Role of (La,Gd) co-doping on the enhanced dielectric and magnetic properties of BiFeO3 ceramics

The effect of the La³⁺ and Gd³⁺ co-doping on the structure, electric and magnetic properties of BiFeO₃ (BFO) ceramics are investigated. For the compositions (x=0 and 0≤y≤0.15) in the perovskite structured La_xGd_yBi_1−(x+y)FeO₃ system, a tiny residual phase of Bi₂Fe₄O₉ is noticed. Such a secondary phase is suppressed with the incorporation of ‘La’ content (x). The magnitude of dielectric constant (ε_r) increases progressively by increasing the ‘La’ content from x=0 to 0.15 with a remarkable decrease of dielectric loss. For x=0.15, the system La_xGd_yBi_1−(x+y)FeO₃ exhibits highest remanent magnetization (M_r) of 0.18 emu/g and coercive magnetic field (H_C) of ~1 T in the presence of external magnetic field of 9 T at 300 K. The origin of enhanced dielectric and magnetic properties of La_xGd_yBi_1−(x+y)FeO₃ and the role of doping elements, La³⁺, Gd³⁺ has been discussed.     

Dynamical behavior of step-like transition of La0.5Sr0.5Mn1−xTixO3 in a widened field sweep rate

The magnetization behavior of Ti⁴⁺ doped perovskite manganites La_0.5Sr_0.5Mn_1−xTi_xO₃ (x=0.15, 0.175 and 0.2) was investigated. Experimental results show that Ti⁴⁺ dopant suppresses the antiferromagnetic charge ordering and leads to a step-like magnetization behavior below 3 K. The step-like transitions and the critical magnetic fields are strongly dependent on the Ti doping level, magnetic field sweep rate and temperature. Above 3 K, the step-like transitions transform to broad ones. In addition, under pulsed high magnetic fields with an ultrafast field sweep rate of ~10³ T/s, the sharp step-like transitions observed in the static magnetization measurements become smooth metamagnetic transition at low temperatures. This feature is correlated with the collapse of the balance between the magnetic energy and elastic energy in the phase separation system within a martensitic-like scenario.     

Influence of Ce addition on the structural,magnetic, and magnetocaloric properties in La0.7−xCexSr0.3MnO3 (0≤x≤0.3) ceramic compound

We report improvement in the magnetocaloric properties of Ce-doped lanthanum manganites. Polycrystalline La_0.7−xCe_xSr_0.30MnO₃ (0≤x≥0.3) samples were prepared using the conventional solid-state reaction method with phase purity and structure confirmed using X-ray diffraction. Temperature dependent magnetization measurements and Arrott analysis reveal second order ferromagnetic transition in parent sample and as well as in doped sample with Curie temperature decreasing progressively with increasing Ce-concentration from ~370 K for x=0.0 to 310 K for x=0.30. Magnetic entropy change (ΔS_M) was calculated by applying the thermodynamic Maxwell equation to a series of isothermal field dependent magnetization curves. A large ΔS_M associated with the ferromagnetic–paramagnetic transition in La_0.7−xCe_xSr_0.30MnO₃ samples has been observed. The value of ΔS_M was found to increase with Ce-doping up to x=0.15 and the highest value of 2.12 J kg⁻¹ K⁻¹ (at ΔH=2 T) was observed for La_0.55Ce_0.15Sr_0.30MnO₃ sample near the Curie temperature of 356 K. Also, improved relative cooling power of ~122 J kg⁻¹ was observed for the same sample. Due to the large magnetic entropy change and high Curie temperature, the La_0.55Ce_0.15Sr_0.30MnO₃ sample is suggested to be used as potential magnetic refrigerants for magnetic refrigeration technology above room temperature.     

