Search

In the current research work, half-doped perovskite cobaltite Nd0.5Ba0.5CoO3 (NBCO) was prepared by the sol–gel process. We study the structural, morphological, magnetocaloric and magnetic properties of NBCO. The X-ray diffraction patterns showed that the NBCO sample crystallizes in the orthorhombic distorted symmetry with the Pmmm space group. Magnetic measurements indicated the existence of a paramagnetic (PM)–ferromagnetic (FM) transition at TC = 125 K. A ferromagnetic-to-antiferromagnetic transition is observed at T = 25 K. The second-order transition was confirmed by both Banerjee criterion and construction of the universal curve. Indeed, the magnetic entropy change (-ΔSM) value estimated by Landau and Hamad theories is close to that obtained using the classical Maxwell relation. The value of the relative cooling power (RCP) under a magnetic field of 5 T is 114 J. kg−1; therefore, this compound can be considered as a potential candidate for application in the field of magnetic refrigeration.

Polycrystalline Mg0.5Co0.5Fe1.6Al0.4O4 ferrite was prepared using sol–gel method. This sample was characterized by powder X-ray diffraction (XRD), Scanning electron microscopy, and impedance spectroscopy. XRD analysis combined with the Rietveld refinement confirmed the cubic-spinel structure (SG: $$Fd\stackrel{-}{3}m$$ ) for the prepared sample. Electrical conductivity obeying the Jonscher power law indicates that the prepared material exhibits semiconductor behavior, and the conduction process follows the “non-overlapping small polaron tunnelling, NSPT” model between neighbors’ sites. The behavior of dielectric constants such as permittivity and loss coefficient has been interpreted based on the Maxwell–Wagner’s theory of interfacial polarization. The curves of imaginary parts of impedance (Z″) and modulus (M″) show dielectric-relaxation phenomenon in the sample with activation energy near to that determined from the dc conductivity study. Nyquist plots (− Z″ vs. Z′) show a monotonic decrease in both grain resistance (Rg) and grain boundary resistance (Rgb) with increasing temperature such as Rgb ≫ Rg. This result confirms that the transport mechanism in Mg0.5Co0.5Fe1.6Al0.4O4 compound is governed by the grain boundaries effect.

We have reported the sol-gel synthesis and microstructural, magnetic, magnetocaloric, and critical behaviors of La0.7Ba0.15Sr0.15CoO3 cobaltite. The XRD data refined by Rietveld technology confirms that a sample has a pure perovskite phase with rhombohedral structure. We tested the magnetization curve near the phase transition temperature (TC) from ferromagnetic (FM) to paramagnetic (PM), and made the Arrott diagram which confirmed the existence of the secondary phase transition. The obtained magnetic constants from hysteresis loop offer to the sample the possible use in some interesting technological applications. The M(μ0H, T) isothermal magnetizations have been analyzed to estimate the critical exponents (β, γ, and δ). We found that β exponent is close to that of the mean-field model; however, γ and δ exponents belong to an unconventional model. The reliability of the estimated critical exponents was checked from the magnetic field variations of the magnetic entropy change (− ΔSm) and the relative cooling power (RCP).

Based on the mean-field theory, the magnetic and magnetocaloric behavior of La0.6Sr0.4Mn0.9V0.1O3 manganese oxide has been analyzed. The Bean-Rodbell equation of state presents a second-order magnetic phase transition with a η factor of η = 0.71. Using the experimental data of magnetization M (H, T), the molecular mean-field parameter is found to be λ1 = 2.19 T g emu−1. The Brillouin function makes it possible to determine the total angular moment J, the saturation magnetization MS, and the Lande factor g for La0.6Sr0.4Mn0.9V0.1O3 sample. We simulated the magnetization as a function of the magnetic field and the temperature by applying the mean-field theory, as well as the variation of the magnetic entropy change ΔSM(T). As observed, the simulated results close with experimental data. The magnetic entropy changes for the samples were also estimated by using the mean-field scaling theory, and the results show a difference between the theoretical and experimental values.

