Room-temperature Multiferroics Research Articles

Control of Crystallization Pathways in the BiFeO3-Bi2Fe4O9 System.

Bismuth ferrites, specifically perovskite-type BiFeO3 and mullite-type Bi2Fe4O9, hold significant technological promise as catalysts, photovoltaics, and room-temperature multiferroics. However, challenges arise due to their frequent cocrystallization, particularly in the nanoregime, hindering the production of phase-pure materials. This study unveils a controlled sol-gel crystallization approach, elucidating the phase formation complexities in the bismuth ferrite oxide system by coupling thermochemical analysis and total scattering with pair distribution function analysis. We tune the crystallization pathways in the BiFeO3-Bi2Fe4O9 system by adjusting the metal to complexing agent ratio and pH during precursor preparation with a fixed Bi/Fe ratio of 1:2. Although all precursors undergo an amorphization process during heating, our results demonstrate a consistent correlation between the crystallization pathway and the initial structural entities formed during gel formation. Pair distribution function analysis reveals structural differences in the intermediate amorphous structures, which preferentially crystallize into either BiFeO3 or Bi2Fe4O9. This study offers mechanistic insights into the formation processes in the system and synthetic guidance for the controlled synthesis of pure Bi2Fe4O9 and mixed BiFeO3/Bi2Fe4O9 nanomaterials. Additionally, it elucidates the unusual growth behavior and structural size dependence of Bi2Fe4O9, particularly highlighting significant distortions in the local structure likely induced by the proximity of Bi's stereoactive lone electron pairs at small sizes.

Anisotropic magnetocapacitance of antiferromagnetic cycloids in BiFeO3

Distinguishing different antiferromagnetic domains by electrical probes is a challenging task, which in itinerant compounds can be achieved, e.g., via the anisotropic magnetoresistance. Here, we demonstrate that in insulators, the anisotropic magnetocapacitance can be exploited for the same purpose. We studied the magnetic field dependence of the dielectric response in BiFeO3, one of the few room-temperature multiferroics. We observed a sizeable dielectric anisotropy upon the rotation of the modulation vector of the antiferromagnetic cycloid in the plane normal to the rhombohedral axis. Importantly, this anisotropy is characteristic of the cycloidal mono-domain state even in zero magnetic field, thus facilitating the determination of the antiferromagnetic domain population. This approach can be utilized to electrically distinguish between antiferromagnetic domains even in complex magnets, such as modulated spin structures, via the magnetodielectric coupling.

Ultrathin Ternary FeCoNi Alloy Nanoarrays in BaTiO3 Matrix for Room-Temperature Multiferroic and Hyperbolic Metamaterial.

Multifunctional vertically aligned nanocomposite (VAN) thin films exhibit considerable potential in diverse fields. Here, a BaTiO3-FeCoNi alloy (BTO-FCN) system featuring an ultrathin ternary FCN alloy nanopillar array embedded in the BTO matrix has been developed with tailorable nanopillar size and interpillar distance. The magnetic alloy nanopillars combined with a ferroelectric oxide matrix present intriguing multifunctionality and coupling properties. The room-temperature magnetic response proves the soft magnet nature of the BTO-FCN films with magnetic anisotropy has been demonstrated. Furthermore, the anisotropic nature of the dielectric-metal alloy VAN renders it an ideal candidate for hyperbolic metamaterial (HMM), and the epsilon-near-zero (ENZ) wavelength, where the real part of permittivity (ε') turns to negative, can be tailored from ∼700 nm to ∼1050 nm. Lastly, room-temperature multiferroicity has been demonstrated via interfacial coupling between the magnetic nanopillars and ferroelectric matrix.

