Room-temperature Single-phase Research Articles

Magnetoelectric coupling at microwave frequencies observed in bismuth ferrite-based multiferroics at room temperature

• Room-temperature single phase multiferroic materials was obtained • Magnetoelectric coupling was observed at microwave (GHz) frequency • The coupling suggests a dynamic interaction between magnetic moments and ferroelectric polarization 0.63Bi 0.99 La 0.01 Fe 1- x Mn x O 3 -0.33BaTiO 3 -0.04Na 0.5 Bi 0.5 TiO 3 ( x = 0.10, 0.15, and 0.20) ceramics have been prepared by solid state reaction, and their structural, ferroelectric, ferromagnetic, piezoelectric, and magnetoelectric properties are studied. Except for the x = 0.2 composition, the X-ray diffraction and SEM-EDX analysis on the sintered ceramics show that the samples are single-phase, rhombohedral perovskites at room temperature. The introduction of Mn ions leads to a small deterioration in the ferroelectric and piezoelectric properties, whilst the room-temperature magnetization increases upon doping. The optimal multiferroic properties are obtained for the x = 0.15 composition. The magnetoelectric coupling in this sample was evidenced by the anomalies occurring simultaneously in the dielectric permittivity and magnetic permeability spectral profiles in the microwave region. Such a coupled phenomenon can be attributed to a dynamic interaction between magnetic moments and ferroelectric polarization.

Structural and charge carrier dynamics study of Dy stabilized La6MoO12 ionic conductors

Room temperature single phase stabilization of different metals (Zr, Cd, and Ca) doped La6MoO12 compound was achieved by incorporating Dy2O3 using the solution combustion method. Rietveld refined X-ray diffraction profiles showed cubic fluorite (Fm3¯m) structure for all the compositions. The optical bandgap, calculated from ultraviolet-visible spectra, exhibited blue shifts with decreasing dopant ionic radius. The photoluminescence spectra confirmed the charge carrier's various trapping or defect states. The complex impedance plot exhibited non-Debye-type relaxation with a negative temperature coefficient of resistance (NTCR). It also indicated that the dopant might act as an acceptor or donor depending upon its valency. The ac conductivity was found to follow the small polaron hopping mechanism. A thermally activated single relaxation process was observed. The composition La5.4Dy0.4Zr0.2MoO12.1 showed the highest conductivity, 1.05 × 10−4 Ω−1 cm−1 at 680 °C. The scaling of spectra indicated temperature and composition independent charge carrier mechanism.

Significantly enhanced hybrid improper ferroelectricity of Ca3Ti2O7 ceramics by the oxygen vacancy engineering

Ca3Ti2O7 materials with hybrid improper ferroelectricity call tremendous interest due to their great potential in designing novel room-temperature single-phase multiferroics with a strong magnetoelectric coupling effect. However, achieving Ca3Ti2O7-based ceramics with superior room-temperature ferroelectricity remains a crucial challenge. Herein, Ca3Ti2O7 ceramics were prepared by a tartaric acid sol–gel method and sintered in air and oxygen atmospheres, respectively. It was found that the long-time high-temperature sintering in air could lead to the formation of CaTiO3/Ca3Ti2O7/CaTiO3 sandwich structure, and oxygen-enriched sintering could effectively avoid the formation of CaTiO3 phase on the surface. The superior hybrid improper ferroelectricity is attained in Ca3Ti2O7 ceramics sintered in oxygen atmosphere with a higher remnant polarization (3.63 μC/cm2) and lower coercive electric field (72.6 kV/cm) obtained by positive-up and negative-down method, which is better than the ceramic sample sintered in air. The significantly better ferroelectricity is benefited from the synergistic effects of its larger distortion of oxygen octahedron, larger grain size, and lower oxygen vacancy concentration. Due to its low leakage current density, Ca3Ti2O7 ceramics sintered in oxygen atmosphere exhibit the remanent polarization as high as 7.40 μC/cm2 under the conditions of 1 Hz and 200 kV/cm obtained by dynamic hysteresis measurement mode. Furthermore, there are obvious stripe ferroelectric domains, which provide direct evidence for the excellent hybrid improper ferroelectricity of Ca3Ti2O7 ceramics. These findings demonstrate that the oxygen vacancy engineering is an effective strategy to improve hybrid improper ferroelectricity of Ca3Ti2O7 materials.