In this work, an overview of the Weiss molecular mean-field theory, the Bean–Rodbell model and the Landau theory is presented, providing the theoretical background for simulating the magnetocaloric properties for La0.6Ca0.2Na0.2MnO3 manganite. Results showed that sample exhibits second-order ferromagnetic (FM)–paramagnetic (PM) magnetic phase transition and relatively higher values of magnetic entropy change (−∆SM). In application point of view, this material can be used in magnetic refrigeration technology. The theoretical values of −∆SM determined using each theory agree well with the experimental ones estimated from Maxwell relations. In other part, a good agreement in the spontaneous magnetization values, Mspont(T), estimated from (−∆SM vs. M2) and (H/M vs. M2) data was found. Also, the values of the critical exponent (β) found from both methods are close and check that the mean-field model is adequate to study the MCE in La0.6Ca0.2Na0.2MnO3 sample.

The impendence spectroscopy properties of polycrystalline Nd0.67Pb0.33Mn0.9Al0.1O3 perovskites prepared by sol–gel method at two different sintering temperatures (900°C and 1100°C), have been inves...

Ni0.4Cd0.3Zn0.3Fe2O4 ferrites have been prepared through the use of sol-gel method at 900 °C and 1100 °C. Rietveld refinements of XRD patterns indicate that the prepared samples crystallize in the cubic spinel structure. Lattice constant and grain size are found to increase with sintering temperature. Magnetic measurements show that the maximum magnetization (Ms) rises, whereas both coercivity (Hc) and remanence (Mr) decrease when increasing the sintering temperature. Frequency and temperature dependence of electrical conductivity, electrical modulus, and electrical impedance have been studied using impedance spectroscopy technique. As the sintering temperature increases, the conductivity of the samples increases. The variation of imaginary part of modulus (M″) displays the presence of an electrical relaxation phenomenon and non-Debye nature. Nyquist representations have been analyzed using an electrical equivalent circuit. The obtained results reveal that the conduction mechanism of the samples is achieved basically of the grain boundary contribution.

In this research paper, we have carried out an in-depth study of the critical phenomena at the FM–PM phase transition for Nd0.5Ba0.5CoO3 (NBCO). The FM–PM phase transition is identified as second order for our material. Moreover, the critical exponents of NBCO compound were estimated based on multiple techniques such as Kouvel–Fisher, modified Arrott plot method, and critical isotherm analysis. Therefore, the average values of the critical exponent and the critical temperature obtained by the various methods are (βmoy = 0.506; γmoy = 1.011; δmoy = 3.042; TC = 125.543). The obtained critical exponents are in agreement with the mean-field model. Furthermore, the exponent n as a function of temperature reveals the non-homogeneous nature of NBCO compound. According to the mean field theory, the magnetic entropy change (ΔSM vs. M2) was invested to determine the spontaneous magnetization (Mspont) values. Results match reasonably well with those obtained from the classical extrapolation of Arrott plots (μ0H/M vs. M2).

Dielectric and conduction properties were investigated on La1 − xSrxFeO3 (0 ≤ x ≤ 0.5) ceramics synthesized by solid-state reaction method. X-ray powder diffraction pattern of all prepared samples and their Rietveld refinement revealed that all samples crystallize in orthorhombic structure with Pnma space group. Sr substitution induced the volume unit cell to decrease and the crystallite size to vary in a non-monotonic manner. Scanning electron microscopy images revealed also Sr-dependent morphology of the studied ceramics. Fourier transform infrared (FTIR) spectra evidenced structural distortion effects on νFe–O–Fe, σFe–O–Fe and νLa–O vibrations and showed the presence of the carbonate ion impurity for the compositions x = 0.2, 0.3, 0.4 and 0.5. Ac conductivity analysis was accomplished according to equation: $$\sigma_{ac} \left( \omega \right) = \frac{{\sigma_{s} }}{{1 + \tau^{2} \omega^{2} }} + \frac{{\sigma_{\infty } \tau^{2} \omega^{2} }}{{1 + \tau^{2} \omega^{2} }} + A \omega^{n}$$ for the parent compound and its doped one La0.9Sr0.1FeO3. However, this analysis was realized by using Jonscher law: $$\sigma_{ac} \left( \omega \right) = \sigma_{dc} + A \omega^{n}$$ for the compositions x = 0.2, 0.3, 0.4 and 0.5. It revealed that the hopping process occurred through long distance for 0 ≤ x ≤ 0.3 compositions, whereas for x = 0.4 and 0.5 compositions the hopping occurred between neighboring sites. Complex impedance analysis performed on these ceramics by means of different electrical equivalent circuits owing to their different morphologies allowed probing grain boundary effect on ac conductivity.