Sr1-xLaxFe12O19 (0.0≤x≤0.5) hexaferrites: Synthesis,characterizations, hyperfine interactions and magneto-optical properties

A simple sol-gel auto combustion process was used to synthesize La³⁺ substituted M-type strontium hexaferrite, Sr_1-xLa_xFe_12-xO₁₉ (0.0≤x≤ 0.5). Structural, magnetic, and optical behavior as a function of La³⁺ substitutions were investigated by Fourier transformed infrared spectroscopy (FT-IR). X-ray powder diffraction (XRD). Scanning electron microscopy (SEM), Mössbauer spectroscopy, vibrating sample magnetometer (VSM), and Diffuse reflectance spectroscopy (DRS). XRD data showed single phase magnetoplumbite structure and Rietveld analysis confirmed P6₃/mmc space group for all the series. The average crystallite size was found to be in the range of 43.2–48.4 nm. The variation in line width, isomer shift, quadrupole splitting, relative area and hyperfine magnetic field values have been determined from ⁵⁷Fe Mössbauer spectroscopy data. The fittings accounted for the Fe²⁺/Fe³⁺ charge compensation mechanism at the 2a site due to replacement of Sr²⁺ by La³⁺. The saturation magnetization (σ_s) decreases from 57,21 to 63,23 emu/g and remnant magnetization (σ_r) decreases from 35.6 emu/g to 28.7 emu/g with increasing La substitution. The decrement is sharper at coercive field (H_c) from maximum value of 5325 to minimum 1825 Oe. Demagnetizing factor (N) is 3 times more for the x=0.3, 0.4, and 0.5. However all samples exhibit ferromagnetic behavior at room temperature. Magnetic anisotropy of Hexaferrites was detected as uniaxial and effective anisotropy constants (K_eff) were between 5.93×10⁵ and 4.76×10⁵ Ergs/g. The high magnitudes of anisotropy fields (H_a) in the range of 13863–15574 Oe reveal that all hexaferrites are magnetic hard materials. Tauc plots were applied to extrapolate the direct optical energy band gap (E_g) of hexaferrites. The E_g values decreased from 1.83 to 1.34 eV with increasing La content.     

Synthesis and negative thermal expansion properties of solid solutions Yb2−xLaxW3O12 (0 ≤ x ≤ 2)

A new series of rare earth solid solutions Yb_2?xLa_xW₃O₁₂ were successfully synthesized by the solid-state method. Effects of substituted ion lanthanum on the microstructures and thermal expansion properties in the resulting Yb_2?xLa_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analyzer (TGA), field emission scanning electron microscope (FESEM) and thermal mechanical analyzer (TMA). Results indicate that the structural phase transition of the Yb_2?xLa_xW₃O₁₂ changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phases can form in the composition range of 0 ≤ x < 0.5 with orthorhombic structure and 1.5 < x ≤ 2 with monoclinic one. High lanthanum content leads to a low hygroscopicity of Yb_2?xLa_xW₃O₁₂. Negative thermal coefficients of the Yb_2?xLa_xW₃O₁₂ (0 ≤ x ≤ 2) also vary from ?7.78 × 10^?6 K^?1 to 2.06 × 10^?6 K^?1 with increasing substituted content of lanthanum.     

Physical and magnetic properties of Mn–Zr doped La–Sr ferrite prepared by the auto-combustion route

This is the first report ever on (Mn²⁺–Zr⁴⁺) doped M-type lanthanum strontium hexaferrite with general formula, Sr_0.85La_0.15(MnZr)_xFe_12−2xO₁₉ where x=0.0, 0.25, 0.50, 0.75, and 1.0, prepared by citrate auto-combustion method. These ferrites were characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM), Energy dispersive X-ray spectroscopy (EDX) and Vibrating sample magnetometer (VSM). X-ray diffraction patterns show the formation of high purity hexaferrite phase without other secondary phases for all the synthesized samples. It was observed from magnetic hysteresis data that the coercive force is reduced from 5692.5 Oe to 1669.2 Oe with increase in doping contents but the net magnetization of the samples varies slightly from 60.6 to 55.2 emu/gm. High saturation magnetization (M_s), low coercivity (H_c) and remanence magnetization (M_r) values of these materials make them particularly suitable for data recording.     