Effect of pH value on multiferroic properties of barium ferrite ceramics prepared by sol–gel method

In this paper, BaFe12O19 (BaM) ceramics at different pH values were prepared by sol–gel method. Effects of pH value on the microstructure, electrical as well as magnetic properties were systematically studied. XRD indicate that the ceramics were mainly BaM. However, with the pH value increasing further, the BaFe2O4 impurity phase appears gradually in both powders and ceramics due to the degree of complexation between citric acid and metal cations and other reasons. SEM results show that the grain size of ceramics decreases from 2.28 μm to 1.35 μm with the pH value increase from 4 to 8. It is found that room temperature dielectric constant, residual polarization, and saturation magnetization of the ceramics is the largest when the pH value of the solution is 4, which are 0.108 μC/cm2 and 32.829 emu/g, respectively. The dielectric constant and residual polarization of ceramics decrease with the increase of pH value. The ferroelectric and magnetic hysteresis loops have confirmed room-temperature multiferroic properties of the prepared ceramics. It laid the foundation for the further development of new room temperature single-phase multi-iron materials.

Superior hybrid improper ferroelectricity and enhanced ferromagnetism of Ca3(Ti1-xCox)2O7 ceramics through the superexchange interaction

Ca3(Ti1-xCox)2O7 ceramics were prepared by a tartaric acid sol-gel method and sintered in an oxygen atmosphere. The introduction of Co2+/Co3+ as acceptor dopants leads to the formation of more oxygen vacancies and defect dipoles in Ca3(Ti1-xCox)2O7 ceramics. Oxygen vacancy and defect dipoles lead to the transition of dielectric, leakage, and ferroelectric behaviors of Ca3(Ti1-xCox)2O7 ceramics. The coexistence of hybrid improper ferroelectricity and ferromagnetism at room temperature in Ca3(Ti1-xCox)2O7 ceramics has been successfully realized through the superexchange interaction of Co–O–Co. Ca3(Ti1-xCox)2O7 ceramics exhibit superior ferroelectricity (the remnant polarization is 3.29 μC/cm2) and enhanced ferromagnetism (the remnant magnetization reaches 6.4×10−3 emu/g). This strategy based on the introduction of transition metal ions with unfilled 3d shells at B sites is an important approach to realize novel room-temperature single-phase multiferroic materials for Ca3Ti2O7-based materials.

Room-temperature multiferroic behaviour in Co/Fe co-substituted layer-structured Aurivillius phase ceramics

Room-temperature single-phase multiferroic materials have attracted the considerable attention of may scientists due to their technological applications in the next-generation electronic devices. Here, we report the structural evolution and its relationship with the ferroelectric and magnetic properties of Aurivillius Bi3.25Sm0.75Ti(3-x) (Fe,Co)xO12 (BSmT:FCx); for (0≤x ≤ 0.4) ceramics to elucidate the room-temperature multiferroics induced by Fe/Co co-substitution. The XRD analysis and structural refinements reveal that incorporating of Co, and Fe ions into Ti sites of BSmT host lattices gives rise to a single orthorhombic phase with no evidence of secondary phases. The X-ray photoelectron spectroscopy (XPS) spectra indicate the existence of oxygen vacancies in the investigated samples. Pure BiT samples exhibit diamagnetic behavior, while BSmT:FCx; for (0≤x ≤ 0.4) exhibits ferromagnetic behavior at room temperature. As a result, the sample with x = 0.4 displayed the highest remnant and saturation magnetization values. It is believed that the occurrence of magnetism causes by the magnetic double exchange interaction between the non-equivalent magnetic ions and oxygen. The reversible polarization induced by an electric field indicates the ferroelectric character of the ceramics at room temperature. The co-substituting samples show unsaturated P-E hysteresis loops, which could be caused by the domain pinning induced by the accumulation of oxygen vacancies near the domain boundaries. We expect these compounds are promising for developing electronic devices for future applications.

Non- d0 ferroelectricity from semicovalent superexchange in bismuth ferrite

Empirical ${d}^{0}$ rule, i.e., magnetic unpairing $d$ occupation tends to be chemically incompatible for ferroelectrics, has for long inhibited the availability of room-temperature single-phase multiferroics. Here, we readdress the ferroelectricity of type-I multiferroic $\mathrm{BiFe}{\mathrm{O}}_{3}$, and conclusively clarify hybrid origins of lone-pair electrons' instability. Surprisingly, the sole lone-pair mechanism cannot account for overall polarization. Thus, beyond the popular belief, we propose that, with half-filling ${t}_{2g}^{3}{e}_{g}^{2}$ configuration and stereochemical activity of Bi's lone pair preparticipating, renormalized nonempty Fe $3d$ orbitals render $\mathrm{BiFe}{\mathrm{O}}_{3}$ a strong p-d charge-transfer insulator, and further induce non-${d}^{0}$ ferroelectricity through antiferromagnetic semicovalent superexchange. The magnetoelectric coupling scenario is numerically elaborated by mean-field p-d model, and first-principles generalized gradient approximation $\mathrm{GGA}+U$ calculations. The magnetically induced ferroelectricity also reconciles the competitivity of octahedral rotations and ferroelectricity, which turns into cooperative behavior as octahedral rotations straighten the $\mathrm{Fe}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Fe}$ bonds and strengthen superexchange interactions. Such mechanism is in principle ubiquitous in magnets with superexchange interaction and manipulatable to hybridization engineering.

