Long-range ferromagnetic order in perovskite manganite La0.67Ba0.25Ca0.08Mn(1−x)TixO3 (x = 0.00, 0.05 and 0.10)

In this study, we report the effects of Ti on the critical behavior of La_0.67Ba_0.25Ca_0.08MnO₃ samples prepared by the flux method. Moreover, the critical exponents β, γ and δ are estimated through numerous techniques such as the modified Arrott plot, the Kouvel–Fisher method and critical isotherm analysis of the magnetic measurements on record near the Curie temperature. Compared to standard models, the estimated critical exponents are close to the theoretical values of the mean-field model for these samples. In order to estimate the spontaneous magnetization at a given temperature, we used a process based on the analysis, in the mean-field theory, of the magnetic entropy change (−ΔS_M) vs. magnetization (M). An excellent agreement was found between the spontaneous magnetization determined from −ΔS_Mvs. M² and the classical extrapolation from the Arrott curves (μ₀H/M vs. M²), thus confirming that −ΔS_M is a valid approach to estimate the spontaneous magnetization in this system. The accuracy of the critical exponent values was confirmed with the scaling hypothesis. The magnetization curves fall onto one of two sides, below and above T_C.

In this study, we report the effects of Ti on the critical behavior of La_0.67Ba_0.25Ca_0.08MnO₃ samples prepared by the flux method.     

Microstructural analysis,magnetic properties,magnetocaloric effect,and critical behaviors of Ni0.6Cd0.2Cu0.2Fe2O4 ferrites prepared using the sol–gel method under different sintering temperatures

This work focuses on the microstructural analysis, magnetic properties, magnetocaloric effect, and critical exponents of Ni_0.6Cd_0.2Cu_0.2Fe₂O₄ ferrites. These samples, denoted as S1000 and S1200, were prepared using the sol–gel method and sintered separately at 1000 °C and 1200 °C, respectively. XRD patterns confirmed the formation of cubic spinel structures and the Rietveld method was used to estimate the different structural parameters. The higher sintering temperature led to an increased lattice constant (a), crystallite size (D), magnetization (M), Curie temperature (T_C), and magnetic entropy change (−ΔS_M) for samples that exhibited second-order ferromagnetic–paramagnetic (FM–PM) phase transitions. The magnetic entropy changed at an applied magnetic field (μ₀H) of 5 T, reaching maximum values of about 1.57–2.12 J kg⁻¹ K⁻¹, corresponding to relative cooling powers (RCPs) of 115 and 125 J kg⁻¹ for S1000 and S1200, respectively. Critical exponents (β, γ, and δ) for samples around their T_C values were studied by analyzing the M(μ₀H, T) isothermal magnetizations using different techniques and checked by analyzing the −ΔS_Mvs. μ₀H curves. The estimated values of β and γ exponents (using the Kouvel–Fisher method) and δ exponent (from M(T_C, μ₀H) critical isotherms) were β = 0.443 ± 0.003, γ = 1.032 ± 0.001, and δ = 3.311 ± 0.006 for S1000, and β = 0.403 ± 0.008, γ = 1.073 ± 0.016, and δ = 3.650 ± 0.005 for S1200. Obviously, these critical exponents were affected by an increased sintering temperature and their values were different to those predicted by standard theoretical models.

This work focuses on the microstructural analysis, magnetic properties, magnetocaloric effect, and critical exponents of Ni_0.6Cd_0.2Cu_0.2Fe₂O₄ ferrites.     

Magnetic and magnetocaloric properties of La0.55Bi0.05Sr0.4CoO3 and their implementation in critical behaviour study and spontaneous magnetization estimation

In this work, we present the results of the magnetic, critical, and magnetocaloric properties of the rhombohedral-structured La_0.55Bi_0.05Sr_0.4CoO₃ cobaltite. Based on the modified Arrott plot, Kouvel–Fisher, and critical isotherm analyses, we obtained the values of critical exponents (β, γ, and δ) as well as Curie temperature (T_C) for the investigated compound. These components were consistent with their corresponding values and they were validated by the Widom scaling law and scaling theory. The obtained critical exponents were close to the theoretical prediction of the mean-field model values, revealing the characteristic of long-range ferromagnetic interactions. The magnetic entropy, heat capacity, and local exponent n(T, μ₀H) of the La_0.55Bi_0.05Sr_0.4CoO₃ compound collapsed to a single universal curve, confirming its universal behaviour. The estimated spontaneous magnetization value extracted through the analysis of the magnetic entropy change was consistent with that deduced through the classical extrapolation of the Arrott curves. Thus, the magnetic entropy change is a valid and useful approach to estimate the spontaneous magnetization of La_0.55Bi_0.05Sr_0.4CoO₃.