Magnetic properties of Er(Co,Mn)O3 perovskites

The erbium-based manganite ErMnO₃ has been partially substituted at the manganese site by Co in the general formula ErCo_xMn_1−xO₃. The perovskite orthorhombic structure is found from x(Co) = 0.3 up to x(Co) = 0.7, provided that the synthesis is performed under oxygenation conditions to favour the presence of Co³⁺. Magnetic properties show unusual phenomena, correlated with the presence of different magnetic entities (i.e., Er³⁺, Co²⁺, Co³⁺, Mn³⁺, Mn⁴⁺): the overall magnetic moment reverses its sign when the sample is cooled under an external magnetic field, while the magnetization loops performed at T < 4 K show intersecting branches at low fields and a sudden jump at high fields. A phenomenological model of two interacting sublattices, coupled by an antiferromagnetic exchange interaction, explains the inversion of the overall spin, while the high-field discontinuity is explained in terms of dynamical models.     

Synthesis and magnetic properties of La3+-Co2+ substituted strontium ferrite particles using modified spray pyrolysis–calcination method

The purpose of the research was to improve the intrinsic magnetic properties of strontium ferrite by substituting lanthanum and cobalt for strontium and iron. The salt-assisted ultrasonic spray pyrolysis (SA-USP) following calcination process were used to from La-Co substituted strontium ferrite particles (La_xSr_1-xFe_12-yCo_yO₁₉), and their compositional dependent magnetic properties systemically investigated. All the samples were calcined at 1050 °C for 1 h in an air atmosphere to yield single-phased hexagonal particles several hundred nanometers to microns in size. A saturation magnetization of 70.76 emu/g and a coercivity 7265 Oe were obtained at a composition of La_0.25Sr_0.75Fe_11.75Co_0.25O₁₉. The amount of Co was reduced to obtain an optimized saturation magnetization of 71.40 emu/g and a coercivity of 7572 Oe at a composition of La_0.25Sr_0.75Fe_11.8Co_0.2O₁₉.     

Magnetic properties of Cu1+xMn2−xO4 and Ni1+xMn2−xO4 solid solutions

Magnetic properties of two spinel oxides solid solutions, Cu_1+xMn_2−xO₄ and Ni_1+xMn_2−xO₄, are reported. These series are characterized by two magnetic transitions: the upper one, of ferrimagnetic type, occurs at about 85 K (for copper-based) and at 105–110 K (for nickel-based spinels), independently of the x-content; the lower transition may be related to a Néel-type collinear ordering and takes place at 30 and 45 K, respectively. Application of moderate fields (H > 250 Oe) make both transitions to merge into one broad maximum in the magnetization, which takes place at lower temperature when applying larger fields. Magnetization cycles with temperature (ZFC/FC) or field (loops) allowed us to well characterize the ordered state. The effective moment follows the expected behavior when manganese ions are being substituted by ions of lower magnetic moment (Ni²⁺ and Cu²⁺).     

New LiNi0.5PrxFe2−xO4 nanocrystallites: Synthesis via low cost route for fabrication of smart advanced technological devices

Praseodymium substituted nano-crystalline Li-Ni spinel ferrites with different Pr³⁺ contents were synthesized by micro-emulsion method. X-ray diffraction (XRD), scanning electron spectroscopy (SEM) and vibrating sample magnetometery (VSM) techniques were employed to study the impact of substitution of the Pr³⁺ on the structure, surface morphology and magnetic parameters. XRD confirmed the formation of the single phase spinel ferrites of all compositions of LiNi_0.5Pr_xFe_2−xO₄ nanocrystallites. The crystallite size determined from XRD data by Scherrer formula was calculated in range from 40 nm to 70 nm. However the nanoparticles size estimated by SEM was found 35–115 nm. The room temperature VSM measurements were carried out in the applied field range from “−10,000 Oe” to “10000” Oe. Saturation magnetization (M_S) (41 emu/g) and coercivity (H_C) values (156.9 Oe) of LiNi_0.5Fe₂O₄ were improved by the addition of rare earth Pr³⁺ cations. The value of Hc is low, which is a strong indication of soft ferrites. The synthesized LiNi_0.5Pr_xFe_2−xO₄ ferrites may be utilized for low core losses on transformers.     

