Characterization of multiferroic PbFe0.5Nb0.5O3 and PbFe0.5Ta0.5O3 ceramics derived from citrate polymeric precursors

Ceramics of the perovskite multiferroics PbFe_0.5Nb_0.5O₃ (PFN) and PbFe_0.5Ta_0.5O₃ (PFT) were synthesized from new citrate polymeric precursors. X-ray tests pointed to trace amounts of the pyrochlore phase. SEM studies revealed the heterogeneous grain size distribution for PFN and the homogeneous one for PFT. Dielectric studies pointed to one diffuse T-C phase transition at 378 K for PFN and two diffuse M-T and T-C phase transitions, at 200 and 235 K, for PFT, respectively. X-ray photoelectron spectroscopy studies of PFN reveal that all ions exist in one valence state, however, with two chemical shifts for Pb²⁺. Two valence states for the majority of ions of PFT seem to be connected with a higher volume fraction of the amorphous grain-boundary phase. The electronic energy band gap for both compounds is approximately 2.8 eV. Two magnetic transitions, ie, from the paramagnetic to the antiferromagnetic phase and then to the spin-glass phase, were observed at 156 and 10 K for PFN, and at 145 and 15 K for PFT, respectively.     

Temperature-induced magnetic compensation and negative magnetization in a cluster spin glass Gd2Co0.5Mn1.5O6 system

Experiments for orthorhombic double perovskite Gd₂Co_0.5Mn_1.5O₆ revealed intrinsic effects of magnetic compensation characterized by M(T_comp) = 0 at T_comp = 20 K and negative magnetization depicted by M(T) < 0 under positive magnetic fields, which were experimentally investigated by different protocols of direct current magnetization measurements. Compared to Gd₂CoMnO₆, the excessive ratio of Mn in Gd₂Co_0.5Mn_1.5O₆ promotes the antisite disorder and preferably generates magnetic clusters owing to the intrinsic inhomogeneity. The clusters exhibit spin glass (SG) properties as demonstrated by alternating current susceptibility and aging measurements. A possible physical mechanism for evolution of the spin configuration with temperature is proposed. The effects of magnetic compensation and negative magnetization are attributed to the negative exchange coupling among the abundant ferromagnetic clusters. The additional pinning force provided by the cluster SG is an essential factor to prevent the flipping of the spins from aligning with the applied magnetic field.     

Field tuning magnetic phase transition in Dy0.5Tb0.5FeO3 single crystal

RFeO₃ is an ideal candidate for the fabrication of spintronic devices, and its rare earth site doping is an important means to regulate and produce many unique spin behaviors. In this work, we report a high-quality Dy_0.5Tb_0.5FeO₃ single crystal grown by optical floating zone method and its magnetic properties are studied explicitly. A type-Ⅱ spin switching effect and suppressed spin reorientation transition are found in the Magnetization vs Temperature curves in Dy_0.5Tb_0.5FeO₃. In the temperature region of spin reorientation transition, the transition of Fe³⁺ magnetic sublattice configuration becomes very complicated by the influence of competitive interaction with magnetic Dy³⁺/Tb³⁺ ions. Interestingly, a field tunable double-hysteresis loop is observed near the antiferromagnetic transition temperature of rare earth ions sublattice, which has never been reported in RFeO₃ family compounds. By studying the dependence of double-hysteresis loop at different temperatures, the magnetic phase transition and spin reorientation transition induced by the applied magnetic field are inferred. This novel physical phenomenon helps us understand the mechanism of spin configuration transition better at low temperatures.     

A study on the structural,optical, magnetization reversal and bipolar magnetic switching behavior of SmCr0.85Mn0.15O3 nanoparticles

