Room Temperature Multiferroic Materials Research Articles

Improved magnetoelectric coupling of Co–Ti substituted barium hexaferrite at room temperature and related electrical investigations

This paper reports a systematic study of the electrical, multiferroic and magnetoelectric coupling properties of Co–Ti substituted Barium hexaferrites, having the chemical composition BaFe12-2xCoxTixO19 (x = 0–2.0; in steps of 0.5), synthesized using the sol gel auto-combustion method. X-ray Diffraction (XRD) studies and Rietveld refinement confirmed the successful incorporation of Co–Ti ions in the hexaferrite lattice with all the samples displaying Pb63/mmc space group single phase structure. Scanning Electron Microscope (SEM) images revealed the inhibition of crystal growth in a particular direction with Co–Ti substitution. The conduction mechanisms of the samples were analysed from the conductivity measurements and were found to follow Jonschers power law. The complex impedance spectroscopy analysis and the Cole-Cole plot fitted diagrams of the samples indicated the enhancement of bulk resistivity and grain boundary resistivity with substitution. Saturation magnetizations of the substituted samples decreased and were ascribed to the replacement of Fe ions by Co–Ti ions at different crystallographic sites. The observed reduction in anisotropy constant indicated the disturbance in the magnetic axis. The multiferroic nature of the substituted samples at room temperature was confirmed from the soft ferroelectric loops. Room temperature magnetoelectric coupling coefficient was found to be largely enhanced with substitution, achieving an exceptionally high value of 182mV/cmOe for the x = 1.5 sample. This may prove useful for potential use in emerging device applications as a single phase room temperature multiferroic material.

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.

Near room temperature hexagonal multiferroic (Yb0.25Lu0.25In0.25Sc0.25)FeO3 high-entropy ceramics

In this work, h-RFeO3 multiferroic high-entropy ceramics are designed by introducing high-entropy concept with four elements (Yb, Lu, In, Sc) for R site. (Yb0.25Lu0.25In0.25Sc0.25)FeO3 high-entropy ceramics are prepared by the standard solid-state reaction methods. The samples prepared with precursors were identified to crystalize in the pure hexagonal structure with polar space group (P63cm) at room temperature. By the analysis of the variable XRD patterns, it is speculated that the ferroelectric transition temperature of the sample is above 773 K. Meanwhile, three magnetic anomalies are determined for high-entropy ceramics, which is the antiferromagnetic transition at Néel temperature TÑ280 K, the spin-reorientation temperature TSR∼145 K and the reasonable ferromagnetic-like clusters without long ordering in TN < T < TA (∼397 K). Besides, it is observed that the peak intensity of the pyroelectric current around TN can be tuned by magnetic field, which is indicated the magnetoelectric coupling effect in (Yb0.25Lu0.25In0.25Sc0.25)FeO3 high-entropy ceramics. These results indicate the ferroelectric and antiferromagnetic orders are coexist in the h-RFeO3 high-entropy ceramics as promising near room temperature multiferroic materials, and it is an effective way to obtain a new type single-phase multiferroic materials.

Room temperature ferroelectricity and ferromagnetism in Ni-substituted Bi5Ti3FeO15

Aurivillius-type bismuth layer-structured ferroelectric (BLSF) Bi5Ti3FeO15 (BTF) has recently attracted considerable attention as a typical multiferroic material because ferroelectric and magnetic orders coexist, but bulk BTF exhibits antiferromagnetic (AFM) orders and negligible intrinsic magnetoelectric (ME) coupling effects. In this study, nickel-substituted Bi5Ti3FeO15 (Bi5Ti3Fe0.5Ni0.5O15, abbreviated as BTF-Ni) was synthesized using a solid-state reaction method to explore and enhance both the magnetic and ferroelectric properties of BTF. Polarization-electric field P-E loops indicate that the BTF-Ni exhibits considerable maximum polarization P m of 11.9 μC/cm2 and remnant polarization P r of 5.8 μC/cm2, but still keeps a very high ferroelectric Curie temperature (FE T c) of 1029 K, which are much superior to those of pure BTF. Moreover, magnetization-magnetic field M-H loops indicate that BTF-Ni exhibits significant ferromagnetic properties with a large saturation magnetization M s of 60 memu/g, low coercive field H c of 31 Oe at room temperature, and a high ferromagnetic Curie temperature (FM T c) of 698 K, whereas pure BTF has an antiferromagnetic Néel temperature (T N) of 80 K. Our work suggests that nickel-substituted BTF is a potential room-temperature magnetoelectric multiferroic material.