First-principles study on the p-orbital multiferroicity of single-layer XN (X = Ge, Sn, Pb)

Two-dimensional (2D) multiferroic materials have triggered a burst of interest owing to their wide applications in nanoelectronics. Specifically, p-orbital multiferroicity is strongly desired but has been very few reported. Here, based on first-principles calculations, we unveil a new type of 2D multiferroic material: the single-layer (SL) XN (X = Ge, Sn, Pb). Our findings show these materials are ferromagnetic semiconductors with strong magnetoelectric coupling and wide bandgaps. The multiferroicity is related to the unpaired p-orbital electrons of N atoms and the buckled crystal structure. All the single layers show easy-plane magnetocrystalline anisotropy, and the magnetic anisotropy energy increases significantly with the atomic number increasing. Our Monte Carlo simulations suggest the Curie temperature TC is 205.44 K, 200.45 K, and 287.88 K for SL GeN, SnN, and PbN, respectively. By applying tensile strain, the TC can be further increased to 225.39 K, 245.35 K, 369.99 K, respectively. Additionally, biaxial strain can induce semiconductor-to-half-metal and ferroelectricity-to-paraelectricity transition in the single layers. We aspire that our work contributes to the exploration of room-temperature p-orbital multiferroicity.

A Route to Two-Dimensional Room-Temperature Organometallic Multiferroics: The Marriage of d-p Spin Coupling and Structural Inversion Symmetry Breaking.

Two-dimensional (2D) room-temperature multiferroic materials are highly desirable but still very limited. Herein, we propose a potential strategy to obtain such materials in 2D metal-organic frameworks (MOFs) by utilizing the d-p direct spin coupling in conjunction with center-symmetry-breaking six-membered heterocyclic rings. Based on this strategy, a screening of 128 2D MOFs results in the identification of three multiferroics, that is, Cr(1,2-oxazine)2, Cr(1,2,4-triazine)2, and Cr(1,2,3,4-trazine)2, simultaneously exhibiting room-temperature ferrimagnetism and ferroelectricity/antiferroelectricity. The room-temperature ferrimagnetic order (306-495 K) in these MOFs originates from the strong d-p direct magnetic exchange interaction between Cr cations and ligand anions. Specifically, Cr(1,2-oxazine)2 exhibits ferroelectric behavior with an out-of-plane polarization of 4.24 pC/m, whereas the other two manifest antiferroelectric characteristics. Notably, all three materials present suitable polarization switching barriers (0.18-0.31 eV). Furthermore, these MOFs are all bipolar magnetic semiconductors with moderate band gaps, in which the spin direction of carriers can be manipulated by electrical gating.

Investigating magnetic, and magneto-electric properties of Nd doped [(1-y) Bi1-xNdxFeO3-yPbTiO3] solid solution

Magneto-electronic devices applications of room-temperature multiferroics have been made possible by recent developments. But the potential use of pure BFO in multifunctional devices is constrained by its antiferromagnetic nature and need substitution-induced modification to improve magnetic and magnetoelectric properties. In the present paper, attempt has been made to improve the magnetic and magnetoelectric properties of the BiFeO3-PbTiO3 solid solution by incorporating Bismuth cation with Lead(II) ion and Ferric cation with Titanium cation into the formulation of Nd doped,(1-y)Bi1-xNdxFeO3-yPbTiO3where x = 0.05,0.10,0.15,0.20 and y = 0.1, 0.2,0.3 and 0.4. The impact of different PbTiO3 and Nd compositions on magnetoelectric and magnetic characteristics of solid solution has been investigated. The results showed that, degree of magnetization continuously increases with increasing Nd doping by affecting Fe-O-Fe angles. However, the magnetization drops as the volume of PbTiO3 rises, that may be attributed to a rise in the tetragonal phase of solid solutions. The improvement of the magnetoelectric coupling coefficient and change of antiferromagnetic behaviour to ferromagnetic behaviour of solid solutions has been observed with increasing Nd doping. This enhance the ferromagnetic behaviour of the compound.

Two-Dimensional Asymmetric Multiferroics: Unique Way toward Strong Magnetoelectric Coupling and Multistate Memory.

Two-dimensional (2D) materials have provided a fascinating platform for exploring novel multiferroics and emergent magnetoelectric coupling mechanisms. Here, a novel 2D asymmetric multiferroic based on Janus 2D multiferroic MXene-analogous oxynitrides (InTlNO2) is presented by using first-principles calculations. We find three inequivalent phases for InTlNO2, including two metallic phases (p1 and p2) and one semiconducting phase (p3) with a band gap of 0.88 eV. All phases are room-temperature multiferroics with different Curie temperatures, leading to tunability by phase transitions. We show that there is a 90° rotation of the magnetic anisotropy easy axis between p1 and p2, where p1 favors the in-plane and p2 the out-of-plane easy axis. Therefore, the magnetic anisotropy can be tuned by reversing the out-of-plane polarization. Our strategy provides a unique way toward strong magnetoelectric coupling and multistate memory.