We investigate the effect of Sb5+ doping at Mn-site in La0.67Ba0.33Mn1−xSbxO3 (LBMO-Sbx) on the magnetic and magneto-electrical properties. The variation of the magnetization M versus temperature T, under an applied magnetic field of 0.05 T, reveals a ferromagnetic–paramagnetic transition for all samples. The resistivity and magneto-transport measurements are performed using standard four-probe assembly with and without magnetic fields. The temperature dependence of electrical resistivity shows that all samples undergo a sharp metal–semiconductor (M–SC) transition at a temperature (TM–SC), accompanying the ferromagnetic–paramagnetic transition. The peak resistivity ρmax is noted at the metal–semiconductor transition temperature (TM–SC) and lowering in TM–SC is observed for higher concentrations of Sb5+. The resistivity data have been analyzed in two parts. Firstly, in the metallic region below TM–SC the resistivity data is fitted with three degree polynomial. Secondly, in the semiconducting region above TM–SC data have been fitted with Small Polaron Hopping models. Above all, the magnetoresistance study showed a peak which has a high value around the M–SC transition temperature. The dependence of resistivity on the temperature and magnetic field data is used to deduce the magnetic entropy change.

This work focuses on the structural and electrical properties of CaMn1−xFexO3−0.5x (x = 0.0 and 0.20) perovskite compounds prepared using the ceramic method. XRD patterns confirm the formation of the orthorhombic structure with Pnma space group for the samples, and Rietveld method was used to estimate their different structural parameters. Refinement results show that the cell parameters and unit cell volume increase with increasing Fe content. The complex impedance spectroscopy technique was used to investigate the frequency and temperature dependence of electrical conductance and electrical impedance. The total conductance curves of the sample are found to obey Jonscher power law GT(ω) = Gdc + Gac = Gdc + Aωn with an increase in frequency exponent (n) as temperature increases. Electrical impedance results show the presence of the electrical relaxation phenomenon for the samples. The activation energy deduced from the analysis of the conductance curves matches very well with the value estimated from the relaxation time, indicating that relaxation process and electrical conductivity are attributed to the same defect. The Nyquist representations have been analyzed using a proposed electrical circuit, and the results show that the grain boundary contribution is responsible to the conduction mechanism of the prepared samples.

The effects of Ni substitution on structural, magnetic and magnetocaloric properties of Mg0.6Cu0.4−xNixFe2O4 (x = 0 and 0.2) ferrites synthesized via sol–gel method has been investigated in this work. XRD patterns confirm the formation of cubic spinel structure with $$Fd\bar {3}m$$ space group. Both samples present ferromagnetic (FM)–paramagnetic (PM) phase transitions of second order type with an increment of Curie temperature (TC) from 630 to 670 K for x = 0 and 0.2, respectively. The addition of Ni leads to the increase of the values of maximum magnetic entropy change ( $$- \Delta S_{M}^{{max}}$$ ) and relative cooling power (RCP) from (1.09 J kg−1 K−1 and 136 J kg−1, for x = 0) to (1.38 J kg−1 K−1 and 173 J.kg−1, for x = 0.2) for an applied magnetic field of H = 5 T. The magnetic entropy changes for the samples were also estimated by using the mean-field scaling theory, and the results show a difference between the theoretical and experimental values.

This work focuses on the structural, magnetocaloric effect, and critical exponent investigations of La0.6Sr0.2Na0.2MnO3 manganite prepared via sol–gel method. XRD analysis confirms the rhombohedral structure with $$ R\overline{3}c $$ symmetry for this sample, and Rietveld method was used to estimate its different structural parameters. The sample presents FM-PM phase transition of second-order type at TC = 309 K. The maximum of magnetic entropy change (− $$ \Delta {S}_M^{max} $$ ) and relative cooling power (RCP) reach values of about 3.57 J kg−1 K−1 and 214.46 J kg−1, respectively (at μ0H = 5 T). These values indicate that La0.6Sr0.2Na0.2MnO3 sample can be used as active magnetic refrigerator. Critical exponents (β, γ, and δ) of the sample have studied by analyzing the M(μ0H, T) isotherms using different techniques. The estimated values of these critical exponents are found to be different to those predicted for standard theoretical models.

