Synthesis,structure, CO oxidation,and H2 production activities of CaCu3−xMnxTi4−xMnxO12 (x = 0, 0.5, and 1.0)

Synthesis of nanocrystalline pristine and Mn-doped calcium copper titanate quadruple perovskites, CaCu_3?xMn_xTi_4?xMn_xO₁₂ (x = 0, 0.5, and 1.0) by modified citrate solution combustion method has been reported. Powder X-ray diffraction patterns attest the phase purity of the perovskite materials. Average particle sizes of all the materials obtained from the Scherrer's formula are in the range of 55–70 nm. The specific surface areas for all the perovskites obtained from BET isotherms are found to be low as expected for the condensed oxide systems and fall in the range of 13–17 m² g^?1. Transmission electron microscopy studies show a reduction in particle size of CaCu₃Ti₄O₁₂ with increase in Mn doping. Ca and Ti are present in +2 and +4 oxidation states in all the materials as demonstrated by X-ray photoelectron spectroscopy analyses. Cu²⁺ gets reduced in CaCu₃Ti₄O₁₂ with higher Mn content. Mn is observed to be present only in +3 oxidation state. All the materials have been examined to be active in CO oxidation as well as H₂ production from methanol steam reforming. CaCu₃Ti₄O₁₂ with ~14 at.% Mn is found to show best catalytic activities among these materials. A comprehensive analysis of the catalytic activities of these perovskites toward CO oxidation and H₂ production from MSR reveal the cooperative activity of copper-manganese in the doped perovskites and it is more effective at lower manganese content.     

Effect of Mn-doping on the structure and electrical properties of (Pb0.325Sr0.675)TiO3 ceramics

Pb_0.325Sr_0.675Ti_1-xMn_xO₃ ceramics (x?=?0, 0.001, 0.005, 0.01, and 0.05) were successfully prepared by traditional solid-state reaction method. It was found that the lattice constant calculated through Rietveld refinement initially increased and then decreased with increasing Mn content, which was attributed to the variation in valence state of Mn and Ti ions. The microstructure gradually varied from the coexistence of large grains and fine grains for x?=?0 to the uniform grain for x?=?0.05 by increasing the doping Mn ions. With increasing Mn content from x?=?0 to x?=?0.05, the Curie temperature (Tc) dramatically decreased from 25?°C to ??40?°C and dielectric maximum decreased from 27,100 to 13,200. Pb_0.325Sr_0.675Ti_1-xMn_xO₃ ceramics with x?=?0.001 showed the lowest dielectric loss of 0.006 with a relatively high dielectric peak value of ~ 21,000. The grain boundaries resistance obtained from the complex impedance decreased with the increase of Mn content. The decrease in resistance was ascribed to oxygen vacancies and electronics produced by the change of ionic valence state. X-ray photoemission spectroscopy revealed that Ti ions were Ti⁴⁺ and the valences of Mn ions were deduced to be mainly in the form of Mn²⁺ and/or Mn³⁺ for ceramics with low content of Mn, while the Ti ions were in the form of Ti³⁺ and Ti⁴⁺ and Mn ions were diverse valence states with the coexistence of Mn²⁺, Mn³⁺, and Mn⁴⁺ for ceramics with x?=?0.01 and 0.05.     

Magnetic Field‐Induced Ferroelectric Switching in Multiferroic Aurivillius Phase Thin Films at Room Temperature

Single‐phase multiferroic materials are of considerable interest for future memory and sensing applications. Thin films of Aurivillius phase Bi₇Ti₃Fe₃O₂₁ and Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈ (possessing six and five perovskite units per half‐cell, respectively) have been prepared by chemical solution deposition on c‐plane sapphire. Superconducting quantum interference device magnetometry reveal Bi₇Ti₃Fe₃O₂₁ to be antiferromagnetic (T_N = 190 K) and weakly ferromagnetic below 35 K, however, Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈ gives a distinct room‐temperature in‐plane ferromagnetic signature (M_s = 0.74 emu/g, μ₀H_c =7 mT). Microstructural analysis, coupled with the use of a statistical analysis of the data, allows us to conclude that ferromagnetism does not originate from second phase inclusions, with a confidence level of 99.5%. Piezoresponse force microscopy (PFM) demonstrates room‐temperature ferroelectricity in both films, whereas PFM observations on Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈ show Aurivillius grains undergo ferroelectric domain polarization switching induced by an applied magnetic field. Here, we show for the first time that Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈ thin films are both ferroelectric and ferromagnetic and, demonstrate magnetic field‐induced switching of ferroelectric polarization in individual Aurivillius phase grains at room temperature.     

