pressure‐induced magnetic change from C‐type antiferromagnetic state to ferromagnetic state and then to paramagnetic configuration occurred after continuous compression of the volume. Essentially, the fundamental reason for the transition of the magnetic structure lies in the change of the internal atomic structure, such as the pressure induced abrupt changes of Co─O bond length and <Co─O─Co> bond angle. Meanwhile, an obvious volume collapse was observed when the pressure was applied to 51–56 GPa. Finally, as the pressure increases, the magnetism changes from C‐type antiferromagnetic state to paramagnetic configuration, accompanying the transformation from high spin state with b2g2eg2a1g1b1g1 configuration in pyramid structure to low spin state with t2g6eg0 in octahedral field. Therefore, our results demonstrate that applied hydrostatic pressure in BiCoO3 can trigger the transformation of structure, magnetism, charge, and spin state.

Localized Flexoelectric Effect Around Ba(CuNb) Nano‐Clusters in Epitaxial BiFeO3 Films for Enhancement of Electric and Multiferroic Properties

AbstractRoom‐temperature (RT) multiferroic materials have received significant research attention for various potential applications; however, their properties are not suitable for real‐world implementation. In this study, a nano‐scale localized flexoelectric effect is introduced to enhance the RT multiferroic performance of epitaxial bismuth iron oxide (BiFeO3; BFO) thin films by embedding 10 mol% Ba(Cu1/3Nb2/3)O3 (BCN) nano‐clusters into the host BFO film, which originally has a rhombohedral crystal structure. By utilizing nano‐clustering, a large out‐of‐plane coherent strain is localized around the nano‐clusters, resulting in a highly strained tetragonality of the BFO structure; subsequently, the films exhibit peculiar types of domains and domain walls, such as nano‐scale rotational vortices and antiparallel dipole configurations. These peculiar domain structures, which originate from the localized flexoelectric effect at the nano‐scale, enable excellent ferroelectric, ferromagnetic, and RT multiferroic magnetoelectric coupling. This study reveals that the local variation in the localized flexoelectric field around nano‐clusters considerably impacts the formation of unusual domain‐wall structures. This suggests that the controlled introduction of nano‐clusters with different crystal structures is promising for achieving the desired multiferroic properties.

Structural, Microstructural, Dielectric and Ac-conductivity of CaTi1-xFexO3 Multiferroic Materials Synthesized by Solid State Method

In this study, the structural, microstructural, dielectric and conductivity properties of Fe-doped CaTiO3 (CT) ceramic were investigated. These ceramics were successfully synthesized using the conventional solid-state method. The X-ray diffraction and Rietveld refinement results showed that the ceramics at x=0.0, 0.1, 0.2 and 0.3 crystalized in the orthorhombic phase with the Pmmm space group. While for x=0.4, 0.5 and 0.6, the phase formation changes from orthorhombic to tetragonal phase with P4/mmm group space. The SEM results revealed a semi-spherical shape of grain for lower Fe content (≤0.4), while at x=0.5 and 0.6 an agglomeration phenomenon is observed and the observed density of the sample at x=0.5 is higher than the other ceramics. The dielectric properties were studied as function of frequency and temperature. The dielectric permittivity (ɛ’r) decreases as function of frequency at lower frequency region and remain almost independent of frequency at high frequency region in the whole temperature range of R.T–520 °C. Each converges at high frequency (>105 Hz) for all the temperatures. The evolution of ɛ’r as function of temperature for pure CT ceramic showed a stability at large range of temperature (from R.T to 250°C). For substituted ceramics, we obtained a phase transition which shifted to the lower temperature with the increase of Fe content. And the value of ɛ’r, increases for Fe-doped CT ceramics and reaches a maximal value for x=0.5 which is 50 time bigger than the pure ceramic. The AC conductivity, σAC, is found to increase rapidly as a function of frequency, at low frequency region, and increases linearly at high frequency region, which confirms the hopping of electrons related to the conduction mechanism. The σAC evolution is found to follow the Jonscher’s law and the s exponent decreased with the increasing temperature which is related to the correlated barrier height (CBH) model. And the σAC values increase with the increases of Fe content.