Structural, magnetic and magnetocaloric properties of perovskite manganite La0.55Pr0.1Sr0.35Mn1-xTixO3 (LPSMTxO, x ​= ​0; 0.05) synthetized by sol–gel method have been studied. X-ray diffraction studies have shown that all samples have been crystallized in rhombohedral symmetry with R 3 ¯ c space group. The magnetic study revealed that the samples have exhibited a ferromagnetic (FM) – paramagnetic (PM) phase with decreasing Curie temperature (TC) from 310 ​K to 288 ​K when Ti doping level increases. At μ0H ​= ​3T, the maximum value of the magnetic entropy change ( − Δ S M m a x ) varies from 3.58 ​J/kg.K to 2.78J/kg.K for x ​= ​0.00 and 0.05 respectively. Besides, by performing the Landau theory, the deviation between experimental and theoretical calculations have indicated the noncontributing of elastic, magnetoelastic and magnetoelectronic coupling in the magnetocaloric properties of La0.55Pr0.1Sr0.35Mn1-xTixO3. Accordingly, important results of the relative cooling power (RCP) for the two investigated compounds were obtained proving that they can be suggested as good candidates for magnetic refrigeration technology.

Structural analysis, magnetocaloric properties, and theoretical investigations of the magnetocaloric effect were carried out in the crystalline La0.7Ba0.15Ag0.15MnO3 manganite prepared using sol-gel route. The phase purity and structure of this sample were checked by X-ray diffraction technique and Rietveld analysis. From magnetic measurements, the ferromagnetic to paramagnetic (FM-PM) phase transition was observed around TC = 255 K. The maximum change in magnetic entropy ( $$ \Delta {S}_M^{max} $$ ) and relative cooling power (RCP) extracted from magnetic measurements were 3.48 J × kg-1 × K-1 and 225 J × kg-1 at an applied magnetic field of 5 T. These magnetocaloric parameters offer to the sample the possible use in the magnetic refrigeration technology. The magnetic entropy simulation by using different theories such as the Weiss molecular mean-field theory and the Landau theory shows good correlation between the theoretical values of −∆SM(T) and the experimental ones estimated from Maxwell relation.

We have reported the solgel synthesis, the structural analysis, and the correlation between magnetic and electrical properties of La0.7Ba0.15Ag0.15MnO3 manganite. The phase purity and structure of this sample were checked by X-ray diffraction technique and Rietveld analysis. M(T) measurement shows a ferromagnetic (FM)-to-paramagnetic (PM) phase transition of second-order type at Curie temperature TC = 255 K. The obtained magnetocaloric parameters offer to the sample the possible use in the magnetic refrigeration technology. The resistivity data, ρ(μ0H, T), show the presence of metal–semiconductor transition and a maximum magnetoresistance (MR) value of the order of 32% at magnetic field of 5 T. The phenomenological percolation model was used to study the conduction mechanism of the sample. It has been also shown that, from analyzing the temperature and magnetic field dependences of the electrical resistivity, it is possible to predict the behavior of the magnetization as well as the change of magnetic entropy near the phase transition temperature. The obtained agreement with experimental data for this analysis is quite reasonable.

Cr-substituted Cd–Zn spinel ferrites of different compositions (Cd0.5Zn0.5Fe2−xCrxO4) were prepared using solgel method for x = 0 and 0.5. The XRD analysis and Rietveld refinements indicate that samples crystallize in the cubic $$Fd\overline{3}m$$ spinel structure and the lattice parameters decrease with Cr substitution. Two principal absorption bands related to the stretching vibration of tetrahedral and octahedral sites were shown from FTIR spectra. The spontaneous magnetization, coercivity and remanance were also investigated for the prepared samples. DC conductivity was found to decrease with the substitution of Cr. The studies of imaginary part of permittivity and tangent loss reveal that the prepared materials may be good candidates for some interesting electronics devices. An electrical relaxation phenomenon with non-Debye nature was observed from the variations of imaginary parts of modulus and impedance. The estimated activation energies values from relaxation time and dc-conductivity are very close. Electrical parameters deduced from the Nyquist plots using an equivalent circuit show that the conduction process in the prepared samples is due to the grain boundaries contributions.

In this paper, we have used the Landau theory to simulate the MCE in the Nd0.67Ba0.33MnO3 manganite. The expanding of the Gibb’s free energy as a function of magnetization to the eighth order has given better results than the developing at sixth order in modeling the magnetic entropy change. The second-order phase magnetic transition of Nd0.67Ba0.33MnO3 sample has been proved. Under low magnetic fields, a phenomenological model has been used to determine some magnetic properties for the sample. Theoretical calculations have been compared to the experimental results.