Percolation like transitions in phase separated manganites La0.5Ca0.5Mn1-xAlxO3-δ

We show that the replacement of Mn with Al strongly affects the magnetization and electrical transport behaviors in La_0.5Ca_0.5Mn_1-xAl_xO_3-δ (x = 0, 0.05, 0.07 and 0.09). Nonmagnetic Al³⁺ ions substitution at Mn sites dilutes Mn³⁺-O^2--Mn⁴⁺ network, thus suppresses the ferromagnetic metallic state in La_0.5Ca_0.5MnO_3-δ and causes a phase separation phenomenon, in which ferromagnetic phase coexists with antiferromagnetic charge ordered phase at low temperatures. With applying sufficiently high magnetic field, step-like metamagnetic transitions were observed in x = 0.05–0.09 systems below helium temperature, in which the antiferromagnetic charge ordered phase collapsed into ferromagnetic phase. Corresponding to the sharp step-like metamagnetic transitions, the resistivity decreases dramatically with increasing magnetic field, exhibiting a percolative insulator-metal transition. The variation of temperature and magnetic field changes the relative fractions of ferromagnetic and charge ordering phases, and percolative insulator-metal transition occurs due to the development of percolation paths between the growing FM domains.     

Complex permittivity and relaxation processes in CaCu3Ti(4−x)MnxO12

The effects of Mn-doping on CaCu₃Ti₄O₁₂ (CCTO), i.e. the electrical properties of CaCu₃Ti_(4−x)Mn_xO₁₂ were studied in order to get a deep insight into the origin of the colossal dielectric constant of CCTO. It was found that 1 mol% substitution of Mn for Ti decreases the dielectric constant largely to a factor of 1/100, supporting the point of view that the native defects are responsible for the dielectric response of CCTO.     

Effect of Fe/Ta doping on structural,dielectric, and electrical properties of Bi4Ti2.5Fe0.25Ta0.25O12 ceramics

Aurivillius single‐phase Bi₄Ti_2.5Fe_0.25Ta_0.25O₁₂ ceramics was prepared via high‐temperature solid‐state reaction method. The detailed structural analysis of the doped Bi₄Ti₃O₁₂ compound was carried out by Rietveld refinement of the full XRD Pattern. Dielectric and electrical properties were studied in a wide range of temperature and frequency by dielectric/impedance spectroscopies. The comprehensive analysis of frequency spectrum reveals the occurrence of two relaxation behaviors in the ceramics at low frequency and high frequency, respectively. A phase transition was observed at ~650°C in Bi₄Ti_2.5Fe_0.25Ta_0.25O₁₂ somewhat lower than the ferroelectric transition temperature of Bi₄Ti₃O₁₂. The possible reason for the decrease of ferroelectric transition temperature was discussed based on the structural analysis. The present results could be useful for designing and/or modifying properties of Bi₄Ti₃O₁₂‐related ceramics.     

High entropy BaFe12-x(Ti/Mn/Ga/In)xO19 (x = 1–7) oxides: Correlation of the composition,entropy state,magnetic characteristics,and terahertz properties

The BaFe_12-x (Ti/Mn/Ga/In)_xO₁₉ (x = 1–7) high-entropy oxides (HEOs) were obtained by solid-phase synthesis. The correlation of the chemical composition (the level of the Fe replacement), structural parameters, magnetic characteristics, and terahertz (THz) properties was investigated. All studied samples were single-phase (SG: P6₃/mmc). Lattice parameters showed a monotonic increasing trend with an increase of (x). The key role in increasing the lattice parameters of HEOs may belong to the influence of In³⁺ ions, which are much larger in size than Fe³⁺ ions. As one of the possible reason for explanation of the lattice parameters behavior we hypothesized that partial charge transformation of the Mn³⁺ state to the Mn²⁺ state takes place in order to maintain electrical neutrality. Other cations (Ti/Ga/In) have stable oxidation state. The anisotropic nature of the increase in lattice parameters was demonstrated. No strong correlation between chemical composition and microstructural parameters of the investigated HEOs was observed. The magnetic characteristics of the BaFe_12-x (Ti/Mn/Ga/In)_xO₁₉ (x = 1–7) HEOs were investigated at broad intervals of magnetic fields and temperatures. The behavior of the magnetic characteristics was shown to be the result of the magnetic structure frustration. In addition, terahertz electrodynamic properties were studied by measuring spectra of complex dielectric permittivity at frequencies 0.2–1.2 THz in the temperature interval 20–300 K. The spectra indicate the presence of a higher-frequency infrared phonon resonance, whose damping decreases with cooling, as well as an excitation below 0.2 THz that freezes out at low temperatures, the origin of which is associated with the polycrystalline nature of the materials.     