Electric-field-controlled magnetism due to field-induced transition of Pna21/R3c in Bi1-xGdxFeO3 ceramics

BiFeO3 has been recognized as one of the most important room-temperature single-phase multiferroic materials, but it still suffers from several drawbacks especially the weak magnetoelectric coupling. In the present work, the electric-field-controlled magnetism is achieved in Bi1-xGdxFeO3 system, which involves a field-induced transition of Pna21/R3c at the morphotropic phase boundary region, and the magnetic state is switched between cycloidal state and canted antiferromagnetic state. The electric-field-controlled magnetism becomes reversible with the help of annealing, which is confirmed by magnetic hysteresis loops and the quantitative ratio of the involved phases for the as-sintered, as-poled and as-annealed samples. Compared with the systems of Bi1-xNdxFeO3 and Bi1-xSmxFeO3, it is easier to tune the symmetry from R3c to Pna21 with lower rare earth-content, and the field-induced transition is more apparent and subsequently leads to more significant electric-field-controlled magnetism in a wider composition range.

Role of annealing temperature in tailoring Ce-Doped LaFeO3 features

Recently, LaFeO3 has become the focus of intense research effort because it is a room-temperature single-phase multiferroic. Here, La0.80Ce0.20FeO3 nanoparticles have been prepared as a single phase on the nanoscale by a flash combustion methodology. The powder was annealed at different temperatures to investigate the influence of the annealing temperature on the physico-chemical features. X-ray diffraction analysis of La0.80Ce0.20FeO3 samples confirmed an orthorhombic structure with the space group Pbnm. The morphologies of the samples were studied by field-emission scanning electron microscopy (FESEM). The samples have a sponge-like morphology, and the pore size decreases with increasing annealing temperature. The molar magnetic susceptibilities of the samples were measured as a function of temperature, and showed an antiferromagnetic trend. For the sample annealed at 500 °C, the value of the effective magnetic moment, μeff, decreased as the Néel temperature, TN, increased. The dielectric properties and conductivities of the samples indicated a semiconductor-like nature, with two different conduction mechanisms. The morphological features of the investigated samples suggest their potential use in gas-sensing applications, water detoxification, and Li-ion batteries.

Remarkable enhancement in hybrid improper ferroelectricity of Ca3Ti2O7 ceramics by a simple sol-gel process

Ca3Ti2O7 with Ruddlesden-Popper structure is a typical hybrid improper ferroelectric with large polarization. But the room-temperature ferroelectricity of Ca3Ti2O7 ceramics prepared by a solid-state method is not good enough. In this work, Ca3Ti2O7 ceramics with remarkably enhanced hybrid improper ferroelectricity have been successfully fabricated by a simple tartaric acid sol-gel method. The ceramic sample exhibits the highest remnant polarization (4.32 μC/cm2) to our knowledge and lower coercive electric field (108.7 kV/cm) at room temperature. The enhanced ferroelectricity of Ca3Ti2O7 ceramics results from the larger antiferroelectric distortion displacements and the larger grain with irregular polygon features. These findings are beneficial for developing room-temperature single phase multiferroic materials with strong magnetoelectric coupling and excellent ferroelectricity.

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.

Structural, strong magnetic exchange, and room temperature multiferroic properties of single-phase 0.75BiFeO3–BaFe1/2Nb1/2O3 bulk ceramics

The ceramic 0.75BiFeO3–BaFe1/2Nb1/2O3 (0.75BF–BaFN) exhibits enhanced magnetisation and polar order regions at room temperature. This study investigated the microscopic structure and the magnetic, ferroelectric, magneto-ferroelectric, and optical properties of 0.75BF–BaFN by conducting experiments and performing density functional theory (DFT) calculations. Evidence of weak amplitude R-type octahedral rotations was observed using transmission electron microscopy (TEM). X-ray diffraction (XRD) refinement revealed that the P1 space group more appropriately described the structure of 0.75BF–BaFN. The weak influence of the magnetic field H on the polarisation of 0.7BF–BaFN and 0.75BF–BaFN ceramics was investigated. A notable magnetic enhancement was observed in 0.75BF–BaFN ceramics with a saturation magnetisation of 0.19 μB per Fe. DFT calculations predicted that the stable magnetic configuration of 0.75BF–BaFN is a G-type antiferromagnetic (AFM) structure. This study found stronger magnetic interactions between first-neighbour Fe ions (Ja = 12 meV) compared to pure BiFeO3. With a significant remanent moment, strong exchange coupling, and magnetic field control of polarisation, 0.75BF–BaFN may facilitate the development of room temperature single-phase magnetoelectric components.