We have reported the results of structural, magnetic, and magnetocaloric characterizations as well as the critical behaviors of the polycrystalline Nd0.67Pb0.33Mn0.9Al0.1O3 manganites. These samples, denoted as S900 and S1100, were prepared using the sol–gel method and sintered separately at 900 °C and 1100 °C, respectively. XRD analysis shows good crystallization for the samples in the $$ R\overline{3}c $$ rhombohedral structure. From M(T) curves, second-order ferromagnetic–paramagnetic (FM–PM) phase transitions appear at the Curie temperatures (TC) around 130 K and 155 K for S900 and S1100, respectively. The estimated values of magnetic entropy change (−ΔSm) at μ0H = 5 T were found to be 3.23 and 4.41 J/kg.K for S900 and S1100, respectively. The corresponding values of relative cooling power (RCP) are equal to 242 and 245 J/kg. These values are relatively higher, making our samples promising candidates for magnetic refrigeration technology. We found that the critical exponents (β, γ, and δ) agree well with those of the mean-field and the 3D-Heisenberg models for S900 and S1100, respectively.

La0.6Ca0.2Na0.2MnO3 manganite was prepared using sol–gel method. Rietveld refinement of XRD pattern indicates that the prepared sample crystallizes in the rhombohedral $$R\overline{3}c$$ structure. The impendence spectroscopy properties of the sample were studied by performing a series of measurements as a function of temperature and frequency of conductivity, dielectric constants, modulus, and impedance. From these measurements, the metal–semiconductor behavior was observed for the sample at a transition temperature of the order of 240 K. The studies of imaginary part of permittivity and tangent loss reveal that the prepared material may be a good candidate for low-frequency energy storage devices. An electrical relaxation phenomenon with non-Debye nature was observed in variations of imaginary parts of modulus and impedance. Electrical parameters deduced from the Nyquist plots analyses using an equivalent circuit reveal that the conduction process for the sample was caused due to the grain boundaries contribution.

The results of structural, magnetic, critical, and magnetocaloric characterizations of La0.6Ca0.2Na0.2MnO3 perovskite have been reported. XRD analysis of the sample synthesized using sol-gel method shows good crystallization in the $$ R\overline{3}c $$ rhombohedral structure. From M(T) curve, a second-order ferromagnetic-paramagnetic (FM-PM) phase transition appears at the Curie temperature (TC) around 275 K. Critical exponents (β, γ, and δ) have been evaluated using different techniques such as modified Arrott plots (MAP), Kouvel-Fisher (KF) method, and critical isotherm (CI). We found that the critical exponents of the prepared sample agree well with those of the mean-field model. We have also estimated the magnetic entropy change (−ΔSM) and relative cooling power (RCP) for the sample which obey power laws as −∆SM = a(μ0H)n and RCP = A(μ0H)N, respectively. ΔSM and RCP values were relatively higher, making the sample promising candidate for magnetic refrigeration technology. The estimated values of the magnetic ordering parameters (n and N) are used to confirm the reliability of the evaluated critical exponent.

In this work, the structural, optical, and dielectric properties for Mg0·6Cu0·2Ni0·2Cr2O4 chromite sample prepared by sol-gel process were investigated. The XRD characterization for this sample confirmed the development of the face-centered cubic spinel structure with F d 3 ‾ m space group. From the UV–Vis optical absorbance, the synthesized chromite shows significantly lower direct band gap energy and has an appropriate absorption rate in the visible light region. The variations of penetration depth, extinction coefficient, refractive coefficient, dispersion energy parameters, and dielectric permittivity constants were also interpreted. The Non-Overlapping Small Polaron Tunneling (NSPT) and the Correlated Barrier Hopping (CBH) are the suitable models to describe the conduction process for the prepared sample. The behavior of dielectric constants has been interpreted based on the Maxwell-Wagner's theory of interfacial polarization. The behavior of imaginary part of impedance (Z″) shows a dielectric-relaxation phenomenon in the sample with activation energies near to those determined from the conductivity study.

In this work, we have used many theoretical models to simulate the magnetocaloric effect (MCE) in the charge ordered P r 0.5 S r 0.5 M n O 3 (PSMO) manganite. This sample undergoes a first order antiferromagnetic charge-ordered (AFM/CO) to a ferromagnetic (FM) transition at TCO∼165 K followed by a second order FM to paramagnetic (PM) transition at Curie temperature, TC∼255 K. The Mean-field theory and the Bean-Rodbell model are called to study deeply the MCE in FM-PM transition. Based on the mean-field theory, we proposed a numerical approach to study the evolution of magnetization versus temperature for the first order AFM/CO – FM transition. By using a phenomenological model, we have compared theoretical and experimental MCE for the above two transitions.