Physical mechanism of Ge doping enhanced Ruddlesden-Popper structure quasi-2D Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> ceramic hybrid improper ferroelectricity

Hybrid improper ferroelectricity with quasi-two-dimensional (quasi-2D) structure has attracted much attention recently due to its great potential in realizing strong magnetoelectric coupling and room-temperature multiferroicity in a single phase. However, recent studies show that there appears high coercive field and low remnant polarization in ceramics, which severely hinders the applications of this material. In this work, high-quality Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> and Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub> ceramics with a Ruddlesden-Popper (R-P) structure are successfully prepared, and their crystal structures and electrical properties are investigated in detail. It is found that the Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> ceramic exhibits a lower coercive field that is close to that of Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> single crystal. Moreover, via a small amount of Ge doping, the polarization reaches 0.34 μC/cm<sup>2</sup> for Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> and 0.61 μC/cm<sup>2</sup> for Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub>. Combining crystal lattice dynamic studies, we analyze the Raman and infrared responses of the samples, showing the information about the tilting and rotation of the oxygen octahedra in the samples. The improved ferroelectricity after doping may be attributed to the increased amplitude of the tilt mode and the reduced amplitude of rotation mode. Besides, the enhanced ferroelectric properties through Ge doping and its mechanism are further investigated by the Berry phase approach and the Born effective charge method. Furthermore, via the UV-visible spectra, the optical bandgap is determined to be 3.91 eV for Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> ceramic and 3.95 eV for Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub> ceramic. Using the Becke-Johnson potential combined with the local density approximation correlation, the bandgap is calculated and is found to be in close agreement with the experimental result. And the electronic excitations can be assigned to the charge transfer excitation from O 2p to Sn 5s (Ge 4s). The effects of Ge doping on the ability of Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> to gain and lose electrons and the bonding strength of Sn-O bond are analyzed via two-dimensional charge density difference. In conclusion, this study provides insights into the synthesis method and modulation of ferroelectric properties of hybrid improper ferroelectrics Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub>, potentially facilitating their widespread applications in various capacitors and non-volatile memory devices.

Rhenium substitution effects on the structural, morphological and magnetic properties of bismuth ferrite BiFe(1-x)RexO3

In order to fully realize room temperature multiferroicity, a release of remnant magnetization in the BiFeO3 (BFO) system is still needed. Herein, we describe novel Re-substituted BiFe1-xRexO3 compounds that were synthesized using an aqueous sol-gel procedure when ethylene glycol was used as a complexing agent. Rhenium substitution was used to disrupt the modulated magnetic structure due to its aliovalent nature and maintain electroneutrality in case of Bi evaporation. Obtained compounds were investigated using X-ray powder diffraction (XRD) analysis coupled with Rietveld refinement, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), as well as magnetometry measurements. The obtained data revealed that only tiny amounts of Re (≤0.75 %) can be introduced instead of Fe ions while maintaining a single-phase structure. Larger inclusions lead to the formation of two impurity phases with Pbam and I213 space groups. However, the typical R3c bismuth ferrite crystal structure remained dominant. SEM analysis indicated that the inclusion of Re ions into the crystal structure caused an increase in average particle size. XPS analysis confirmed the designed chemical compositions of the samples as well as the small amount of Fe2+, which is formed to compensate for the Re ions in the 6+ and 7+ states. Magnetometry measurements revealed that even sub 1 % of Re substitution led to a presence of non-zero remnant magnetization, which reaches 0.0449 Am2kg−1 at room temperature in the compound with x = 0.0075.