Phase Stabilities of High Entropy Alloys

High entropy alloys exhibit a composition design route that different from conventional alloys. However, whether the high configurational entropy is the key factor that stabilizes the room temperature single phase solid solution is still under debate. Here we use a perturbation model to evaluate their phase stabilities. According to the calculation results of 7085 alloys, it clearly indicates that a stable single phase is rare when constituent elements increase. The effect of increased entropy cannot balance the fast-increased driving force for the formation of intermetallics. Although it is not favored by thermodynamics, the frequently observed room temperature single-phase solid solutions can still form due to dynamic effect, which from the other side, provides a method to tune the microstructure and therefore the mechanical properties.

Nonvolatile eight-state memory prototype based on single-phase multiferroic hexaferrite at room temperature

Abstract Multiferroic materials exhibit both ferroelectricity and ferromagnetism, and their magnetoelectric effects can be modulated by both magnetic and electric fields. Thus, they exhibit great potential in next-generation data memory devices. In this study, we implemented an eight-state memory prototype based on room-temperature single-phase multiferroic Sr 3 Co 2 Fe 24 O 41 . Data were imported by a poling procedure with electric and magnetic fields and derived visa electric polarization, P m , or the magnetoelectric coefficient, α m . This memory prototype possesses the advantages of a single structure, being easy to integrate, having a high storage capacity and a high stability. In this research, we enriched the magnetoelectric coupling performance of multiferroic materials, which demonstrates their possibilities for applications in the memory field.

High Rate Li-Ion Batteries with Cation-Disordered Cathodes

Summary Cation disordering in electrode materials for Li-ion batteries is deemed as an undesired structural feature for achieving high electrochemical activity owing to sluggish Li diffusion in the structure. Therefore, extensive efforts have been made to develop electrode materials having a well cation-ordered structure. We report a new class of cation-disordered material that can achieve reasonable Li diffusion by controlling the way of the cation distribution rather than the degree of cation disordering itself. The fully cation-disordered LiFeSO4F, activated by changes in the synthesis temperature and the Li/Fe ratio, delivers a substantially improved electrochemical performance. It exhibits a superior rate capability up to 100 C (36-s discharge) with ∼ 60 mAh/g and excellent capacity retention for 2,500 cycles at 5 C charge/20 C discharge rate. These results extend the scope of electrode materials to include cation-disordered materials and provide new opportunities for designing them for high-performance Li-ion batteries.

Crystal structures, dielectric properties, and phase transition in hybrid improper ferroelectric Sr3Sn2O7-based ceramics

Hybrid improper ferroelectricity is induced by the coupling of oxygen octahedral rotation and tilt, and it may provide a promising way to develop new room-temperature single-phase multiferroic materials with a strong magnetoelectric effect. Though ferroelectric polarizations are confirmed by the electric hysteresis loops at room temperature, the ferroelectric-paraelectric phase transition has not been well understood. In the present work, the crystal structures, dielectric properties, and phase transition behaviors of Ruddlesden-Popper Sr3–xCaxSn2O7 ceramics are thoroughly determined. Ferroelectricity is confirmed through the polarization-electric field hysteresis loops at room temperature, and the Curie temperature is identified by the variable-temperature differential scanning calorimetric analysis and dielectric characterization. The polarizations and Curie temperatures of the present ceramics increase with increasing the calcium content. An improper ferroelectric transition is experimentally evidenced by a small dielectric constant and a step variation of dielectric constant around the Curie temperature, and the thermal hysteresis of dielectric constant confirms the first order transition nature. Moreover, the fitting result of Landau phase transition theory also suggests the first-order improper ferroelectric transition nature. The variable-temperature Raman result implies a tetragonal paraelectric phase above TC. The present work confidently confirms the first-order hybrid improper ferroelectric transitions in the present ceramics, and ferroelectric properties can be readily tuned by substituting the cations at the A-site.