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).

Spin Hall Magnetoresistance in Pt/BiFeO3 Bilayer

Multiferroic materials are general antiferromagnets with negligibly small net magnetization, which strongly limits their magnetoelectric applications in spintronics. Spin Hall magnetoresistance (SMR) is sensitive to the orientation of the Néel vector, which can be applied for the detection of antiferromagnetic states. Here, we apply SMR on the unique room-temperature antiferromagnetic multiferroic material BiFeO3 (BFO). The angular dependence of SMR in a bilayer of epitaxial BFO (001) and heavy metal Pt is studied. By rotating the sample under a magnetic field of 80 kOe in the film plane, the resistance shows the maximum when the field is perpendicular to the current while it shows the minimum when the field is along the current. This can be well explained by the SMR in the bilayer of heavy metal/antiferromagnet with the relative orientation between the Néel vector and current direction. In contrast, the angular dependence of the resistance of Pt directly deposited on a SrTiO3 (001) substrate shows a 90° shift with the magnetic field rotating in the film plane, which originates from the Hanle magnetoresistance of Pt. The obtained spin mixing conductance at the Pt/BFO interface clearly confirms the efficient spin transmission. Our results provide a possible solution for applications of antiferromagnetic multiferroic materials in spintronics.

Structural origin of room-temperature ferroelectricity in spark-plasma sintered DyCrO3 and LaCrO3

Identification of novel multiferroic materials with high-ordering temperatures remains at the forefront of condensed matter physics research. Recently, room-temperature ferroelectricity of structural origin, arising from off-centering displacements of Gd and Cr ions, has been identified in spark-plasma sintered (SPS) ${\mathrm{GdCrO}}_{3}$ [Suryakanta Mishra et al., Phys. Rev. B 104, L180101 (2021)]. Here, using a similar synthesis protocol (involving SPS), we have been able to engineer room-temperature ferroelectricity (FE) from a similar mechanism in two otherwise nonferroelectric $\mathit{R}{\mathrm{CrO}}_{3}$ (R= rare-earth) compounds, namely, ${\mathrm{DyCrO}}_{3}$ (which is reported as a quantum paraelectric) and ${\mathrm{LaCrO}}_{3}$ (which is already known to be paraelectric). Room-temperature FE in SPS-${\mathrm{LaCrO}}_{3}$ and SPS-${\mathrm{DyCrO}}_{3}$ is confirmed through various electrical, calorimetric, and synchrotron-based structural investigations. Out of these two emergent room-temperature FE materials, SPS-${\mathrm{LaCrO}}_{3}$ also undergoes a high-temperature antiferromagnetic ordering at 290 K, thus coming very close to becoming the first room-temperature multiferroic material in this promising family of $\mathit{R}{\mathrm{CrO}}_{3}$ compounds.

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.