SmCr_0.85Mn_0.15O₃ (SCMO) nanoparticles with Pnma space group have been synthesized by the sol-gel auto-combustion method with an average crystallite size of 70 nm. The structural analysis indicates a comparable unit cell volume with that of the bulk compound. The structural distortion is calculated by various factors such as cell distortion factor, octahedral distortion, dodecahedral distortion, strain and tolerance factor, which arise due to the Jahn-Teller (JT) effect and a tilting in the octahedra. The obtained distortion values suggest a larger distortion in the structure due to the nano-size effect compared with the bulk counterpart. The diffuse reflectance spectrum confirms the semiconducting nature of SCMO nanoparticles with an optical bandgap of 2.9 eV, higher than that of the bulk compound. The extinction coefficient of the SCMO nanoparticles exhibits a larger value than that of the bulk, suggesting a higher absorbing character of the nanoparticles. The temperature-dependent magnetization reveals a decrease in the Néel temperature on Mn substitution. The negative magnetization is also observed below the compensation temperature, T_comp. Moreover, a stable magnetic switching behavior is demonstrated by varying temperatures under ZFC and FC protocols. The semiconducting nature and the stable magnetic switching behavior of SCMO nanoparticles make them suitable for optoelectronic, photovoltaic, and magnetic switching applications.     

Magnetic and magnetoelectric properties of Bi0.5Nd0.5FeO3 ceramics

Ceramic solid solution described by formula Bi_0.5Nd_0.5FeO₃ was prepared by standard solid-state sintering method. X-ray diffraction proved the formation of a single-phase sample with orthorhombic structure typical of NdFeO₃. Vibrating sample magnetometry revealed weak ferromagnetic behavior of the ceramics up to the Curie temperature T_c = 642 K. Local atomic configurations and local magnetic fields were investigated by ⁵⁷Fe Mössbauer spectroscopy. It was shown that substitution of a single Nd³⁺ ion by Bi³⁺ ion in the orthorhombic NdFeO₃ lattice causes a decrease of hyperfine magnetic field experienced by ⁵⁷Fe ion by the value of 0.186 T. Measurements of the magnetoelectric effect showed that the maximal registered value of the magnetoelectric voltage coefficient was equal to 0.46 mV/(cm·Oe).     

Anisotropic electrical and magnetic properties in textured Bi5Ti3FeO15 ceramics

Textured Bi₅Ti₃FeO₁₅ (BTFO) ceramics were prepared by molten salt synthesis at 850 °C followed by plasma activated sintering at 800 °C. The structure and the anisotropic electrical and magnetic properties of the ceramics were investigated. The textured BTFO ceramics exhibited [001] preferred orientation with a high degree of texture fraction (Lotgering factor is 0.82), as confirmed from XRD patterns, FE-SEM and TEM micrographs. The anisotropic dielectric relaxation and conduction characteristics along the parallel and perpendicular directions were investigated using the dielectric and impedance spectroscopies. The grain boundary along the parallel direction had a similar value of capacitance to that of grains, whereas the grain boundary along the perpendicular direction dominated the resistive component. The magnetic performance along the perpendicular direction (H_c ∼ 32.5 Oe) was better than that along the parallel direction (H_c ∼ 17.6 Oe). These results could be useful for enhancing the electrical and magnetic properties of BTFO by texturization.     

Ferroelectric and magnetic properties in (1−x)BiFeO3–x(0.5CaTiO3–0.5SmFeO3) ceramics

Multiferroic ceramics were prepared and characterized in (1?x)BiFeO₃–x(0.5CaTiO₃–0.5SmFeO₃) system by a standard solid‐state reaction process. The structure evolution was investigated by X‐ray diffraction and Raman spectrum analyses. The refinement results confirmed the different phase assemblages with varying amounts of polar rhombohedral R3c and nonpolar orthorhombic Pbnm as a function of the substitution content. In the compositions range of 0.2≤x≤0.5, polar R3c and nonpolar Pbnm coexisted, which was referred to polar‐to‐nonpolar morphotropic phase boundary (MPB). According to the dielectric and DSC analysis results, the ceramics with x≤0.2 changed to diffused ferroelectric, and the ferroelectric properties were enhanced significantly. Two dielectric relaxations were detected in the temperature range of 200‐300 K and 500‐700 K, respectively. The high‐temperature dielectric relaxation was attributed to the grain‐boundary effects. While the low temperature dielectric relaxation obtained in the samples with x=0.3‐0.5 was related to the charge transfer between Fe²⁺ and Fe³⁺. The magnetic hysteresis loops measured at different temperature indicated the enhanced magnetic properties in the present ceramics, which could be attributed to the suppressed cycloidal spin magnetic structure by Ti ions. In addition, the rare‐earth Sm spin moments might also affect the magnetic properties at relatively lower temperature.     