Engineering room temperature multiferroicity and temperature dependent dielectric properties of Cr doped BaTiO3 ceramics

The coexistence of ferroelectricity, ferromagnetism, and piezoelectricity at room temperature in BaTi1-xCrxO3 (0 ≤ x ≤ 0.06) nanostructures is interesting and leads to the multiferroic nature of system. Sol-gel auto-combustion technique has been used for the synthesis of nanostructures. The XRD patterns along with Rietveld refinement analysis, confirm that the samples possess parent tetragonal perovskite structure in the P4mm space group. The oxygen vacancies stoichiometry ratio was estimated through XPS studies. The scanning electron microscopy (SEM) confirms the large grain size in the doped sample. The study of leakage current density reveals an increase in its value with the increase in Cr content due to defects induced in the system. P-E measurements confirm the ferroelectric nature of the samples that diminishes in the doped samples due to the high leakage current. Moreover, the estimated value of energy efficiency (η) was found to be maximum for the undoped sample. The undoped BaTiO3 has a diamagnetic character that is partially altered into weak ferromagnetic under the Cr-doping at the expense of decreasing polarization parameters. The temperature-dependent dielectric constant shows all characteristics of phase transitions in the samples. The doping of Cr enhances the ferroelectric to paraelectric phase transition temperature (Tc). The piezoelectricity remains preserved in the doped samples. Thus, the coexistence of ferroelectricity, ferromagnetism, and piezoelectricity at room temperature finalizes the multiferroic nature of the synthesized samples.

Effect of oxygen vacancies and cationic valence state on multiferroicity and magnetodielectric coupling in (1-x)BaTiO3.(x)LaFeO3 solid solution

A solid solution of (1-x)BaTiO3–(x)LaFeO3 (x = 0, 0.007, 0.015, 0.031, 0.062) has been investigated for room temperature multiferroicity and magnetodielectric effect. The incorporation of La and Fe ions in ferroelectric BaTiO3 leads to increased lattice disorder and generation of oxygen vacancies that induces magnetism. A detailed correlated study using XRD, XPS and Raman spectroscopy is being reported. A typical ferromagnetic and ferroelectric nature of each sample are confirmed by M−H and P–E hysteresis loops, respectively at room temperature. Temperature dependent dielectric studies reveal the decrease of transition temperature from 137 0C for undoped to room temperature for increasing substitution. The changes in the dielectric property with applied magnetic field shows the indications of magnetodielectric effect. For x = 0.015, existence of both proper ferroelectric without lossy properties and ferromagnetic properties in a single tetragonal phase makes it a perfect room temperature multiferroic material. For this sample, the magnetodielectric coupling is the strongest with Magnetocapacitance of ∼2.7% and Magneto loss of ∼0.8% at 10 kHz and 1 T. The multiferroicity and the magnetodielectric effect exhibited by this material has been correlated to the structural properties, electric and magnetic properties, changes in valence states, and oxygen vacancy (Ov).

Room temperature magnetoelectric effect in Sr2FeNbO6 perovskite: A theory supported experimental investigation

Magnetoelectric multiferroic materials are those which can be influenced by both electric and magnetic fields and have wide applications. Single-phase materials that show magnetoelectric effects are very few compared to composites. Recently, several studies have been conducted on magnetic and dielectric properties of single-phase double perovskite Sr2FeNbO6, while magnetoelectric effects have not been reported yet. The present study aims to explore the magnetoelectric effects of Sr2FeNbO6 prepared through solid-state reaction route. Rietveld refinement confirms the orthorhombic symmetry of the material with Pbnm space group. The existence of Fe2+, Fe3+, and Nb5+ ions were investigated by XPS analysis. The room temperature magnetic and electric hysteresis loops display the simultaneous existence of magnetism and ferroelectricity in Sr2FeNbO6. A high Curie temperature of 727 K is obtained from temperature-dependent variation of magnetization, making it suitable for spintronics applications. Further, this room temperature multiferroicity of Sr2FeNbO6 was confirmed using magnetoelectric coupling measurement and obtained a coupling coefficient of 4.4 mV cm−1 Oe−1. The conclusion of Sr2FeNbO6 showing ferrimagnetic nature is obtained through density functional theory (DFT) calculations.