Zinc substitution effect on the structural,spectroscopic and electrical properties of nanocrystalline MnFe2O4 spinel ferrite

This paper reports the structural, morphological, spectroscopic, dielectric, ac conductivity, and impedance properties of nanocrystalline Mn_1-xZn_xFe₂O₄. The nanocrystalline Mn–Zn ferrites were synthesized using a solvent-free combustion reaction method. The structural analysis using X-ray diffraction (XRD) pattern reveals the single-phase of all the samples and the Rietveld refined XRD patterns confirmed the cubic-spinel structure. The calculated crystallite size values increase from 8.5 nm to 19.6 nm with the Zn concentration. The surface morphological analysis using field emission scanning electron microscopy and the transmission electron microscopy confirms the nano size of the prepared ferrites. X-ray photoelectron spectroscopy was used to study the ionic state of the atoms present in the samples. Further, the high-resolution Mn 2p, Zn 2p, Fe 2p, and O 1s spectra of Mn_1-xZn_xFe₂O₄ does not result in the appearance of new peaks with Zn content, indicating that the Zn substitution does not change the ionic state of Mn, Zn, Fe, and O present in nanocrystalline Mn_1-xZn_xFe₂O₄. The investigated electrical properties show that the dielectric constant, tan δ and ac conductivity gradually decrease with increasing Zn substitution and the sample Mn_0·2Zn_0·8Fe₂O₄ has the lowest value of conductivity at 303 K. The ac conductivity measured at different temperatures shows the semiconducting nature of the ferrites. The impedance spectra analysis shows that the contribution of grain boundary is higher compared with the grain to the resistance. The obtained results suggest that the Zn substituted manganese ferrite nanoparticles can act as a promising candidate for high-frequency electronic devices applications.     

Influence of manganese on the structure and magnetic properties of YFeO3 nanocrystal

High purity Mn doped YFeO₃ nanocrystals were readily prepared by sol-gel combustion method, with citric acid as the fuel. The doping effect on the structure and magnetic properties was systematically studied. X-ray diffraction and TEM results indicate that the samples are well crystallized. Orthorhombic YFeO₃ crystallites can be obtained when calcined at 800 °C. In YFe_1−xMn_xO₃ system for x ≤ 0.2, hexagonal structure exists as metastable phase and the calcined temperature has to be elevated to obtain pure orthorhombic phase. Distortion of the crystal structure was also observed, with changes in lattice parameters. The antiferromagnetic coupling was effectively strengthened with increased Mn content. This is possibly originated from the structural distortion which gives rise to enhanced antiferromagnetic superexchange interaction between Fe³⁺-Fe³⁺ and Fe³⁺-Mn³⁺.     

Structure,magnetic and electrical transport properties of the perovskites Sm1−xCaxFe1−xMnxO3

The structure of series Sm_1−xCa_xFe_1−xMn_xO₃ (0.0 ≤ x ≤ 1.0) compounds was investigated. The lattice parameters increase with coupled substitution Sm³⁺ by Ca²⁺ and Mn⁴⁺ for Fe³⁺. The variation of parameter, c, is larger than that of a and b, respectivly. The detailed analysis of magnetic properties of series Sm_1−xCa_xFe_1−xMn_xO₃ (0.1 ≤ x ≤ 0.9) shows that local magnetic interaction between Fe³⁺ and Fe³⁺ and Mn⁴⁺ and Mn⁴⁺ at below magnetic transition temperature is antiferromagnetic. Above magnetic transition temperature the presence of large magnetic cluster is proposed and the sizes of magnetic clusters decrease with Mn⁴⁺. The electrical transport behaviors related with small polaron hopping and variable range hopping models.     