In this work, we present the results of the magnetic, critical, and magnetocaloric properties of the rhombohedral-structured La_0.55Bi_0.05Sr_0.4CoO₃ cobaltite.     

Critical phenomena and estimation of the spontaneous magnetization from a mean field analysis of the magnetic entropy change in La0.7Ca0.1Pb0.2Mn0.95Al0.025Sn0.025O3

In the present work, we have studied the universal critical behavior in the perovskite-manganite compound La_0.7Ca_0.1Pb_0.2Mn_0.95Al_0.025Sn_0.025O₃. Experimental results revealed that all samples exhibit a second-order magnetic phase transition. To study the critical behavior of the paramagnetic–ferromagnetic transition, various techniques such as modified Arrott plots, the Kouvel–Fisher method and critical isotherm analysis were used to determine the values of the Curie temperature T_C, as well as the critical exponents β (corresponding to the spontaneous magnetization), γ (corresponding to the initial susceptibility) and δ (corresponding to the critical magnetization isotherm). The critical exponent values for our sample are consistent with the prediction of the mean field model (β = 0.46, γ = 1.0, and δ = 2.94 at an average T_C = 302.12 K), indicating that the magnetic interactions are long-range. The reliability of the critical exponent values was confirmed by the Widom scaling relation (WSR) and the universal scaling hypothesis. Moreover, to estimate the spontaneous magnetization of La_0.7Ca_0.1Pb_0.2Mn_0.95Al_0.025Sn_0.025O₃ perovskite, we used the magnetic entropy change (ΔS_M), obtained from isothermal magnetization. The results obtained through this approach are compared to those obtained from classical analysis using Arrott curves. An excellent agreement is found between this approach and the one obtained from the extrapolation of the Arrott curves.

In the present work, we have studied the universal critical behavior in the perovskite-manganite compound La_0.7Ca_0.1Pb_0.2Mn_0.95Al_0.025Sn_0.025O₃.     

Evaluation of Nd0.8−xSr0.2CaxCoO3−δ(x = 0, 0.05, 0.1, 0.15, 0.2) as a cathode material for intermediate-temperature solid oxide fuel cells

A series of Nd_0.8−xSr_0.2Ca_xCoO_3−δ(x = 0, 0.05, 0.1, 0.15, 0.2) cathode materials was synthesized by sol–gel method. The effect of Ca doping amount on the structure was examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermal expansion, and X-ray photoelectron spectroscopy (XPS). Electrochemical properties were evaluated for possible application in solid oxide fuel cell (SOFC) cathodes. Results showed that second phase NdCaCoO_4+δ is generated when the Ca doping amount is higher than 0.1. The increase in Ca limits the electronic compensation capacity of the material, resulting in a decrease in thermal expansion coefficient (TEC). With the increase of Ca content, the conductivity increases at first and then decreases, and the highest value of 443 S cm⁻¹ is at x = 0.1 and T = 800 °C. Nd_0.7Sr_0.2Ca_0.1CoO_3−δ exhibits the lowest area specific resistance of 0.0976 Ω cm² at 800 °C. The maximum power density of Nd_0.7Sr_0.2Ca_0.1CoO_3−δ at 800 °C is 409.31 mW cm⁻². The Ca-doped material maintains good electrochemical properties under the coefficient of thermal expansion (CTE) reduction and thus can be used as an intermediate-temperature SOFC (IT-SOFC) cathode.

The increase in Ca for Nd_0.8−xSr_0.2Ca_xCoO_3−δ limits the electronic compensation capacity, resulting in a decrease in CTE. The Ca-doped material maintains good electrochemical properties under CTE reduction and thus can be used as an IT-SOFC cathode.     