Raman and dielectric spectroscopic analysis of magnetic phase transition in Y(Fe0.5Cr0.5)O3 multiferroic ceramics

Here, we report the Raman and dielectric spectroscopic studies as a function of temperature of orthorhombically distorted Y(Fe_0.5Cr_0.5)O₃ (YFC) ceramics, measured from 80 to 300 K. The dc-magnetization measurements under field cooled (FC)-zero field cooled (ZFC) protocol indicate a small onset of magnetic ordering at T_N∼270 K. The field dependent magnetization plot recorded at 50 K, 150 K and 200 K show a clear opening in hysteresis loops. The linear dependence of magnetization plot at high field without any saturation of magnetization indicates the coexistence of weak ferromagnetic (WFM) component within the canting antiferromagnetic (CAFM) matrix. Temperature evolution of Raman line-shape parameter of B_2g(4) phonon mode clearly exhibits an anomalous behavior of phonon shift near T_N∼270 K, indicating the spin-phonon coupling in the ceramics. From the temperature dependent dielectric permittivity (ε(T)) study, two dielectric relaxation peaks are detected below 200 K and above 250 K. The appearance of former relaxation peak is responsible for polaronic conduction mechanism, while the later one is associated with magnetic phase transition which might be relevant to the presence of magnetoelectric coupling in YFC ceramics. The observed P-E hysteresis loops at room temperature indicate weak ferroelectric nature of the ceramics.     

Structure,microstructure, dielectric and piezoelectric properties of (1-x-y)BiFeO3-xPbFe0.5Nb0.5O3-yPbTiO3 ceramics

Ceramics of seven quasi-binary concentration sections of the ternary solid solution system (1-x-y)BiFeO₃-xPbFe_0.5Nb_0.5O₃-yPbTiO₃ were prepared by the conventional solid-phase reaction method in the range of 0.05 ≤ x ≤ 0.325; 0.05 ≤ y ≤ 0.325. By using x-ray diffraction technique, the phase diagram of the system was constructed which was shown to contain the regions of tetragonal and rhombohedral symmetry and the morphotropic phase boundary between them. Grain morphology, dielectric and piezoelectric properties of selected solid solutions were investigated. The highest piezoelectric coefficient d₃₃ = 50 pC/N was obtained. Dielectric characteristics of ceramics revealed ferroelectric relaxor behavior and region of diffuse phase transition from the paraelectric to ferroelectric phase in the temperature range of 600–800 K.     

Negative magnetization and exchange bias effect in Fe-doped CoCr2O4

Polycrystalline ceramics of Co(Cr_1-xFe_x)₂O₄ (0?≤?x?≤?0.12) were experimentally studied based on a series of temperature and time-dependent dc magnetic measurements using different magnetic field histories. Magnetization in field cooling process was continuously decreased for doping content x in the range of 0?≤?x?≤?0.04. Remarkable negative magnetization is observed when x reaches to 0.06 and persists up to x?=?0.1. Two-sublattice model is established and competition of the two magnetic sublattices is responsible for the phenomenon. The magnetic switching effect is realized just by changing the magnitude of the applied magnetic field and double magnetocaloric effects are obtained. These unique features under low magnetic fields show attractive for application in spintronic devices due to that the magnetic state can effectively be tuned through magnetic field or temperature. Besides, the system exhibits both positive and negative exchange bias fields which are considered to be originating from the unidirectional anisotropy of exchange coupling of antiferromagnetic/ferromagnetic phases and spin reorientation of the two sublattices magnetic moments, respectively.     