Synthesis,optical, and magnetic properties of six-layered Aurivillius bismuth ferrititanate

This work reports on the preparation, structure, photochemical, and magnetic properties of six-layered Aurivillius bismuth ferrititanates, that is, Bi₇Ti₃Fe₃O₂₁, Bi₇(Ti₂Nb)Fe₃O_21+δ, and Bi₇(Ti₂Mg)Fe₃O_21−δ nanoparticles. The samples were prepared through the modified citrate complexation and precursor film process. The XRD Rietveld refinements were conducted to study the phase formations and crystal structure. The morphological and chemical component characteristics were investigated using SEM, TEM, and EDX analyses. Bi₇Ti₃Fe₃O₂₁, Bi₇(Ti₂Nb)Fe₃O_21+δ, and Bi₇(Ti₂Mg)Fe₃O_21−δ nanoparticles present an indirect allowed transitions with band energies of 2.04, 2.03, and 2.02 eV, respectively. The hybridized (O2p+Fet_2g+Bi6s) formed the valence band (VB) and electronic components of (Ti–3d+Fe–e_g) formed the conduction band (CB) of this six-layered Aurivillius bismuth ferrititanate. The three samples showed efficient photocatalytic degradation of Rhodamine B (RhB) dyes with the excitation wavelength λ > 420 nm. The optical absorption, photodegradation, and magnetic abilities were improved through microstructural modification on “B” site via partial substitution of Mg²⁺ and Nb⁵⁺ for Ti⁴⁺. The photocatalytic results were discussed based on the layer structure and multivalent Fe ions. Fe^3+/2+ in the perovskite slabs (Bi₅Fe₃Ti₃O₁₉)²⁻ could act as the catalytic mediators in the photocatalysis process. As a photocatalyst, Aurivillius Bi₇(Ti₂Mg)Fe₃O_21−δ nanoparticle is advantageous due to its photocatalytic and magnetically recoverable abilities.     

Microstructures and physical properties of sol–gel derived Ba2FeMnO6 double perovskite

We report microstructures and physical (dielectric, magnetic, and optical) properties of sol–gel derived Ba₂FeMnO₆ (BFMO) double perovskite powders. The BFMO powders belong to hexagonal crystal structure with P-6m2 space group. SEM images reveal the powders are nearly homogeneous distribution with spherical morphology and average particle size of 300 nm. Energy-dispersive spectra gave out the molar ratio of the Ba:Fe:Mn elements equal to 2.16:1.00:1.00. FTIR spectrum verified the [FeO₆] and [MnO₆] octahedra present in the powders. X-ray photoelectron spectroscopy spectra identified the chemical valence states of the constituent elements. The BFMO ceramics displayed a strong frequency dispersion dielectric behavior. A relaxor-like dielectric behavior appeared around ∼475 K due to the contributions of oxygen vacancies of$(V_{\ddot{o}})$ and the $({\mathrm{Fe}}_{\mathrm{Mn}}^{\prime }-)/({\mathrm{Mn}}_{\mathrm{Fe}}^{\prime }-)$ defect dipoles. A ferrimagnetic behavior was observed in the powders at 5 K with M_S = 0.14 μ_B/f.u. and H_C = 1.51 kOe. The magnetic Curie temperature (T_C) was 381 K, whereas the Neel temperature (T_N) was 31 K. The ferrimagnetic behavior is governed by the Mn³⁺–Fe⁴⁺ double-exchange interaction via the mediated oxygen between them. The BFMO powders have a direct optical bandgap of 1.65 eV, which originates from the electron transferring from O 2p to Mn 3d (and/or Fe 3d) levels. The unique combination of high temperature ferrimagnetism and semiconductivity in the BFMO powders makes them particularly appealing for magnetic spintronics and photovoltaics.     

Structural,spectral, dielectric and photocatalytic studies of Zr-Ni doped MnFe2O4 co-precipitated nanoparticles

In this study the Mn_1–2xZr_xFe_2−yNi_yO₄ nanoparticles fabricated by co-precipitation technique were investigated. Thermo-gravimetric analysis (TGA) exhibited the annealing temperature of the nanoparticles ~990 °C. Cubic spinel structure of Mn_1–2xZr_xFe_2−yNi_yO₄ nanoparticles was confirmed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) analysis. Crystallite size was calculated by XRD data and found in the range of 32–58 nm. Photocatalytic activity of Mn_0.92Zr_0.04Fe_1.88Ni_0.12O₄/graphene nanocomposites was tested by degrading methylene blue (MB) under visible light irradiation. The MB was almost completely degraded in the presence of Mn_0.92Zr_0.04Fe_1.88Ni_0.12O₄-graphene nanocomposites under visible light irradiation. Dielectric parameters were also investigated in the frequency range 1×10⁶–3×10⁹ Hz. An overall decrease in the values of dielectric constant, dielectric loss and tangent loss was observed on account of the substitution of Zr and Ni with Mn and Fe cations.     