Synthesis, Structural, Morphology and Magnetic Properties: Effect of La on Multiferroic Nature of BiFeO3 Nanoparticles

Objectives: The present study focuses on developing La-doped BiFeO3 multiferroic materials for dielectric absorber applications. Methods: The samples are characterized by X-ray Diffractometer (XRD). Further, the morphology is examined using field emission electron microscopy (FESEM) and Transmission Electron Microscopy (TEM). The dielectric parameters and ac-electrical parameters are carried out by impedance spectroscopy. The magnetic properties are studied using a vibrating sample magnetometer VSM, P-E loop. Findings: XRD confirms the rhombohedral (trigonal) structure of BLFO nanoparticles. The grain and particle size values are determined by SEM and TEM, respectively. The dielectric and ac-electrical parameters for all the samples are thoroughly discussed at room temperature. The Maxwell-Wagner or interfacial polarization is noticed at low input field frequencies for x=0.2-0.8. The impedance analysis shows the contribution of grain and its boundary in the electrical conduction mechanism. The multiferroic behavior is examined using M-H loops, P-E loops, and μi-f plots. Novelty: The samples are synthesized by the hydrothermal process to prepare La-doped BiFeO3 nanomaterials. Keywords: Nanoparticles, Electron Microscopy, Multiferroic, Magnetization, Polarization, Permeability

Study on the Microstructure and Performance of the Multi-Field Composite-Assisted Laser Cladding of Nickel-Based Tungsten Carbide Coatings

Improving the hardness and wear resistance of die cutting tools is an important issue in the study of the service life of die cutting equipment. Using laser cladding technology, nickel-based composite coatings with varying BiFeO3 contents were prepared on a 45 steel substrate, because BiFeO3 can have an effect on the dilution rate and microstructure of the sample; morover BiFeO3 is a new type of multiferroic material with certain magneto-electric coupling effects which can be prepared for the study of added magnetic fields. The microstructure and morphology were characterized to determine the optimal BiFeO3 content. Based on the optimal addition of BiFeO3, a comparative analysis was conducted to investigate the effect of different magnetic field strengths under a composite energy field on the microstructure, hardness, and wear resistance of Ni-based WC cladding layers. The results show that the optimal addition of BiFeO3 was 5 wt%. At this concentration, there were no significant porosity defects in the coating, and the dilution rate was appropriate (4.77%). Additionally, the interface bonding strength was also increased. With optimal BiFeO3 addition, stirring with different magnetic field strengths was applied to the cladding layer, and the results show that the aspect ratio of the cladding layer gradually increased with increasing the alternating magnetic field strength. When the magnetic field strength in the composite energy field was 40 mT, the microstructure was fine and uniform, the hardness of the cladding layer reached the highest level, about 925.2 HV1.0, the wear resistance was also the best, the friction coefficient of the cladding layer was about 0.54, and the width of the wear mark was about 0.53 mm.

Synthesis, structural, improved magnetic and electrical properties of Sr2+ doped BiFeO3 multiferroic materials

Synthesis, structural, improved magnetic and electrical properties of Sr2+ doped BiFeO3 multiferroic materials

Fully spin-polarized two dimensional polar semi-metallic phase in Eu substituted GdI2 monolayer

Abstract The coexistence of seemingly mutually exclusive properties such as ferromagnetism, ferroelectricity and metallicity in atomically thin materials is the requirement of the hour in electronics as the Moore’s law faces an impending end. Only a few 2D multiferroic materials have been predicted/realized so far. The polar metals with simultaneous presence of polarity and conductivity are also equally rare. Here, we predict, based on first-principles calculations that an Eu-substituted rare-earth halide GdI2 monolayer showcases ferromagnetism, ferroelasticity while being polar and a fully spin-polarized semi-metal at the same time. The ferroelasticity and polarity are shown to be coupled making it possible to switch the polar direction using external mechanical stress. Further, it is observed that an application of biaxial tensile strain of 5% causes the spin easy-axis to shift from out-of-plane to in-plane direction. Thus, spin easy axis gets coupled with the direction of polarization in the strained monolayer making the switching of magnetization also possible using external strain. Simultaneous coexistence and coupling of the ferroic orders in a metallic 2D material makes the Eu substituted GdI2 monolayer an incredibly rare material for nano-electronics and spintronics applications.

Exploration of drag on the thin film of micropolar fluid flow within a permeable medium

Advances in thin film acquired many industrial applications because of their use in several products like electronic devices, magnetic recording media, LED, optical coating, etc. Additionally, thin film has an extensive role in the study of multiferroic materials and superlattices in quantum phenomena. Therefore, it is an urge to predict the importance of drag employing Darcy-Forchheimer on the thin film flow of micropolar liquid through an expanding plate embedding with a permeable medium. Moreover, the current model enriches by incorporating the impact of Soret because of the cross-diffusion process and the radiative heat. By using the appropriate transformed variables, the designed problem is distorted, and further, these sets of equations are handled numerically employing Runge-Kutta shooting technique. The computation is carried out by deploying in-house built-in function bvp4c in MATLAB. The significant operational behavior of the manipulated quantities is accessible graphically and deliberated in the discussion section. The important findings are elaborated as; the angular velocity distribution shows a greater deceleration for the increasing microrotation and the inertial drag effect. Further, the thermo-diffusion due to Soret also attenuates the concentration profile and the solutal thickness became thinner for the higher heavier species.