Magnetic field modulation of photocurrent in BiFeO&lt;sub&gt;3&lt;/sub&gt; film

BiFeO&lt;sub&gt;3&lt;/sub&gt; (BFO) is a kind of room temperature multiferroic material with bulk photovoltaic effect, and it has been a research hotspot in the field of multifunctional materials in recent years. The coexistence of the coupling among magnetic, optical, electrical properties brings rich and complex physical connotations. In this work, BiFeO&lt;sub&gt;3&lt;/sub&gt; thin film is deposited on FTO substrate by pulsed laser deposition, and the solar cell structure with BiFeO&lt;sub&gt;3&lt;/sub&gt; film used as light absorption layer and Au film serving as electrode is constructed. X-ray diffraction and Raman spectra indicate that the BFO film grown on FTO substrate has a pure phase structure. The experimental results of physical properties indicate that the BFO film possesses good ferromagnetic and ferroelectric properties and obvious photoelectric effect. According to the hysteresis loop, the remanence (&lt;i&gt;M&lt;/i&gt;&lt;sub&gt;r&lt;/sub&gt;) of the sample is 0.8 emu/cm&lt;sup&gt;3&lt;/sup&gt;, and the coercivity (&lt;i&gt;H&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt;) is 200 Oe at 300 K. In terms of ferroelectricity, the saturation polarization intensity of the sample can reach 0.997 μC/cm&lt;sup&gt;2&lt;/sup&gt;, the residual polarization intensity is 0.337 μC/cm&lt;sup&gt;2&lt;/sup&gt;, and the coercive electric field is 12.45 kV/cm. The above results show that the BFO film has good multiferroic properties. Under solar illumination conditions, the photocurrent density up to 208 mA/cm&lt;sup&gt;2&lt;/sup&gt; is obtained when a bias voltage 1 V is applied. More importantly, magneto-photocurrent (MPC) effect is found in the BFO film. No matter whether the magnetic field starts to increase from the positive direction or the negative direction, the MPC usually changes with the magnitude of magnetization. When a 1.3 kOe magnetic field is applied, the magneto-photocurrent change rate up to 232.7% is observed under standard solar illumination condition. The results show that the photocurrent of BFO films is greatly improved by a positive magnetic field and negative magnetic field. This magneto-photocurrent effect in BFO thin film comes from the photo-magnetoresistance effect, that is, the photogenerated electrons become spin photoelectrons under the action of an external magnetic field and receive spin-dependent scattering during moving in the conductive band of the material, thus producing the photo-magnetoresistance effect. In addition, the magneto-photocurrent effect is further enhanced by weakening the domain wall scattering of the spin electrons by the magnetic field. This work provides a reference for the modulation effect of magnetic field and light field on the magnetic, optical and electrical properties in multiferroics, and presents a foundation for the research and application of devices in the field of multifunctional optoelectronic materials.

What lies beneath? Investigations of atomic force microscopy-based nano-machining to reveal sub-surface ferroelectric domain configurations in ultrathin films

Multiferroic materials, encompassing simultaneous ferroelectric and ferromagnetic polarization states, are enticing multi-state materials for memory scaling beyond existing technologies. Aurivillius phase B6TFMO (Bi6TixFeyMnzO18) is a unique room temperature multiferroic material that could ideally be suited to future production of revolutionary memory devices. As miniaturization of electronic devices continues, it is crucial to characterize ferroelectric domain configurations at very small (sub-10 nm) thickness. Direct liquid injection chemical vapor deposition allows for frontier development of ultrathin films at fundamental (close to unit cell) dimensions. However, layer-by-layer growth of ultrathin complex oxides is subject to the formation of surface contaminants and 2D islands and pits, which can obscure visualization of domain patterns using piezoresponse force microscopy (PFM). Herein, we apply force from a sufficiently stiff diamond cantilever while scanning over ultrathin films to perform atomic force microscopy (AFM)-based nano-machining of the surface layers. Subsequent lateral PFM imaging of sub-surface layers uncovers 45° orientated striped twin domains, entirely distinct from the randomly configured piezoresponse observed for the pristine film surface. Furthermore, our investigations indicate that these sub-surface domain structures persist along the in-plane directions throughout the film depth down to thicknesses of less than half of an Aurivillius phase unit cell (˂ 2.5 nm). Thus, AFM-based nano-machining in conjunction with PFM allows demonstration of stable in-plane ferroelectric domains at thicknesses lower than previously determined for multiferroic B6TFMO. These findings demonstrate the technological potential of Aurivillius phase B6TFMO for future miniaturized memory storage devices. Next-generation devices based on ultrathin multiferroic tunnel junctions are projected.

The ferromagnetic resonance (FMR) of SrZ hexaferrite (Sr3Co2Fe24O41), and the tuning of FMR with an external magnetic field