Anomalous behavior above the Curie temperature in (Nd1−xGdx)0.55Sr0.45MnO3 (x = 0, 0.1, 0.3 and 0.5)

Magnetic properties were studied just above the ferromagnetic–paramagnetic (FM–PM) phase transition of (Nd_1−xGd_x)_0.55Sr_0.45MnO₃ with x = 0, 0.1, 0.3 and 0.5. The low-field inverse susceptibility (χ⁻¹) of Nd_0.55Sr_0.45MnO₃ exhibits a Curie–Weiss-PM behavior. For x ≥ 0.1, we observe a deviation in χ⁻¹(T) behavior from the Curie–Weiss law. The anomalous behavior of the χ⁻¹(T) was qualified as Griffiths phase (GP)-like. The study of the evolution of the GP through a susceptibility exponent, the GP temperature and the temperature range of the GP reveals that the origin of the GP is primary due to the accommodated strain. Likewise, the magnetic data reveal distinct features visible only for x = 0.5 at a low magnetic field that can be qualitatively understood as the result of ferromagnetic polarons, entailed by the strong effect of chemical/structural disorder, whose concentration increases upon cooling towards the Curie temperature. We explained the magnetic properties at a high temperature for the heavily Gd-doped sample (x = 0.5) within the phase-separation scenario as an assembly of ferromagnetic nanodomains, antiferromagnetically coupled by correlated Jahn–Teller polarons.

Magnetic properties were studied just above the ferromagnetic–paramagnetic (FM–PM) phase transition of (Nd_1−xGd_x)_0.55Sr_0.45MnO₃ with x = 0, 0.1, 0.3 and 0.5.     

Structure and transport properties of triple-conducting BaxSr1−xTi1−yFeyO3−δ oxides

In this work, Ba_xSr_1−xTi_1−yFe_yO_3−δ perovskite-based mixed conducting ceramics (for x = 0, 0.2, 0.5 and y = 0.1, 0.8) were synthesized and studied. The structural analysis based on the X-ray diffraction results showed significant changes in the unit cell volume and Fe(Ti)–O distance as a function of Ba content. The morphology of the synthesized samples studied by means of scanning electron microscopy has shown different microstructures for different contents of barium and iron. Electrochemical impedance spectroscopy studies of transport properties in a wide temperature range in the dry- and wet air confirmed the influence of barium cations on charge transport in the studied samples. The total conductivity values were in the range of 10⁻³ to 10⁰ S cm⁻¹ at 600 °C. Depending on the barium and iron content, the observed change of conductivity either increases or decreases in humidified air. Thermogravimetric measurements have shown the existence of proton defects in some of the analysed materials. The highest observed molar proton concentration, equal to 5.0 × 10⁻² mol mol⁻¹ at 300 °C, was obtained for Ba_0.2Sr_0.8Ti_0.9Fe_0.1O_2.95. The relations between the structure, morphology and electrical conductivity were discussed.

Ba_xSr_1−xTi_1−yFe_yO_3−δ-based perovskite materials with different barium and iron contents are reported as triple conducting oxides (TCOs), which may conduct three charge carriers: oxygen ions, protons and electrons/holes.     

Influence of Ni content on structural,magnetocaloric and electrical properties in manganite La0.6Ba0.2Sr0.2Mn1−xNixO3 (0 ≤ x ≤ 0.1) type perovskites

We present a detailed study on the physical properties of La_0.6Ba_0.2Sr_0.2Mn_1−xNi_xO₃ samples (x = 0.00, 0.05 and 0.1). The ceramics were fabricated using the sol–gel route. Structural refinement, employing the Rietveld method, disclosed a rhombohedral R$\bar{3}$c phase. The magnetization vs. temperature plots show a paramagnetic–ferromagnetic (PM–FM) transition phase at the T_C (Curie temperature), which decreases from 354 K to 301 K. From the Arrott diagrams M²vs. μ₀H/M, we can conclude the phase transition is of the second order. Based on measurements of the isothermal magnetization around T_C, the magnetocaloric effects (MCEs) have been calculated. The entropy maximum change (−ΔS_M) values are 7.40 J kg⁻¹ K⁻¹, 5.6 J kg⁻¹ K⁻¹ and 4.48 J kg⁻¹ K⁻¹, whereas the relative cooling power (RCP) values are 232 J kg⁻¹, 230 J kg⁻¹ and 156 J kg⁻¹ for x = 0.00, 0.05 and 0.10, respectively, under an external field (μ₀H) of 5 T. Through these results, the La_0.6Ba_0.2Sr_0.2Mn_1−xNi_xO₃ (0 ≤ x ≤ 0.1) samples can be suggested for use in magnetic refrigeration technology above room temperature. The electrical resistivity (ρ) vs. temperature plots exhibit a transition from metallic behavior to semiconductor behavior in the vicinity of T_M–SC. The adiabatic small polaron hopping (ASPH) model is applied in the PM-semiconducting part (T > T_MS). Throughout the temperature range, ρ is adjusted by the percolation model. This model is based on the phase segregation of FM-metal clusters and PM-insulating regions.