We have investigated the effect of sintering temperature on structural, magnetic, magnetocaloric and critical behaviors of Ni0.4Cd0.3Zn0.3Fe2O4 ferrites synthesized using sol-gel method at 900 °C and 1100 °C. X-ray diffraction patterns indicated that samples crystallize in the cubic spinel structure ( Fd 3 ‾ m space group) with an increase of lattice constant and average crystallite size as the sintering temperature increases. Magnetic measurements revealed that that the prepared samples undergo second-order ferromagnetic (FM) to paramagnetic (PM) phase transitions. An increment in magnetization, Curie temperature, magnetic entropy change and relative cooling power has been observed with increasing the sintering temperature. The magnetic entropy change reached maximum values of about 1.11 J.kg −1.K−1 and 1.62 J.kg −1.K−1 for μ0H = 5 T corresponding to relative cooling power (RCP) of 152.09 J.kg −1 and 253.65 J.kg −1 for samples sintered at 900 °C and 1100 °C, respectively. These values are comparable favorably with those of some others ferrites considered as possible candidates for magnetic refrigeration. The estimated critical exponents obtained for sample sintered at 1100 °C are close to the tricritical mean-field model; whereas for sample sintered at 900 °C, the exponents belong to a different universality class. This change of critical exponents is mainly due to the increase of average crystallite size as the sintering temperature increases.

The magnetocaloric effect in Pr0.5Sr0.5MnO3 (PSMO) has been successfully modeled in this work. PSMO undergoes a first-order antiferromagnetic charge ordering (AFM/CO) to a ferromagnetic (FM) transition at $ T_{CO}=T_{N}\sim 165$ K followed by a second-order ferromagnetic (FM) to paramagnetic (PM) transition at the Curie temperature, $ T_{C}\sim 255$ K. The magnetocaloric effect in PSMO has been studied by the simulation of the magnetic entropy change $ (-\Delta S_M)$ for these two transitions by developing a numerical approach based on the Landau free energy within the mean-field approximation. Simulated results agree well with the experimental ones for these two transitions.

The magnetocaloric properties for the Co-doped La0.67Pb0.33(Mn1−xCox)O3 samples with x= 0.01, 0.06, 0.1 and 0.15 under a 0.005 T magnetic field have been theoretically investigated and compared with experimental results. It has been found that the Co doping in this system decreases lightly the magnetocaloric properties. Moreover, the results of Co doping indicate that the magnetocaloric effect in this system is tunable, which is beneficial for manipulating magnetocaloric refrigeration that occurs in various temperature ranges. This makes the La0.67Pb0.33(Mn1−xCox)O3 samples potential candidates for practical applications.

Spinel ferrites having Mg0.6 − xCoxZn0.4Fe2O4 (0 ≤ x ≤ 0.6) compositions were prepared using the sol–gel method. XRD patterns show that samples have the cubic phase of the spinel ferrite with the $Fd\bar {3}m$ space group. The obtained values of the lattice constant and porosity increase with increasing cobalt concentration. FTIR spectra show only the tetrahedral stretching peaks that are shifting towards higher frequencies with increasing Co content. The substitution of Mg by Co leads to the decrease of conductivity of the samples. The behaviors of the real part of permittivity and loss tangent have been investigated using Maxwell theory. Electrical modulus and impedance results show the presence of the electrical relaxation phenomenon for the samples with non-Debye type. The Nyquist representations have been analyzed using a proposed electrical circuit, and the results show that the resistances of the prepared samples resulted mostly from the grain boundary effect.

We have investigated the correlation between magnetocaloric and electrical properties of La0.47Pr0.2Pb0.33MnO3 perovskite prepared using the sol–gel method. Rietveld analysis of X-ray diffr...

Ferrite with nominal composition Ni0.6Cu0.4Fe2O4 was synthesized using Pechini sol-gel method. The X-ray diffraction results indicate that the ferrite sample has a cubic spinel type structure with F d 3 ¯ m space group without any impurity phase. The electrical properties of this ferrite using complex impedance spectroscopy technique have been carried out as a function of frequency at different temperatures. The total conductance curves for the sample are found to obey Jonscher power law (G(ω) = GDC + Aωn) with an increase of frequency exponent (n) as temperature increases. Frequency dependence of imaginary part of impedance (Z″) shows the existence of relaxation phenomenon in our sample. The impedance study using Nyquist representation revealed the appearance of semicircle arcs and an equivalent circuit of the type of (Rg + Rgb//ZCPE) has been proposed to explain the impedance results. Likewise, the analysis of the temperature variation of the imaginary part of modulus (M″) spectra confirms the existence of relaxation phenomena. Activation energies calculated from DC conductance, impedance and modulus spectra are in close agreement. This indicates that the relaxation process and electrical conductivity are attributed to the same defect.