The hexagonal Z-type ferrite Sr3Co2Fe24O41 (SrZ) was first synthesised in 2001 and reported as being a room temperature multiferroic material in 2010, with subsequent investigations into its multiferroic properties, but little into high frequency and microwave properties, and ferromagnetic resonance frequency (FMR), which determines its ability as an electromagnetic (EM) absorber and radar absorbing (RAM) stealth material. It was shown that SrZ existed as a majority or single phase after heating in a narrow temperature range between 1170 and 1190 °C using X-ray diffraction (XRD) and measurement of magnetic hysteresis loops, with the sample appearing to be single phase SrZ at 1190 °C. We measured complex permeability and permittivity of a single phase polycrystalline ceramic sample of SrZ between 500 MHz and 8 GHz (X-band). The sample had a relatively high permittivity >17 over this entire frequency range, and it showed a strong ferromagnetic resonance (FMR) at 2.3 GHz. This FMR could also be tuned by the effect of an external magnetic field, by moving a simple bar magnet progressively closer to a toroidal sample. This incurred a very slight shift in the peak up to 2.48 GHz at distances of 2.5–10 cm from the sample – a tuning of ∼5–6% with applied magnetic fields estimated to be 0.11–0.23 T, which is not insignificant. At a close distance of 0.5 mm we got a high degree of tuning of FMR to 3.4 GHz, a large change of 1.07 GHz (= 46% increase) with an applied magnetic field estimated to be 0.40 T. Despite this, the applied field had no significant effect on permittivity over 0.5–8 GHz. Such results have never been reported before, and are significant, as this would enable tuning of the FMR via simple physical/mechanical movement of a bulk alloy magnet, effectively creating a tuneable microwave filter or absorber.

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.

Effect of annealing temperature on structural, optical, magnetic and electrical properties of CrFeO3 multiferroic nanoparticles: A validation to first-principle calculations

Recently, room-temperature multiferroic materials have attracted great interest as they are suitable for the design of low-power electronic and spintronic devices. In this paper, we have investigated the effect of annealing temperature on the structural, electrical and magnetic properties of CrFeO3 multiferroic nanoparticles prepared by sol-gel auto-combustion method. The structural properties by observing X-ray diffractogram and its thorough Rietveld refinement confirmed the trigonal crystal structure with space group R3̅. The crystallite size calculated from the W-H plot increases from 39.6 nm to 100.5 nm with the increase of annealing temperature. FTIR results confirmed the evolution of ilmenite type ABO3 phase. Electron microscopy images showed increase of grain size due to agglomeration of nanoparticles at higher annealing temperature. EDXA spectra and color mapping demonstrated the elemental purity. Magnetic measurements revealed the antiferromagnetic nature of CrFeO3 multiferroic nanoparticles. Electrical measurements showed the decrease in dielectric constant with the increase in frequency. We have validated the results by running systematically first-principle calculations (Density Functional Theory) on CrFeO3 multiferroic nanoparticles.

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.

Two-Dimensional Organic-Inorganic Room-Temperature Multiferroics.

Organic-inorganic multiferroics are promising for the next generation of electronic devices. To date, dozens of organic-inorganic multiferroics have been reported; however, most of them show a magnetic Curie temperature much lower than room temperature, which drastically hampers their application. Here, by performing first-principles calculations and building effective model Hamiltonians, we reveal a molecular orbital-mediated magnetic coupling mechanism in two-dimensional Cr(pyz)2 (pyz = pyrazine) and the role that the valence state of the molecule plays in determining the magnetic coupling type between metal ions. Based on these, we demonstrate that a two-dimensional organic-inorganic room-temperature multiferroic, Cr(h-fpyz)2 (h-fpyz = half-fluoropyrazine), can be rationally designed by introducing ferroelectricity in Cr(pyz)2 while keeping the valence state of the molecule unchanged. Our work not only reveals the origin of magnetic coupling in 2D organic-inorganic systems but also provides a way to design room-temperature multiferroic materials rationally.

Structure domains induced nonswitchable ferroelectric polarization in polar doubly cation-ordered perovskites

Double perovskites with cation ordering allow the coexistence of ferroelectricity and magnetization in the same phase, and thus become a good candidate for room-temperature multiferroic materials. In this paper, we successfully synthesized A- and B-site ordered double perovskites NaLnBWO6 (Ln = La, Nd, and BMg, Mn) ceramics, and investigated their microstructure by combining X-ray diffraction, aberration-corrected scanning transmission electron microscopy, and optical second harmonic generation measurements. At room temperature, both NaLaMgWO6 and NaNdMgWO6 ceramics possess the space group of P21/m, while NaLaMnWO6 has a polar crystal structure (P21) without inversion symmetry. All three samples exhibit the A-site layered ordering and B-site rock-salt ordering as directly imaged by the HAADF-STEM images. The observed structural domains arising from the cation ordering well explain the nonswitchable ferroelectric polarization due to a large energy barrier. These findings suggest that it is necessary to eliminate structural domains during the sample synthesis for obtaining large ferroelectric polarization in double perovskites.

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.