Role of in-situ electrical stimulation on early-stage mineralization and in-vitro osteogenesis of electroactive bioactive glass composites

Bioactive glass (BAG) has emerged as an effective bone graft substitute due to its diverse qualities of biocompatibility, bioactivity, osteoblast adhesion and enhanced revascularization. However, inferior osteogenic capacity of BAG compared to autologous bone grafts continues to limiting it's wide-spread clinical applications towards repairing of bone fractures and healing. In this study, we have fabricated BAG composites with 0.5 to 2 wt% bismuth ferrite (BF, a multiferroic material) with an aim to generate in-situ electrical charges pertinent to early-stage bone regeneration thus mimicking nature bone, which is a piezoelectric material. The fabricated BAG composites were characterised in terms of microstructures, phase analysis, remanent polarization, wettability and subsequently evaluated for in vitro cell proliferation and osteogenesis with and without magnetic field exposure (200 mT, 30 min./day). Pre-osteoblast cells from mice (MC3T3-E1) seeded on these composites exhibited excellent cell growth without any cytotoxicity, which is further supported by FITC/DAPI staining and a live/dead assay. The results of Alizarin Red S assay and increased levels of Alkaline Phosphatase (ALP) activity, at 21 days of culture, suggest that the BAG-BF composites promote in vitro osteogenic differentiation of pre-osteoblast cells. The enhanced osteogenesis of BAG-BF composites was also confirmed through qRT-PCR analysis, which showed rapid upregulation of osteoblastogenic specific genes namely RunX-2, Collagen-1, Bone Sialo Protein, and ALP after 21 days. Additionally, the osteogenic differentiation was assessed by the Western Blot technique, which revealed significantly higher band intensity of osteogenic markers in BAG-1.5 BF and BAG-2 BF composites than pure BAG. These findings clearly demonstrate that in-situ electrical stimulation and osteoconductive capacity of BF reinforced BAG composites have positive impact on osteoblast cell development, bone formation, and healing.

Tunable Schottky barriers and magnetoelectric coupling driven by ferroelectric polarization reversal of MnI3/In2Se3 multiferroic heterostructures

Two-dimensional (2D) multiferroic materials are recognized as promising candidates for next-generation nanodevices due to their tunable magnetoelectric coupling and distinctive physical phenomena. In this study, we proposed a novel 2D multiferroic van der Waals heterostructure (vdWH) by stacking atomic layers of ferroelectric In2Se3 and ferromagnetic MnI3. Using first-principles calculations, we found that the MnI3/In2Se3 vdWH exhibit robust metallic conductivity across various spin and polarization states, preserving the distinctive band characteristics of isolated In2Se3 and MnI3. However, the alignment of Fermi levels causes the conduction band minimum (CBM) and valence band maximum (VBM) of In2Se3 and MnI3 to shift relative to their original band structures. Remarkably, the MnI3/In2Se3 with the upward polarization state of In2Se3 exhibits an Ohmic contact. Switching the polarization direction of In2Se3 from upward to downward can transform the MnI3/In2Se3 vdWH from an Ohmic contact to a p-type Schottky contact, while also modifying its dipole moment, magnetic strength and direction. Based on these properties of MnI3/In2Se3 vdWH, we designed the field-effect transistors (FETs) with high on/off rates and nonvolatile data storage device. Furthermore, the Schottky barrier heights (SBHs), magnetic moment, and dipole moment of MnI3/In2Se3 vdWH can also be effectively regulated by reducing the interlayer distance. With the continuous reduction of the interlayer distance of MnI3/In2Se3 vdWH, its easy magnetization axis is expected to shift from in-plane to out-of-plane. These findings offer new insights for the design and development of the next-generation spintronic and nonvolatile memory nanodevices.

Pressure-induced multiple structural phase transitions on multiferroic CaMn7O12

CaMn7O12 (CMO) is an outstanding multiferroic material known for its significant magnetically-induced electric polarization. Its strong magnetoelectric (ME) effect, i.e., electric polarization controlled via magnetic fields or vice versa, makes this material suitable for a variety of technological applications. In this study, we employed synchrotron X-ray powder diffraction to explore polycrystalline CMO's pressure-induced structural phase transitions (SPTs). Our results indicate that CMO undergoes two distinct pressure-induced SPTs: the first transition occurs at pressures above 7.0 GPa, changing from a rhombohedral to an orthorhombic structure, and the second occurs around 13.0 GPa, transforming into a monoclinic structure. These findings differ from the pressure-induced behavior of CMO single crystals and highlight CMO as one of the rare quadruple perovskites exhibiting multiple pressure-induced non-isostructural phase transitions. This study expands the understanding of phase stability behavior in multiferroic materials under high-pressure conditions.