Influence of Al-substitution on structures,resistivity and magnetic properties of LaxSr(2−x)Fe(1+y)Mo(1−y)O6 (x=3y,y=0.05, 0.1, 0.15, 0.2)

Two series of single-phased La_xSr_(2−x)Fe_(1+y)Mo_(1−y)O₆ and La_xSr_(2−x)Fe_(1+0.5y)Al_0.5yMo_(1−y)O₆ (x=3y, y=0.05, 0.1, 0.15 and 0.2) double perovskites were prepared by solid-state reaction. The effects of Al-substitution on the structures, resistivity and magnetic properties of La_xSr_(2−x)Fe_(1+y)Mo_(1−y)O₆ were investigated. Although Al-replacement exhibits a negligible influence of on the B-site ordering degree, it results in the suppression of magnetisation caused by non-magnetic Al³⁺ ions. Reduction of grain sizes leads to increased resistivity, thus an optimised magnetoresistance (MR) behaviour is observed. The greatest MR extent improvement can be obtained when y is 0.15 and the MR% of the Al-doped ceramics reaches −10.5% (10 K, 1 T), which is 2 times greater than that of the undoped ceramics (−4.6%, 10 K, 1 T). Interestingly, the Curie temperature (Tc) of both Al-doped and undoped samples maintained relatively constant values of approximately 420 K and 405 K, respectively, which were different results from the data obtained for similar electron-doping systems in the literature.     

(La,Sr)(Mn,Me)O3 manganites doped with d metals: Study of charge compensation mechanisms by crystallographic and magnetic characterizations

A comparison between theoretically calculated unit cell volume and interatomic distances in the system La_0.7Sr_0.3Mn_1−xMe_xO_3+δ (where Me = Cu, Fe, Cr, Ti) and the experimental data obtained by the full-profile Rietveld X-ray analysis as well as an analysis of magnetic properties allowed us to suggest possible mechanisms of charge compensation occurring when d metals substitute for manganese. It has been shown that in the case when copper, iron, chromium and titanium ions substitute for manganese ions in the system La_0.7Sr_0.3Mn_1−xMe_xO₃ charge compensation is described by the model 2Mn³⁺ → Mn⁴⁺ + Cu²⁺, Mn³⁺ → Fe³⁺, Mn³⁺ → Cr³⁺ and Mn⁴⁺ → Ti⁴⁺, respectively. In the latter case, a decrease in oxygen nonstoichiometry occurs with increasing x.     

Influence of La addition on the structural,magnetic and magnetocaloric properties in Sr2−xLaxFeMoO6 (0≤x≤0.3) double perovskite

We report the effect of carrier doping via partial substitution of La³⁺ for Sr²⁺ on the structural, magnetic and magnetocaloric properties of Sr₂FeMoO₆ double perovskite. Polycrystalline Sr_2−xLa_xFeMoO₆ (x=0.0, 0.1, 0.2, 0.3) samples were prepared using the conventional solid state reaction method. Using the X-ray diffraction (XRD) analysis it was established that all the samples crystallized in a tetragonal structure with I4/mmm space group. An increase in the La doping lead to an increase in the lattice parameter ‘a’ and the volume of the unit cell. The lattice parameter ‘c’ however remained unchanged. The temperature variation in magnetization and Arrott analysis suggested a second order of ferromagnetic phase transition in all samples with Curie temperature, T_C increasing from 358 K for x=0.0–365 K for x=0.3. A gradual increase in magnetization was also observed with the increasing La content up to x=0.2. The magnetic entropy change was calculated from the measurement of isothermal magnetization versus magnetic field at different temperatures. The tunability of magnetization and T_C simply by adjusting the concentration of La and synthesis conditions makes Sr₂FeMoO₆ an attractive material for magnetic refrigeration at desired temperature.     

Influence of Na doping on the magnetic properties of LaFeO3 powders and dielectric properties of LaFeO3 ceramics prepared by citric sol-gel method