In this work, the properties of the Ni0.5Mn0.5Cr2O4 chromite spinel prepared using sol-gel method were investigated. The TGA and DSC analyses were used to study the spinel phase formation and the weight loss during the synthesis process for the sample. The XRD and TEM characterizations confirmed the development of the cubic spinel structure. From the FTIR spectrum, the characteristics vibrational modes corresponding to the spinel structure were observed. From the room temperature UV–Vis optical absorbance, the synthesized chromite sample shows a significantly lower direct band gap (Egd= 1.90 eV) and has an appropriate absorption rate in the visible light region. Magnetization (M-T curve) shows a second order magnetic phase transition from the ferrimagnetic (FiM) behavior to the paramagnetic (PM) behavior at the Curie temperature TC= 40 K. By comparing our results to those previously found for the NiFe2O4 spinel, we can conclude that the substitution of Mn for NiFe2O4 increases its magnetization and decreases its coercivity. The magnetocaloric performances achieved for Ni0.5Mn0.5Cr2O4 sample are quite comparable to several literature values found for other chromite spinels.

The effect of Fe-doping at Mn-site on the structural and electrical properties of Nd0.67Ba0.33Mn1−xFexO3 (0 ≤ x ≤ 0.05) perovskites has been investigated. X-ray diffraction patterns show that the structural parameters change slightly due to the fact that the Fe3+ ions replacing the Mn3+ have similar ionic radius. The electrical properties of these samples have been investigated using complex impedance spectroscopy technique. a function of the frequency at different temperatures. When increasing the Fe-content, a decrease of dc conductivity was observed throughout the whole explored temperature range and the deduced activation energy values are found to increase from 128 meV for x = 0 to 166 meV for x = 0.05. The curves of the imaginary part of impedance (Z″) show the presence of relaxation phenomenon in our samples. The complex impedance spectra show semicircle arcs at different temperatures and an equivalent circuit of the type of Rg + (Rgb//Cgb) has been proposed to explain the impedance results.

We have investigated the effects of barium deficiency on structural, magnetic and magnetocaloric properties of La0.6Nd0.1Ba0.3−xMn0.9Cr0.1O3 (x = 0, 0.05 and 0.1) manganites prepared using the solid-state reaction. All the studied samples crystallize in the orthorhombic structure phase with Pnma space group. Magnetization versus temperature showed that the samples exhibit a second-order paramagnetic to ferromagnetic transition with a decrease of the Curie temperature (TC) from 275 K for x = 0–235 K for x = 0.1. The increase of barium deficiency leads to the decrease of the maximum magnetic entropy change ΔSMmax and relative cooling power values from (4.40 J.kg−1 K−1 and 262.94 J.kg−1, for x = 0) to (2.78 J.kg−1 K−1 and 159.41 J.kg−1, for x = 0.1) for an applied magnetic field μ0H = 5 T. These values are compared favorably with those of some other reported manganites, making our samples promising candidates for the magnetic refrigeration technology.

Magnetic entropy change (−ΔS M ) of Nd0.67 Ba0.33Mn0.98Fe0.02O3 perovskite have been analyzed by means of theoretical models. An excellent agreement has been found between the (ΔSM) values estimated by Landau theory and those obtained using the classical Maxwell relation. In order to estimate the spontaneous magnetization M s pont(T), we used the mean-field theory to analyses the (ΔSM) vs. M 2 data. The obtained M s pont(T) values are in good agreement with those found from the classical extrapolation from the Arrott plots(H/M vs. M 2), confirming that the magnetic entropy is a valid approach to estimate the spontaneous magnetization in our system. At a relatively low magnetic field, a phenomenological model has been used to estimate the values of the magnetic entropy change. The results are in good agreement with those obtained from the experimental data using Maxwell relation.