Weak trimer distortion and planar spin configuration in hexagonal Lu0.6In0.4FeO3

Improper ferroelectricity and canted ferromagnetism in antiferromagnetically ordered hexagonal ferrites with A2-type spin configuration have been intensely studied due to their potential for room-temperature multiferroicity with strong magnetoelectric couplings. However, the subtle interplay between the magnetic structure and trimer structural distortion, which is a critical ingredient for ferroelectricity, has not been clearly verified in experiments due to the lack of control over trimer distortion. In this study, we report on Lu0.6In0.4FeO3, which exhibits weaker trimer distortion primarily related to the smaller tilting of the FeO5 bipyramids. The reduced vertical displacement of the equatorial oxygen of FeO5 located at the center of the trimer lowers the magnitude of the in-plane Dzyaloshinskii–Moriya vector component, resulting in the absence of canted ferromagnetism with the planar A1-type spin configuration, rather than the canted A2-type observed in other hexagonal ferrites. Our findings demonstrate that the degree of trimer distortion plays an important role in determining the spin configuration in related hexagonal systems.

Room temperature multiferroicity of hexagonal LuFeO3 and its enhancement by co-doping in Lu0.9Co0.1Fe0.9Ti0.1O3 nanoparticle system

Antiferromagnetic LuFeO3 can be good multiferroic by having ferroelectricity in its non-centrosymmetric hexagonal phase. But, it is hard to stabilize this metastable phase, preventing the stable orthorhombic phase. In this work, metastable hexagonal LuFeO3 (LFO) nanoparticle was stabilized in chemical sol-gel route in pure phase and co-doped with Co and Ti in the same route to synthesize Lu0.9Co0.1Fe0.9Ti0.1O3 (LCFTO) nanoparticles. Room temperature multiferroicity of bare LFO was established through relevant characterization. And motive behind the co-doping is to enhance the magnetoelectric behavior of the bare system to synthesize a new monophasic type II magnetoelectric multiferroic. Structural investigation by thorough Rietveld analyses of the recorded X-ray diffractograms, confirmed the formation of pure hexagonal (P63cm) phase of both bare and doped LFO. However, deviations in various structural & microstructural parameters were observed in the doped system, which is mainly responsible for the enhancement of magnetic and electric properties of the sample. Presence of antiferromagnetic transition at ∼604 K confirmed the room temperature magnetic ordering of bare LFO. Interestingly, LCFTO shows a drastic enhancement of magnetic property than the bare one in all concerns, where the maximum magnetization at the maximum applied field is enhanced by nearly 36 times at room temperature. Detailed high-temperature dielectric investigation shows, good dielectric strength (∼261) of LFO gets enhanced highly in LCFTO (∼1053) and a high relaxation time having negligible loss factor with an indication of ferro to paraelectric transition above room temperature. Current density vs. electric field (J-E) curve suggests the presence of polarization at room temperature with negligible leakage loss. Direct measurement of ferroelectric loop shows the ferroelectricity (Pmax∼0.064 µc/cm2) of bare LFO at room temperature and a well improvement in the doped LCFTO (Pmax∼0.151 µc/cm2). The presence of room temperature magnetoelectric coupling, confirmed by magnetocapacitance measurements, results in a high value (∼5 %) of magnetocapacitance in the doped system which is also much higher than the bare one (<∼1 %) as expected. All these properties confirm the magnetoelectric multiferroicity of bare h-LuFeO3 at room temperature. And, co-doping in hexagonal LuFeO3 nanoparticle system results in a considerable improvement in its magnetoelectric behavior, so this co-doped system can be a promising and potential magnetoelectric multiferroic for the future generation magnetoelectric devices.

γ-BaFe2O4: a fresh playground for room temperature multiferroicity

Multiferroics, showing the coexistence of two or more ferroic orderings at room temperature, could harness a revolution in multifunctional devices. However, most of the multiferroic compounds known to date are not magnetically and electrically ordered at ambient conditions, so the discovery of new materials is pivotal to allow the development of the field. In this work, we show that BaFe2O4 is a previously unrecognized room temperature multiferroic. X-ray and neutron diffraction allowed to reveal the polar crystal structure of the compound as well as its antiferromagnetic behavior, confirmed by bulk magnetometry characterizations. Piezo force microscopy and electrical measurements show the polarization to be switchable by the application of an external field, while symmetry analysis and calculations based on density functional theory reveal the improper nature of the ferroelectric component. Considering the present findings, we propose BaFe2O4 as a Bi- and Pb-free model for the search of new advanced multiferroic materials.