The emergence of ferromagnetism in perovskite oxide LaFeO₃ nanoparticles and colossal dielectric response in ceramics has inspired researchers to study the effect of various dopants on the magnetic and dielectric properties of LaFeO₃ powders and ceramics. However, the influence of alkali element Na doping has not been studied yet, and the origin of such ferromagnetic behavior is still ambiguous. The primary objective of the present work is to elucidate the effect of Na doping on the magnetic properties of La_1−xNa_xFeO₃ (x=0, 0.1 and 0.2) powders and dielectric properties of corresponding ceramics prepared by citric sol-gel method. FE-SEM results showed that the introduction of Na dopant actually resulted in the formation of nonstoichiometric La_1−xFeO₃ and x/2 Na₂O. Compared to the canted AFM behavior for the pure powder, ferromagnetic behavior with enhanced magnetization of 2.11 emu/g at 10 kOe could be obviously observed at room temperature for the powder with x=0.2. XPS measurement suggested nonstoichiometric Fe/La ratio which leads to the distortion of lattice structure and enlarged canting angle between the two AFM coupled Fe sublattice should be responsible for the enhancement of magnetization in the Na-doped samples. Meanwhile, the introduction of Na dopant lowered the growth temperature of grains of the parent LFO and resulted in larger average grain size, which in turn leaded to great enhancement of ε′ into the order of 10⁵ at 100 Hz at the cost of high tanδ for the Na-doped ceramics.     

Large relative cooling power of Bi-doped La0.8−xBixSr0.08(Ca0.55Ba0.45)0.12MnO3 (x=0.0, 0.1 and 0.3) perovskites: Magnetic and magnetocaloric properties

We report magnetic and magnetocaloric properties of La_0.8−xBi_xSr_0.08(Ca_0.55Ba_0.45)_0.12MnO₃ (x=0.0, 0.1 and 0.3) perovskite manganites. Polycrystalline samples have been synthesized in air by the sol gel method at a sintering temperature of 1150 °C. Powder X-ray diffraction data show that samples are phase pure and their cell parameters slightly decrease with increase in Bi content. Scanning electron micrographs show that the average particle size increases with increase in Bi content. The temperature dependent magnetization measurements show that the Curie temperatures decrease from 315 K (x=0.0) to 140 K (x=0.3) with increase in Bi content. The isothermal magnetization data is used to estimate the magnetic entropy changes (−$\Delta S_M$) and their calculated values are 1.12 J kg⁻¹ K⁻¹, 1.96 J kg⁻¹ K⁻¹ and 1.62 J kg⁻¹ K⁻¹ for x=0.0, 0.1 and 0.3 samples respectively under an applied magnetic field of 2.0 T. The corresponding values of relative cooling power (RCP) are 90 J kg⁻¹, 180 J kg⁻¹, 136 J kg⁻¹ for x=0.0, 0.1 and 0.3 samples respectively. These results of magnetocaloric effect in our samples suggest that they are promising materials for the magnetic refrigeration applications.     

Piezoelectric and dielectric properties of Ce substituted La2Ti2O7 ceramics

The ferroelectric and dielectric properties of cerium (Ce) substituted La₂Ti₂O₇ (LTO) have been investigated. Single phase, dense La_2?xCe_xTi₂O₇ (x = 0.15, 0.25, 0.35) ceramics were prepared by spark plasma sintering. The solubility limit of Ce in La_2?xCe_xTi₂O₇ was found to be between 0.35 and 0.5. The a-, b- and c-axes of the unit cell decrease with increasing Ce substitution. The Curie point (Tc) of La_2?xCe_xTi₂O₇ (x = 0, 0.15, 0.25, 0.35) decreases and dielectric constant and loss increase with increasing Ce substitution. Cerium can increase the d₃₃ of La₂Ti₂O₇. The highest d₃₃ was 3.9 ± 0.1 pC/N for La_1.85Ce_0.15Ti₂O₇ textured ceramic.     

Effect of Ni2+ substitution on structural,magnetic, dielectric and optical properties of mixed spinel CoFe2O4 nanoparticles