Mg 0.6−x Ni x Co 0.4 Fe 2 O 4 (x = 0 and x = 0.2) ferrites are elaborated under same conditions using the Pechini sol-gel technique. The obtained samples reveal a single phase without any detectable of secondary phase. But, the introduction of nickel in the parent ferrite system affects the grains shape, lattice parameters, volumetric mass density and the electrical resistivity. The X-ray diffraction patterns indicate that the investigated ferrites reveal a single phase without any detectable of secondary phase. Also, grains in the nickel doped compound have prismatic shape which indicates a better crystallization than in undoped one. From electrical measurements, a dramatically decrease of resistivity is observed at low temperature range. A decrease with 7 decades is observed in the temperature range 100 K-500 K. Both samples exhibit a semiconductor character in an important temperature range 80 K-700 K. The nickel effect on the electrical resistivity is well pronounced at higher temperature region (T > 150 K). Also, the nichel addition reduces the activation energy. Such variation is associated to the chemical composition, cristallite size, porosity, grain boundary and density effects. In a specific temperature range (ΔT), the ac-conductivity of the investigated materials becomes almost frequency independent. For x = 0 and x = 0.2, such range increases from ΔT = 100 K to ΔT = 220 K respectively. For x = 0.2 and for f ≤ 160 kHz, such range is extended from ambient temperature to 700 K to be ΔT = 400 K. This behaviour may be important to make the material suitable for some desired applications. Impedance analysis confirms the contribution of grain boundary on transport properties and proves the presence of relaxation phenomenon in nickel-magnesium ferrite. The temperature dependence of the average normalised change parameter is in good agreement with the temperature dependence of the dc-resistivity and the ac-conductivity of the investigated samples.

Spinel ferrites T0.6Co0.4Fe2O4 (T= Ni, Mg) were prepared by Pechini sol-gel technique at 1323 K. The X-ray diffraction results indicate that the ferrite samples have a cubic spinel type structure with Fd 3 m space group. FT-IR showed two absorption bands (ν1 and ν2) that are attributed to the stretching vibration of tetrahedral and octahedral sites. The electrical properties of the studied samples using complex impedance spectroscopy technique have been investigated in the frequency range 40 Hz – 107 Hz and in the temperature range 200–420 K. The total conductivity curves for samples are found to obey Jonscher power law σ(ω) =σdc +Aωn with an increase of the frequency exponent (n) as temperature increases. The effect of frequency, temperature and composition on dielectric constant (e") and dielectric loss (tan δ) has been discussed in terms of hopping of charge carriers between Fe2+ and Fe3+ ions. Activation energy has been estimated from both temperature dependent of dc conductivity and relaxation time data, which indicates that same kinds of charge carriers are governing both the processes. Nyquist plots of impedance show semicircle arcs for samples and an electrical equivalent circuit of the type of Rg+(Rgb//ZCPE) has been proposed to explain the impedance results.

Structural and electrical properties of La0.67Ca0.33Mn1-xCrxO3 (x = 0 and 0.2) manganites are studied in this work. The samples were prepared using Pechini sol-gel method at 1173 K. Rietveld analysis of X-ray diffraction (XRD) patterns shows the formation of pure crystalline phase with orthorhombic Pnma structure with a decrease of the unit cell volume and crystalline domain size when increasing Cr-content. Electrical properties of the samples have been investigated using complex impedance analysis in 180–320 K temperature range with varying frequency between 40 and 106 Hz. For both samples, frequency dependence of imaginary part of impedance (Z″) shows the existence of relaxation phenomenon. The impedance study using Nyquist representation revealed the appearance of semicircle arcs and an equivalent circuit of the type of R1 + (R2//ZCPE) has been proposed to explain the impedance results. The conductance curves were analyzed by Jonscher power law and the deduced activation energy values are close to those calculated from relaxation time indicating that relaxation process and conductivity have the same origin.

Ni0.6Cd0.2Cu0.2Fe2O4 ferrites were synthesized using sol-gel method under different sintering temperatures. XRD patterns with the Rietveld refinement indicate that samples crystallize in the cubic spinel structure. The increase of sintering temperature leads successively to the increase of lattice constant, average crystallite size, intensities of absorption bands, magnetization, and electrical conductivity of the prepared ferrites. Dielectric constants decrease with frequency and their behaviors have been investigated using the interfacial polarization theory predicted by Maxwell. The modulus analysis shows the presence of electrical relaxation phenomenon and non-Debye nature for the samples. An appropriate electrical equivalent circuit was used to analyze the Nyquist plots, and the results show that the conduction mechanism of the synthesized ferrites is mainly due to the grain boundary contribution.

International audience