High temperature crystallographic and low temperature magnetic phase transitions in Fe doped Sr2RuMnO6

Fe doped Sr₂RuMnO₆ (SRMO) double perovskites (Sr₂RuMn_1-xFe_xO₆, x = 0, 0.1, 0.2 and 0.3) were prepared by solid-state route. Both x-ray diffraction and Raman spectroscopy were performed to investigate the crystal structure of the synthesized double perovskites. Rietveld refinement of the x-ray diffraction patterns confirmed a phase transition from tetragonal to cubic space group as a function of doping concentration of iron. Raman spectroscopy at room temperature and group theory analysis revealed the phonon modes associated with the space group of the samples. The temperature dependent Raman spectroscopy showed an anharmonic behaviour of the phonon modes of the Fe doped SRMO samples. The temperature evolution of the phononic modes in the range of 300 K–620 K is predominantly influenced by the lattice degrees of freedom. The presence of several oxidation states Mn (2⁺, 3⁺ and 4⁺) and Fe (3⁺ and 4⁺) was confirmed by an X-ray photoemission spectroscopy analysis of the highest doped sample (x = 0.3). The magnetic properties measurements showed that the samples were completely paramagnetic at room temperature. The samples exhibit antiferromagnetism at very low temperatures and we conclude that they exhibit ferrimagnetic ground state in the mid temperature region.     

Improved dielectric properties in A′‐site nickel‐doped CaCu3Ti4O12 ceramics

The improved dielectric properties and voltage‐current nonlinearity of nickel‐doped CaCu₃Ti₄O₁₂ (CCNTO) ceramics prepared by solid‐state reaction were investigated. The approach of A′‐site Ni doping resulted in improved dielectric properties in the CaCu₃Ti₄O₁₂ (CCTO) system, with a dielectric constant ε′≈1.51×10⁵ and dielectric loss tanδ≈0.051 found for the sample with a Ni doping of 20% (CCNTO20) at room temperature and 1 kHz. The X‐ray photoelectron spectroscopy (XPS) analysis of the CCTO and the specimen with a Ni doping of 25% (CCNTO25) verified the co‐existence of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺. A steady increase in ε′(f) and a slight increase in α observed upon initial Ni doping were ascribed to a more Cu‐rich phase in the intergranular phase caused by the Ni substitution in the grains. The low‐frequency relaxation leading to a distinct enhancement in ε′(f) beginning with CCNTO25 was confirmed to be a Maxwell‐Wagner‐type relaxation strongly affected by the Ni‐related phase with the formation of a core‐shell structure. The decrease of the dielectric loss was associated with the promoted densification of CCNTO and the increase of Cu vacancies, due to Ni doping on the Cu sites. In addition, the Ni dopant had a certain effect on tuning the current‐voltage characteristics of the CCTO ceramics. The present A′‐site Ni doping experiments demonstrate the extrinsic effect underlying the giant dielectric constant and provides a promising approach for developing practical applications.     

Electric and dielectric study of cobalt substituted MgMn nanoferrites synthesized by solution combustion technique

Cobalt substituted Mg–Mn nanoferrites with formulae Mg_0.9Mn_0.1Co_xFe_2?xO₄, x=0.0, 0.1, 0.2 and 0.3, have been synthesized for the first time by the solution combustion technique. The effects of Co²⁺ ions on the dc resistivity, dielectric constant and dielectric loss tangent of Mg–Mn nanoferrites at room temperature are presented in this paper. X-ray diffraction confirmed the formation of a single phase spinel structure. Particle size was found to increase, 20.9–23.9 nm, with increasing Co²⁺ concentration. The dc resistivity was increased by two order of magnitude with substitution of Co²⁺ ions while the dielectric constant was found to be decreasing with the increasing concentration of cobalt ions. The value of dc resistivity obtained for Mg_0.9Mn_0.1Fe₂O₄ nanoferrite in our work is greater than the value obtained for the same composition prepared by the conventional ceramic technique. Further, the dielectric constant and dielectric loss tangent were observed to be decreasing with the increase in frequency.     