We present a detailed study on the physical properties of La_0.6Ba_0.2Sr_0.2Mn_1−xNi_xO₃ samples (x = 0.00, 0.05 and 0.1).     

Pr3+ doping at the A-site of La0.67Ba0.33MnO3 nanocrystalline material: assessment of the relationship between structural and physical properties and Bean–Rodbell model simulation of disorder effects

Bulk nanocrystalline samples of (La_1−xPr_x)_0.67Ba_0.33MnO₃ (0.075 ≤ x ≤ 0.30) manganites with a fixed carrier concentration are prepared by the sol–gel based Pechini method. Rietveld refinement of the X-ray diffraction patterns, shows the formation of single-phase compositions with rhombohedral symmetry. Upon Pr³⁺ doping at the A-site, the unit cell volume and the B–O–B bond angles are reduced. FTIR spectra present a prominent absorption peak of the in-phase stretching mode (B_2g mode) rising from the vibration of the Mn–O bond. Raman spectra at room temperature reveal a gradual shift toward lower frequencies in (E_g) phonon mode with increasing Pr³⁺ concentration. The M(T) measurements shows a clear ferromagnetic (FM)–paramagnetic (PM) phase transition with increasing temperature. An increase in resistivity and activation energy and a decrease in the metal–semiconductor transition (T_M–SC) and Curie temperatures (T_C) was observed as a consequence of Pr³⁺ doping. The results are discussed according to the change of A-site-disorder effect caused by the systematic variations of the A-site average ionic radius 〈r_A〉 and A-site-cation mismatch σ², resulting in the narrowing of the bandwidth and the decrease of the mobility of e_g electrons. The magneto-transport behavior in the whole measured temperature and a magnetic field can be described by a percolation model, which is in agreement with the limited experimental data of the samples for x = 0.075, 0.15 and 0.30. The experimental results confirm that A-site substitution with Pr³⁺ destroys the Mn³⁺–O²⁻–Mn⁴⁺ bridges and weakens the double exchange (DE) interaction between the Mn³⁺ (t³_2ge¹_g, S = 2) and Mn⁴⁺ (t³_2ge⁰_g, S = 3/2) ions. On the other hand, the Bean and Rodbell model has been successfully used to simulate the magnetization data of the samples with x = 0.15 and x = 0.22. The random replacement of La³⁺ by Pr³⁺ is shown to induce more disorder in the system, which is reflected in the increase of the fitted disorder parameter and spin value fluctuation. At a temperature close to room temperature, the maximum magnetic entropy change (ΔS_Max) and the relative cooling power (RCP) of La_0.52Pr_0.15Ba_0.33MnO_2.98 are found to be, respectively, 1.34 J kg⁻¹ K⁻¹ and 71 J kg⁻¹ for a 1.5 T field change.

Bulk nanocrystalline samples of (La_1−xPr_x)_0.67Ba_0.33MnO₃ (0.075 ≤ x ≤ 0.30) manganites with a fixed carrier concentration are prepared by the sol–gel based Pechini method.     

Gd3+ and Bi3+ co-substituted cubic zirconia; (Zr1−x−yGdxBiyO2−δ): a novel high κ relaxor dielectric and superior oxide-ion conductor