Symmetry Modulation and Enhanced Multiferroic Characteristics in Bi1‐xNdxFeO3 Ceramics

AbstractBiFeO3 is recognized as the most important room temperature single phase multiferroic material. However, the weak magnetoelectric (ME) coupling remains as a key issue, which obstructs its applications. Since the magnetoelectric coupling in BiFeO3 is essentially hindered by the cycloidal spin structure, here efforts to improve the magnetoelectric coupling by destroying the cycloidal state and switching to the weak ferromagnetic state through symmetry modulation are reported. The structure is tuned from polar R3c to polar Pna21, and finally to nonpolar Pbnm by forming Bi1‐xNdxFeO3 solid solutions, where two morphotropic phase boundaries (MPBs) are detected. Greatly enhanced ferroelectric polarization is obtained together with the desired weak ferromagnetic characteristics in Bi1‐xNdxFeO3 ceramics at the compositions near MPBs. The change of magnetic state from antiferromagnetic (cycloidal state) to ferromagnetic (canted antiferromagnetic) is confirmed by the observation of magnetic domains using magnetic force microscopy. More interestingly, combining experiments and first‐principles‐based simulations, an electric field‐induced structural and magnetic transition from Pna21 back to R3c is demonstrated, providing a great opportunity for electric field‐controlled magnetism, and this transition is shown to be reversible with additional thermal treatment.

Synthesis and ferroic and multiferroic studies on Bi1-xNdxFe0.99Co0.01O3 compositions

The magnetoelectric materials have been being studied due to the scientific chalenges and the very promissing possibility of applications in electronic devices. BiFeO3 is room temperature single-phase multiferroic material. However there are some issues regarding conductivity and undesired parasitic phases that cause the experimentally reduced electrical polarization and low or no net magnetization in this material. In order to solve these questions we studied the effect of partial replacement of Bi3+ by Nd3+ in the Bi1-xNdxFe0.99Co0.01O3 compositions. X-ray diffraction patterns confirmed single phased rhombohedrally distorted perovskite structure in obtained ceramics. Scanning electron microscopy images reveal that the ceramic grains are almost irregular spheres with a 2 μm in diameter, and samples present equal porosity. With the increasing of the Nd3+ substitution the real part of the dielectric constant increased in almost 10 times but decreases the magnetization. As a consequence, the Bi1-xNdxFe0.99Co0.01O3 ceramics presented an induced magnetoelectric voltage and magnetodielectric effects.

Research progress of multiferroicity in Bi-layered oxide single-crystalline thin films

Room temperature multiferroics with a single phase is very rare, and magnetic elements doped Bi-layered Aurivillius oxides are an important family of room temperature single phase multiferroics. However, due to the lack of single crystalline samples, the multiferroic related researches of these materials are mostly based on polycrystalline bulk or thin film samples. And the multiferroic characterizations are performed mostly by using the bulk type of samples. Therefore the studies of the origin and mechanism of the multiferroicity of these materials are extremely difficult. Recently, multiple magnetic elements doped singlecrystalline thin films have been successfully prepared, which makes it possible to study the physics mechanism of the Bi-layered Aurivillius oxides of multiferroicity. The current study shows that most of the single-crystalline thin films exhibit in-plane orientated spontaneous ferroelectric polarization and very weak room temperature magnetism. Moreover, at low temperatures the single-crystalline films exhibit a second magnetic transition. The resonant inelastic X-ray scattering experiments indicate that the doped structure exhibits a changed crystal field split, which may enhance the weak ferromagnetism through Dzyaloshinskii-Moriya interaction. On the other hand, the polarized neutron reflectivity experiments reveal that the single-crystalline thin film possesses much weaker room temperature magnetism than the bulk sample, which indicates that the origin of the magnetism and the magnetoelectric coupling in the single-crystalline samples are different from those in the polycrystalline samples. The current study of the multiferroicity in the single-crystalline Bi-layered Aurivillius thin film opens the road to designing better multiferroic systems of the Aurivillius materials.

Hybrid improper ferroelectricity and multiferroic in Ruddlesden-Popper structures

Hybrid improper ferroelectricity (HIF) is a secondary ferroelectric ordering induced by the coupling between oxygen octahedral in-plane rotation and out-of-plane tilt in a metal-oxide containing the perovskite structure units. Investigation of HIF will greatly extend the connotation and denotation of ferroelectric physics and material science, and it is expected to develop the room temperature single phase multiferroic material with large polarization and strong magnetoelectric coupling, owing to its intrinsic characteristic of the electric-field control of magnetism through HIF in magnet. In the present paper, the recent primary progress of HIFs and the multiferroics with Ruddlesden-Popper structures is reviewed, and the perspective of the future development is also presented.