Magnetoelectric features in the magnetic hysteresis of modified multiferroic BiFeO3: Release of latent magnetization induced by cationic modification

BiFeO3 is a well-known room temperature multiferroic material. However, the presence of antiferromagnetic magnetic order and high leakage current limit its utilization in magnetoelectric devices. This study deals with conclusive identification of the presence of magnetoelectric coupling by carefully selecting higher bond strength cationic substitution and using contactless measurement. Complementary to the magnetic studies, detailed structural analyses are carried out to determine the B-site bond length, B-site bond angle, ionic size mismatch and electronic structure of the substituent. The structural modification due to cationic substitution leads to release of latent magnetization that is locked within the cycloid. Non-monotonic temperature dependence of magnetic coercivity is observed and the results are confirmed for various compositions. These results are explained using Ginzburg-Landau Mean field model. Crossover from anomalous to usual trend of magnetic coercivity as a function of temperature is presented covering the temperature regime beyond Neel temperature of BiFeO3. Results are explained in terms of effective anisotropy which includes contribution associated with the electric polarization. This proves the existence of magnetoelectric effect. Magnetic switching is shown to be affected by the presence of ferroelectric order. The inclusion of normal ferromagnetic response in the high temperature regime makes this a unique study. The importance of this study lies in the fact that such magnetoelectric characteristic cannot be explored through the conventional electrical response measurements as the presence of high leakage current in BFO makes such measurements extremely difficult.

Colossal dielectric response, multiferroic properties, and gas sensing characteristics of the rare earth orthoferrite LaFeO3 ceramics

A great impact of the multifunctional or multiferroic materials on device applications has gained tremendous attention from scientific research groups in the field of materials science. The multiferroic materials bear superior properties of alternating the ferroic orders in a pre-designed way to match the device specifications. Materials in which there is a coupling between the primary ferroic orders (ferroelectric, ferromagnetic, ferroelastic) are termed multiferroics. This paper shows the investigation of a room temperature multiferroic material bearing a chemical composition LaFeO3 (LFO) also known as rare-earth orthoferrites. The particles of LFO are synthesized employing mixed oxide processing with heat treatment. The Rietveld analysis of the material shows that the LFO crystallizes having an orthorhombic symmetry and Pbnm space group. The complex impedance spectra indicate the distribution of relaxation time and non-Debye-type behavior. The dielectric parameters measured at various frequencies and temperatures suggest the formation of a colossal dielectric constant and low loss. The elemental analyses of the material show that there is no impurity present in the material. The pellet of the LFO arranged in a metal-insulator-metal arrangement is tested for gas sensing application. The LFO material has an excellent selectivity towards NO2 gas at relatively low operating temperatures ranging from 50 °C to 300 °C along with a sub-ppm limit of detection. A set of cyclic testing confirms that LFO can carry out stable and lengthy gas sensing operations while demonstrating outstanding adsorption and desorption characteristics. Such superior gas sensing properties and dielectric characteristics can pave the way toward the ultra-high sensitive gas sensors based on lead-free orthoferrites perovskites.

Enhanced dielectric, ferroelectric and magnetic properties of Ba4Sm2Fe2Nb8O30 RT multiferroics prepared by microwave sintering

High density Ba4Sm2Fe2Nb8O30 (BSFN) multiferroics ceramics with tetragonal tungsten bronze structure had been prepared by microwave sintering (MS) for 30min and conventional sintering (CS) methods for 4 h at 1275 °C. Single tungsten bronze phase and equiaxial grains are obtained for the MS BSFN ceramics, while a small amount secondary phase of SmNbO4 is observed in the CS BSFN ceramics with columnar grains. Compared to Ba4Sm2Fe2Nb8O30 ceramics prepared by CS method, enhanced dielectric, ferroelectric and magnetic properties are achieved for the MS BSFN ceramics. The values of electric polarization Pr and coercive electric field Ec are 2.11 μC/cm2 and 7.14 kV/cm for the MS BSFN ceramics, respectively. Meantime, the magnetic polarization Mr of 0.410emu/g and coercive magnetic field Hc of 2930Oe are also obtained for the MS BSFN ceramics. Based on the density, crystal structure, point defect and grain, the reasons of enhanced dielectric, ferroelectric and magnetic properties are discussed for the MS BSFN ceramics It is indicated that Ba4Sm2Fe2Nb8O30 is an intrinsic room temperature multiferroic materials.