Actuation mechanisms in mixed-phase K0.5Bi0.5TiO3-BiFeO3-PbTiO3 ceramics

We report an in-situ synchrotron X-ray diffraction study of K_0.5Bi_0.5TiO₃-BiFeO₃-PbTiO₃ ceramics, which exhibit a T_c of around 450 °C. The electromechanical actuation mechanisms comprise contributions from coexisting tetragonal and rhombohedral phases. The tetragonal {200} grain family exhibited the highest effective lattice strain, up to 8.2 × 10⁻³ at 5 kV/mm. Strong strain anisotropy in the tetragonal phase and field-induced intergranular stresses facilitate a partial transformation from tetragonal (high strain anisotropy) to rhombohedral (low strain anisotropy) at high electric field levels, with an average linear transformation strain of -1.54 × 10^-3. The domain switching behavior was effectively enhanced in both tetragonal and rhombohedral phases after the phase transformation, due to the release of intergranular stress. This observed self-adapting mechanism in tuning intergranular stress through partial phase switching in the morphotropic KBT-BF-PT composition with large lattice distortion could also be exploited in other perovskite systems in order to achieve high performance high temperature piezoelectric ceramics.     

The microstructure and ferroelectric property of Nd-doped multiferroic ceramics Bi0.85Nd0.15FeO3

The microstructures of Bi_0.85Nd_0.15FeO₃ ceramics were investigated by using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), convergent-beam electron diffraction (CBED) and X-ray energy dispersive spectrometry (EDS). The superstructure phase related to 1/4(hh0)_p and 1/4(00 l)_p diffraction spots were observed in the samples. It is found that the superstructure phase can co-exist with R3c phase in a single grain. The bright and dark field images accompanied with SAED patterns provided evidences of the superstructure phase dispersed into the matrix with R3c symmetry and evolution of the domain. The ferroelectric domain wall was observed according to the displacement of Fe³⁺ ions respect to Bi³⁺ sub-lattice in the ferroelectric R3c phase based on HRTEM observations. The space group Pnam of the superstructure was identified by combining SAED and CBED techniques. EDS measurement revealed that the concentration of Nd in the Pnam phase is higher than that in R3c phase. This might mean that the transition from R3c to Pnam structure is due to the inhomogeneous distribution of the Nd concentration and the weakened stereochemical activity of Bi³⁺ lone electron pair, arisen from the increase in Nd content.     

Crystal structure refinement,enhanced magnetic and dielectric properties of Na0.5Bi0.5TiO3 modified Bi0.8Ba0.2FeO3 ceramics

(1-x) Na_0.5Bi_0.5TiO₃–x(Bi_0.8Ba_0.2FeO₃) (x=0.5, 0.6, 0.7, and 0.8) ceramics were synthesized via solid state reaction method. Powder X-ray diffraction investigations performed at room temperature along with Rietveld analysis show all the composites to exhibit a rhombohedral distorted perovskite structure, described by space group R3c. Rietveld refinement confirmed a good agreement between observed and calculated intensities and a low value of goodness of fit (χ²). Magnetic measurements were carried out at room temperature up to a field of 6 kOe. Magnetic properties of BBFO modified NBT ceramics are improved with a significant opening in the M–H hysteresis loop at room temperature. Remanent magnetization and coercive field increased with increase of BBFO concentration. The dielectric response of these samples was analyzed in the frequency range 10 Hz–7 MHz at different temperatures revealing a dispersion in dielectric constant (ε′) and in dissipation factor (tan δ) at lower frequencies. Both ε′ and tan δ increase with increase of BBFO content. The temperature dependence of frequency exponent ′s′ of power law suggests that quantum mechanical tunneling (QMT) model to be applicable at lower temperature and correlated barrier hopping (CBH) mechanism to be appropriate at higher temperature to describe the conduction mechanism in x=0.5 and x=0.6 samples. Further, with increase in BBFO content, the dielectric constant becomes more stable at higher frequencies and temperatures thereby improving the dielectric properties of the material.     

Ceramic processing and multiferroic properties of the perovskite YMnO3-BiFeO3 binary system