Room-temperature multiferroicity in GaFeO3 thin film grown on (100)Si substrate

Room-temperature magnetoelectric multiferroicity has been observed in c-axis oriented GaFeO3 thin films (space group Pna21), grown on economic and technologically important (100)Si substrates by a pulsed laser deposition technique. Structural analysis and comprehensive mapping of the Ga:Fe ratio across a length scale range of 104 reveals coexistence of epitaxial and chemical strain. It induces formation of finer magnetic domains and large magnetoelectric coupling—a decrease in remanent polarization by ∼21% under ∼50 kOe. Magnetic force microscopy reveals the presence of both finer (&amp;lt;100 nm) and coarser (∼2 μm) magnetic domains. Strong multiferroicity in epitaxial GaFeO3 thin films, grown on a (100)Si substrate, brighten the prospect of their integration with Si-based electronics and could pave the way for development of economic and more efficient electromechanical, electrooptic, or magnetoelectric sensor devices.

Room-Temperature Type-II Multiferroic Phase Induced by Pressure in Cupric Oxide.

According to previous theoretical work, the binary oxide CuO can become a room-temperature multiferroic via tuning of the superexchange interactions by application of pressure. Thus far, however, there has been no experimental evidence for the predicted room-temperature multiferroicity. Here, we show by neutron diffraction that the multiferroic phase in CuO reaches 295K with the application of 18.5GPa pressure. We also develop a spin Hamiltonian based on density functional theory and employing superexchange theory for the magnetic interactions, which can reproduce the experimental results. The present Letter provides a stimulus to develop room-temperature multiferroic materials by alternative methods based on existing low temperature compounds, such as epitaxial strain, for tunable multifunctional devices and memory applications.

Room-temperature multiferroicity and magnetoelectric couplings in (Co0.75Al0.25)2(Fe0.75Mg0.25)O4 spinel films

The electric field manipulation of a magnetic order is a key feature of advanced information technologies and the development of room-temperature multiferroic magnetoelectrics remains an important task. In this work, we report novel single-phase magnetoelectric (ME) films consisting of a (Co0.75Al0.25)2(Fe0.75Mg0.25)O4 spinel ferrite. The (Co0.75Al0.25)2(Fe0.75Mg0.25)O4 thin films display a switchable polarization and magnetization with a remnant polarization of about 2.1 µC cm–2 and a remnant magnetization of about 82.0 emu cm–3 at 300 K. Electric field control of magnetism is detected, the magnetization modulation reaches 8.5 % at room temperature with an electric field of 500 kV cm−1, revealing the existence of magnetoelectric coupling. We believe that the substitutions of Al3+ and Mg2+ cations at the spinel sublattices might induce local distortions and local electric field and modulate the superexchange interactions between magnetic ions, which give rise to the ferroelectricity and ME effect in the (Co0.75Al0.25)2(Fe0.75Mg0.25)O4 thin films. The electric field-induced charge modulation may also contribute to the observed magnetoelectric effect, which likely results from the migration and redistribution of oxygen vacancies in the prepared nanofilms. The findings of this work can stimulate research studies on the development of room temperature multiferroics and magnetoelectrics based on the spinel ferrite compounds.

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.

Electronic structure, optical and electrical characteristics of multiferroic [Pb(Fe0.5Nb0.5)O3]0.4–[(Ba0.8Sr0.2)TiO3]0.6

A new compound with formulation 0.4[Pb(Fe0.5Nb0.5)O3] −0.6[(Ba0.8Sr0.2)TiO3] was prepared by solid state route. The structural properties were analyzed by X-ray diffraction in terms of POWD and Rietveld analyses. X-ray photoelectron spectroscopy study provides for electronic structure. Structural vibrations and chemical compositions were analyzed by FTIR and Raman spectroscopy. Morphological and elemental analyses were done by FESEM and EDS studies. UV–Visible analysis was done to study the narrow band gap, effect of oxygen vacancy and Urbach energy. Dielectric characteristics show the diffuse phase transition. The room temperature multiferroicity was confirmed by P-E and M-H loop studies.