Nanoparticles of Co_1−xNi_xFe₂O₄ with x=0.0, 0.10, 0.20, 0.30, 0.40 and 0.50 were synthesized by co-precipitation method. The structural analysis reveals the formation of single phase cubic spinel structure with a narrow size distribution between 13–17 nm. Transmission electron microscope images are in agreement with size of nanoparticles calculated from XRD. The field emission scanning electron microscope images confirmed the presence of nano-sized grains with porous morphology. The X-ray photoelectron spectroscopy analysis confirmed the presence of Fe²⁺ ions with Fe³⁺. Room temperature magnetic measurements showed the strong influence of Ni²⁺ doping on saturation magnetization and coercivity. The saturation magnetization decreases from 91 emu/gm to 44 emu/gm for x=0.0–0.50 samples. Lower magnetic moment of Ni²⁺ (2 µ_B) ions in comparison to that of Co²⁺ (3 µ_B) ions is responsible for this reduction. Similarly, overall coercivity decreased from 1010 Oe to 832 Oe for x=0.0–0.50 samples and depends on crystallite size. Cation distribution has been proposed from XRD analysis and magnetization data. Electron spin resonance spectra suggested the dominancy of superexchange interactions in Co_1−xNi_xFe₂O₄ samples. The optical analysis indicates that Co_1−xNi_xFe₂O₄ is an indirect band gap material and band gap increases with increasing Ni²⁺ concentration. Dispersion behavior with increasing frequency is observed for both dielectric constant and loss tangent. The conduction process predominantly takes place through grain boundary volume. Grain boundary resistance increases with Ni²⁺ ion concentration.     

Catalytic removal of diesel soot particulates over K and Mg substituted La1 − xKxCo1 − yMgyO3 perovskite oxides

K and Mg substituted perovskite catalysts La_1 − xK_xCo_1 − yMg_yO₃ (x = 0–0.4, y = 0–0.2) for soot combustion were prepared by citric acid complexation and characterized by XRD, FT-IR, SEM, TEM, EDS, H₂-TPR, XPS and TG. Soot combustion was remarkably accelerated when K was introduced into LaCoO₃. Then Mg was doped into the K substituted LaCoO₃, soot combustion was further improved for the restrained growth of Co₃O₄ phase. K/Mg substitutions were responsible for enhancing activity of catalysts by improving reducibility as suggested by H₂-TPR studies. Among all the catalysts, La_0.6K_0.4Co_0.9Mg_0.1O₃ exhibited the highest activity.     

Phase structural transition and microwave dielectric properties in isovalently substituted La1−xLnxTiNbO6 (Ln=Ce,Sm) ceramics

Aeschynite-type La_1−xLn_xTiNbO₆ (Ln=Ce, Sm, x=0–1) ceramics were prepared via a conventional solid state method. Analysis of X-ray diffraction, Raman, infrared reflectivity spectra and the microstructures revealed a series of composition-induced phase evolution in sequence: monoclinic→coexistence of monoclinic and orthorhombic→orthorhombic structure, i.e. M→M+O→O. The critical compositions of distinguishing the dominant M or O phase were x=0.15 in La_1−xCe_xTiNbO₆ and x=0.10 in La_1−xSm_xTiNbO₆ ceramics, exactly corresponding to the average ionic radius of rare earth ions (IR) ~1.027 Å. The crystal structure and microwave dielectric properties of RETiNbO₆ (RE=rare earth) ceramics strongly depended on IR. Near-zero τ_f was achieved in the Ce-sample with x=0.153 (ε_r=28.9, Q×f=17,275 GHz@6.54 GHz) as well as in the Sm-sample with x=0.098 (ε_r=28.2, Q×f=19,186 GHz@6.78 GHz). Eventually, RETiNbO₆ would form O euxenite (-τ_f), O aeschynite (+τ_f), and M aeschynite (-τ_f), as IR<0.945 Å, 0.945 Å <IR<1.027 Å, and 1.027 Å<IR<1.032 Å, respectively. The infrared reflectivity study also confirmed that the structural phonon oscillations in the infrared region dominated the dielectric performance in the microwave region for this system.     

Structural,magnetic and Mössbauer study of BaLaxFe12−xO19 nanohexaferrites synthesized via sol–gel auto-combustion technique

BaLa_xFe_12−xO₁₉ (0.05≤x≤0.25) nanohexaferrites were synthesized by sol–gel auto combustion method. X-ray diffraction study revealed the hexagonal structure of the synthesized nanoferrites without any secondary phase and Rietveld analysis confirmed the P63/mmc space group. The crystallite size was observed to increase (49–63 nm) with the increasing substitution of La³⁺ ions. The particle size was observed to be in the range 49–63 nm. The remarkable increase in saturation magnetization upto 78.5 emu/g and an increase in magneto-crystalline anisotropic was observed with the increase in La³⁺ substitution. In addition, for the first time we have reported the Mösssbauer study of BaLa_xF_12−xO₁₉ nanohexaferrites in the present paper.