The perovskite (1−x)YMnO₃−xBiFeO₃ binary system is very promising because of its multiferroic end members. Nanocrystalline phases have been recently obtained by mechanosynthesis across the system, and the perovskite structural evolution has been described. Two continuous solid solutions with orthorhombic Pnma and rhombohedral R3c symmetries were found, which coexist within a broad compositional interval of 0.5 ≤ x ≤ 0.9. This might be a polar-nonpolar morphotropic phase boundary region, at which strong phase-change magnetoelectric responses can be expected. A major issue is phase decomposition at moderate temperatures that highly complicates ceramic processing. This is required for carrying out a sound electrical characterization and also for their use in devices. We present here the application of Spark Plasma Sintering to the ceramic processing of YMnO₃-BiFeO₃ phases. This advanced technique, when combined with nanocrystalline powders, allowed densifying phases at reduced processing temperatures and times, so that perovskite decomposition was avoided. Electrical measurements were accomplished, and the response was shown to be mostly dominated by conduction. Nonetheless, the intrinsic dielectric permittivity was obtained, and a distinctive enhancement in the phase coexistence region was revealed. Besides, Rayleigh-type behavior characteristic of ferroelectrics was also demonstrated for all rhombohedral phases. Magnetic characterization was performed in this region, and antiferromagnetism was shown.     

Oxygen octahedra distortion-induced multiferroic properties of Er and Zr co-doped BiFeO3 nanoparticles

The present work unveiled the distortion of oxygen octahedra influencing magnetic and magnetoelectric properties of novel Bi_1−xEr_xFe_1−yZr_yO₃ (x = 0, .05, .1, y = .02, .05) polycrystalline nanoparticles by sol–gel route. X-ray diffraction patterns analysis reveals that pristine BiFeO₃ and doped BiFeO₃ are crystalized in the rhombohedral structure (R3c). The Fe–O–Fe bond angle of Bi_1−xEr_xFe_1−yZr_yO₃ (x = 0, .05, .1, y = .02, .05) varies between 141° and 159.62° as the concentration of Er (via Bi site) and Zr (via Fe site) ions increases in BiFeO₃. As a result, the tilt angle of oxygen octahedra and the canting angle of spiral spin arrangement increase. Hence, the maximum magnetization varies between .03144 and .37558 emu/g in Er and Zr co-doped BiFeO₃ system. The number of electrons per unit cell of Bi_1−xEr_xFe_1−yZr_yO₃ (x = 0, .05, .1, y = .02, .05) lies between 733.38 and 831, respectively. Further, the number of coherently diffracting domains increases from 3.07 to 5.21, and then it decreases when Er and Zr are increased in BiFeO₃. Consequently, the magnetoelectric coupling coefficient varies between .0265 and .2511 mV/cm Oe, respectively. Particularly, Bi_0.95Er_0.05Fe_0.98Zr_0.02O₃ shows enhanced magnetic and magnetoelectric behaviors compared to other samples.     

Structure and ferroelectric behaviour of Na0.5Bi0.5TiO3-KNbO3 ceramics

Lead-free piezoelectric ceramics, (1?x)Na_0.5Bi_0.5TiO₃-xKNbO₃ (NBT-xKN), with x?=?0.02–0.08 were fabricated by solid-state reaction and sintering. The crystal structures and dielectric properties were measured for different KN contents. All compositions in the unpoled, as-sintered state were found to be single-phase pseudo-cubic. However, typical ferroelectric behaviour, with well-saturated polarisation-electric field hysteresis loops, was observed for certain compositions at high electric field levels. It is shown using high-energy synchrotron X-ray diffraction that the application of the electric field induced an irreversible structural transformation from the nano-polar pseudo-cubic phase to a ferroelectric rhombohedral phase. The changes in lattice elastic strain and crystallographic texture of a poled NBT-0.02KN specimen as a function of the grain orientation, ψ, conform well to those expected for a conventional rhombohedrally distorted perovskite ferroelectric ceramic. The dielectric permittivity-temperature relationships for all compositions exhibit two transition temperatures and a frequency-dependent behaviour that is typical of a relaxor ferroelectric. The transition temperatures and grain size decrease with the increasing KN content.     