Coexistence of ferroelectricity and antiferroelectricity in 2D van der Waals multiferroic

Multiferroic materials have been intensively pursued to achieve the mutual control of electric and magnetic properties. The breakthrough progress in 2D magnets and ferroelectrics encourages the exploration of low-dimensional multiferroics, which holds the promise of understanding inscrutable magnetoelectric coupling and inventing advanced spintronic devices. However, confirming ferroelectricity with optical techniques is challenging in 2D materials, particularly in conjunction with antiferromagnetic orders in single- and few-layer multiferroics. Here, we report the discovery of 2D vdW multiferroic with out-of-plane ferroelectric polarization in trilayer NiI2 device, as revealed by scanning reflective magnetic circular dichroism microscopy and ferroelectric hysteresis loops. The evolution between ferroelectric and antiferroelectric phases has been unambiguously observed. Moreover, the magnetoelectric interaction is directly probed by magnetic control of the multiferroic domain switching. This work opens up opportunities for exploring multiferroic orders and multiferroic physics at the limit of single or few atomic layers, and for creating advanced magnetoelectronic devices.

Oxygen vacancy induced electrical conductivity enhancement in Ca-doped BiFeO3 thin films

Bismuth ferrite, recognized for its exceptional physical properties as a multiferroic material, has recently attracted considerable attention due to its high domain wall conduction. However, challenges in further improving domain wall conductivity and magnetoelectric coupling hamper the practical application of pure-phase BFO thin films. Addressing these limitations, we successfully synthesized Bi1-xCaxFeO3 thin films on LaAlO₃ (001) substrates with varied calcium (Ca) doping levels (x = 0–0.3) through pulsed laser deposition. Our experimental data demonstrate a direct relationship between the films' electrical conductivity and the prevalence of oxygen vacancies (OV). An upward trend in both OV density and electrical conductivity emerges with incremental Ca doping, peaking at x = 0.2. Beyond this doping threshold, a further increase in Ca concentration inversely affects OV density and electrical conductivity. These insights suggest that fine-tuning OV concentration in Ca-doped BFO thin films can significantly modulate their electrical properties, paving the way for the advancement of bismuth ferrite-based electronic devices.

Engineering Topological Spin Hall Effect in 2D Multiferroic Material.

Topological spin Hall effect (TSHE), promoted by coupling between noncoplanar spins and real-space topology, is a significant phenomenon in condensed matter physics. However, the control of TSHE characteristics is missing due to its intrinsic robustness, and such fundamental difficulty prevents it from being used for spintronics up to now. Here, a rational design approach is demonstrated to engineer TSHE in a controllable and reversible fashion. Through symmetry and model analysis, it is unveiled that antiferromagnetic topological charge, as well as Lorentz forces, acted on conduction electrons, can be coupled with Dzyaloshinskii-Moriya interaction chirality for antiferromagnetic bimerons in 2D multiferroic materials. Such coupling guarantees the ferroelectric control of TSHE. Using first-principles calculations and atomic spin model simulations, the validity of this mechanism is further demonstrated in multiferroic monolayer CuCr2Se4 with experimental feasibility. The alter-chirality of the Dzyaloshinskii-Moriya interaction is found to play a crucial role in realizing this mechanism. This results extend TSHE to be used in spintronics and open a new direction for spintronics research.

Evidence of multiferroic behavior in sintered BaTiO3 obtained from high-energy ball-milled powders

Multiferroic BaTiO3 exhibiting ferroelectric and ferromagnetic behavior was synthesized via the high-energy ball milling of pure BaTiO3 (BTO) powders for durations ranging from 15 to 60 min, followed by pressing and sintering at 1200°C X-ray diffraction patterns of all synthesized samples predominantly revealed a BTO phase with a tetragonal structure and a secondary Ba12Fe28Ti15O84 (BFTO) phase. The BFTO phase was formed after milling for more than 30 min because of chemical interactions between the BTO powder and milling media. Vibrating sample magnetometry confirmed the ferromagnetic nature of the sintered pellets. The specific magnetization increased with increasing milling duration, reaching a maximum value of 1.15 emu/g after 60 min of milling. This increase can be attributed to the distortion of the crystal structure and presence of a secondary phase, as confirmed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. Additionally, electrical characterization revealed the dielectric nature of the materials, with relative permittivity ranging from 500 to 1800, maximum spontaneous polarization from 9.77 to 11.31 μC/cm2, coercive field from 3.86 to 11.12 kV/cm, and AC conductivity from 1 x 10−6 to 1 x 10−3 S/cm. The method described in this study is a simple and cost-effective approach to produce multiferroic materials with ferroelectric and relaxor ferroelectric behavior at room temperature, broadening their potential for technological applications.