Effect of Fe doping on structure,magnetic and electrical properties La0.7Ca0.3Mn0.5Fe0.5O3 manganite

The crystal and magnetic structures of La_0.7Ca_0.3Mn_0.5Fe_0.5O₃ compound have been studied by neutron powder diffraction in the temperature range of 10–300 К. The magnetization and electrical resistivity measurements have been also performed in the temperature range of 5–300?K in magnetic fields up to 1?T. These experimental results indicate a formation of a complex magnetic state in which the long-range antiferromagnetic G-type phase coexists with the short-range ferromagnetic clusters. The electrical conductivity of La_0.7Ca_0.3Mn_0.5Fe_0.5O₃ demonstrates an anomalous temperature behavior suggesting a switching between different states. The origin of the unconventional magnetic state, the mechanisms of the electrical conductivity, and correlation between magnetic and transport properties in this manganite have been discussed.     

Innovative methodology for co-treatment of mill scale scrap and manganese ore via oxidization roasting-magnetic separation for preparation of ferrite materials

Mill scale scrap, which contains vast amounts of valuable metals, is a solid waste produced in the iron and steel industry. Conventional mill scale scrap treatment methods for metal extraction are characterized by high energy consumption and low value addition. In this study, co-treatment of mill scale scrap and manganese ore via the oxidization roasting-magnetic separation process was investigated for the synchronous preparation of higher-value materials and recovery of valuable metals. Thermodynamic and magnetism analyses indicated that a higher temperature (>1100 °C) and a MnO₂/Fe₂O₃ molar ratio of 0.75–1 are essential for the preparation of manganese ferrite. The experimental validation revealed that soft magnetic manganese ferrite powders with a purity of 97.5 wt% were obtained when the test was conducted at 1300 °C for 120 min, followed by a two-stage grinding and magnetic separation process; the corresponding yield and the Mn and Fe recoveries were 78.99 wt%, 86.14 wt%, and 84.60 wt%, respectively. During the oxidization process, [Fe²⁺]O was initially oxidized to the anti-form spinel-type structure of [Fe³⁺][Fe²⁺Fe³⁺]O₄, and thereafter, it reacted with the decomposition product of [Mn³⁺][Mn²⁺Mn³⁺]O₄ to form a hybrid spinel-type structure [Me²⁺_xMe³⁺_l-x][Me²⁺_1-xMe³⁺_1+x]O₄ (Me refers to Mn and Fe) via the Mn²⁺/Fe²⁺/Mn³⁺/Fe³⁺ ions exchange at the tetrahedral and octahedral sites. Moreover, the as-purified ferrite can be used as an ingredient for the preparation of high-performance MnZn ferrite.     

Multiferroic (Nd,Fe)-doped PbTiO3 ceramics with coexistent ferroelectricity and magnetism at room temperature

We report the structural, dielectric, elastic, ferroelectric and ferromagnetic properties of multiferroic (Nd, Fe)-doped PbTiO3 perovskite ceramics with composition (Pb_0.88Nd_0.08)(Ti_0.94Fe_0.04Mn_0.02)O₃, prepared by different solid state reaction methods: the first one based on a single-stage calcination (Method I) and the second based on a double-stage calcination (Method II). Structural, dielectric and anelastic measurements evidenced a double phase transition for samples prepared by Method I, which has been attributed to phase separation. This phase separation has been confirmed also by TEM and HRTEM investigations. Samples prepared by Method II showed a single phase transition from paraelectric to ferroelectric phase. We found coexistent ferroelectric and ferromagnetic properties, also at room-temperature, but only for ceramics prepared by Method II. The crucial role of calcination process for avoiding phase separation and obtaining homogeneous structures with ferroelectric and ferromagnetic order is underlined.     

Room temperature multiferroicity in Aurivillius compounds Bi6Fe2−xNixTi3O18 (0≤x≤1)

We investigate the structural, magnetic, ferroelectric, and dielectric properties of Bi₆Fe_2−xNi_xTi₃O₁₈ (0≤x≤1). The coexistence of ferroelectricity and ferromagnetism were observed at room temperature for the Ni-doped samples. The ferromagnetism in Bi₆Fe_2−xNi_xTi₃O₁₈ can be understood in terms of spin canting of the antiferromagnetic coupling of the Fe-based and Ni-based sublattices via Dzyaloshinsky-Moriya interaction. Moreover, the substitution of Ni for Fe was effective for the enhancement of ferroelectric properties. The x=0.6 sample exhibits a maximum remnant polarization P_r of 37.8 μC/cm² because of a lower leakage current. The rather large activation energy in the x=0 and 0.2 samples implies that the relaxation process may be not associated with the thermal motion of oxygen vacancies inside the bulk.