Evolution of magnetic order in multiferroic Pb(Fe2/3W1/3)O3-BiFeO3 solid solution

Multiferroic materials have attracted much interest in the last decade due to both the intriguing fundamental science and the potential applications in spintronics and magnetoelectric data storage devices. In this work, we have investigated and discussed the evolution of the magnetic properties of the multiferroic (1-x)Pb(Fe_2/3 W_1/3)O₃-xBiFeO₃ solid solution ((1-x)PFW-xBFO, x = 0, 0.025, 0.05, 0.075, 0.1 and 0.15). The magnetic phase diagram is established based on the magnetic measurement results, which reveals six magnetically ordered states on the PFW-rich side of the solid solution. The origins of the complex evolution of magnetic order in the PFW-BFO solid solution are discussed from the point view of the variations in both the –Fe–O–Fe– and –Fe–O–W–O–Fe– superexchange routes, which are intimately related to the ratio of magnetic Fe³⁺ ion concentration on the B-site and the changes in the local structural order/disorder and chemical homogeneities. Combining the magnetic phase diagram with the relaxor characteristic phase diagram of the (1-x)PFW-xBFO system, a striking feature is found that the ergodic relaxor (ER) state and the weakly ferromagnetic phase coexist in the composition range of 0.025 ≤ x ≤ 0.1 between the freezing temperature T_f and the Burns temperature T_B.     

Investigation of the mechanism of domain switching in different Na0.5Bi0.5TiO3 microstructures

Several researches have studied the physical properties of hydrothermally-synthesized low dimensional piezoelectric nanostructures. However, the obtained piezoelectric coefficient is not high and the relationship between physical properties and microstructures is still neglected. Here we report the piezoelectric and ferroelectric properties of different lead-free sodium bismuth titanate (Na_0.5Bi_0.5TiO₃, NBT) microstructures synthesised with hydrothermal routes and give visualization of domain structures using piezoresponse force microscopy. The NBT nanowire exhibits better local piezoelectric response compared with NBT spherical aggregates and microcubes and other one-dimensional materials prepared by hydrothermal method and the large piezoelectric coefficient of nanowire was explained by observed regular stripe domain structures. Moreover, it is found that there are different domain configurations at the top and side of the nanowire under the external electrical fields, which don't change the regular stripe domain structure but lead to the movement of domain boundaries. By finite element modeling, it attributes to the different electric potential distributions from tip within the nanowire.     

Sonocatalysis and Photocatalysis in Ba0.5Sr0.5TiO3 ceramics

The photocatalytic, sonocatalytic, and sono-photocatalytic performances of Ba_0.5Sr_0.5TiO₃ (BST) ceramic (synthesized through solid-state reaction route) were investigated for the degradation of an organic dye named methylene blue (MB). The as-prepared BST ceramic powder was characterized using a scanning electron microscope, X-ray diffraction, X-ray photoelectron, and Raman spectroscopy techniques. The optical energy band gap of BST ceramic was found to be ∼3.17 eV. BST has shown significant catalytic activity following sonocatalysis and photocatalysis processes, i.e, ∼48% and ∼65% in 3 h, respectively. The synergic effect of the sonocatalysis and photocatalysis processes had shown an excellent degradation of 81% in 3 h. To determine the reactive species responsible for the degradation of MB dye, a scavenger test was also performed using isopropyl alcohol (IPA), ethylenediaminetetraacetic acid (EDTA), and benzoquinone (BQ) scavengers. The degree of MB dye degradation was quantified by a phytotoxicity test on “Vigna radiata” seeds. Furthermore, the potentiality of BST ceramic was explored for water cleaning applications while irradiating it to solar radiation in real-time conditions.     

Magnetic and ferroelectric domain structures in BaTiO3–(Ni0.5Zn0.5)Fe2O4 multiferroic ceramics

BaTiO₃–(Ni_0.5Zn_0.5)Fe₂O₄ composites prepared by co-precipitation were investigated. The macroscopic magnetic properties derived from the magnetic phase (low coercivity, almost no M(H) hysteretic behavior and high permeability) are preserved in the composite. The dielectric properties are strongly influenced by interface phenomena (Maxwell-Wagner), due to the local electrical inhomogeneity. At low frequencies, the composites present thermally activated conductivity and relaxation, while at 1 MHz permittivity of around 500 and tan δ < 8% is obtained at room temperature. The multiferroic character was demonstrated at nanoscale by the presence of the magnetic and ferroelectric domain structure in the same region. Imprint polarisation in the regions corresponding to the ferroelectric phase is found, as result of an internal electrical field created at the interfaces between the (Ni,Zn)-ferrite and BaTiO₃ regions.