Solid oxide fuel cells (SOFCs) offer several advantages over lower temperature polymeric membrane fuels cells (PMFCs) due to their multiple fuel flexibility and requirement of low purity hydrogen. In order to decrease the operating temperature of SOFCs and to overcome the high operating cost and materials degradation challenges, the Cubic phase of ZrO₂ was stabilized with simultaneous substitution of Bi and Gd and the effect of co-doping on the oxide-ion conductivity of Zr_1−x−yBi_xGd_yO_2−δ was studied to develop a superior electrolyte separator for SOFCs. Up to 30% Gd and 20% Bi were simultaneously substituted in the cubic ZrO₂ lattice (Zr_1−x−yGd_xBi_yO_2−δ, x + y ≤ 0.4, x ≤ 0.3 and y ≤ 0.2) by employing a solution combustion method followed by multiple calcinations at 900 °C. Phase purity and composition of the material is confirmed by powder XRD and EDX measurements. The formation of an oxygen vacant Gd/Bi co-doped cubic zirconia lattice was also confirmed by Raman spectroscopy study. With the incorporation of Bi³⁺ and Gd³⁺ ions, the cubic Zr_1−x−yBi_xGd_yO_2−δ phase showed relaxor type high κ dielectric behaviour (ε′ = 9725 at 600 °C at applied frequency 20 kHz for Zr_0.6Bi_0.2Gd_0.2O_1.8) with T_m approaching 600 °C. The high polarizability of the Bi³⁺ ion coupled with synergistic interaction of Bi and Gd in the host ZrO₂ lattice seems to create the more labile oxide ion vacancies that enable superior oxide-ion transport resulting in high oxide ion conductivity (σ_o > 10⁻² S cm⁻¹, T > 500 °C for Zr_0.6Bi_0.2Gd_0.2O_1.8) at relatively lower temperatures.

The high polarizability of the Bi³⁺ ion coupled with synergistic interaction of Bi and Gd in the host ZrO₂ lattice seems to create the more labile oxide ion vacancies that enable high oxide ion conductivity at lower temperatures.     

Study of magnetic and electrical properties of Pr0.65Ca0.25Ba0.1MnO3 manganite

The magnetic properties and magnetocaloric effect (MCE) in Pr_0.65Ca_0.25Ba_0.1MnO₃ have been investigated supplemented by electrical data. X-ray diffraction shows that the sample crystallizes in the distorted orthorhombic system with the Pnma space group. Pr_0.65Ca_0.25Ba_0.1MnO₃ undergoes paramagnetic–ferromagnetic (PM–FM) phase transition at T_C ∼ 85 K. For a magnetic field change of 5 T, the maximum value of the magnetic entropy change (−ΔS^max_M) is estimated to be 4.4 J kg⁻¹ K⁻¹ around T_C with a large relative cooling power (RCP) value of 263.5 J kg⁻¹. While the modified Arrott plots suggested that the magnetic transition belongs to the second order phase transitions, the universal curves of the rescaled magnetic entropy (ΔS_M) proved the opposite. The electrical properties of Pr_0.65Ca_0.25Ba_0.1MnO₃ have been investigated using impedance spectroscopy techniques. The dc-resistivity (σ_dc) study shows the presence of semiconductor behavior. Ac-conductivity (σ_ac) analysis shows that the conductivity is governed by a hopping process. From the analysis of the alternating regime, the exponent s variation obtained is in good agreement with Mott theory. The impedance spectrum analysis reveals the presence of a relaxation phenomenon. Based on these analyzes, the sample can be modeled by an electrical equivalent circuit.

The magnetic properties and magnetocaloric effect in Pr_0.65Ca_0.25Ba_0.1MnO₃ compound have been investigated supplemented by electrical data.     

Magnetocaloric study,critical behavior and spontaneous magnetization estimation in La0.6Ca0.3Sr0.1MnO3 perovskite

A detailed study of structural, magnetic and magnetocaloric properties of the polycrystalline manganite La_0.6Ca_0.3Sr_0.1MnO₃ is presented. The Rietveld refinement of X-ray diffraction pattern reveals that our sample is indexed in the orthorhombic structure with Pbnm space group. Magnetic measurements display a second order paramagnetic (PM)/ferromagnetic (FM) phase transition at Curie temperature T_c = 304 K. The magnetic entropy change (ΔS_M) is calculated using two different methods: Maxwell relations and Landau theory. An acceptable agreement between both data is noted, indicating the importance of magnetoelastic coupling and electron interaction in magnetocaloric effect (MCE) properties of La_0.6Ca_0.3Sr_0.1MnO₃. The maximum magnetic entropy change (−ΔS^max_M) and the relative cooling power (RCP) are found to be respectively 5.26 J kg⁻¹ K⁻¹ and 262.53 J kg⁻¹ for μ₀H = 5 T, making of this material a promising candidate for magnetic refrigeration application. The magnetic entropy curves are found to follow the universal law, confirming the existence of a second order PM/FM phase transition at T_c which is in excellent agreement with that already deduced from Banerjee criterion. The critical exponents are extracted from the field dependence of the magnetic entropy change. Their values are close to the 3D-Ising class. Scaling laws are obeyed, implying their reliability. The spontaneous magnetization values determined using the magnetic entropy change (ΔS_Mvs. M²) are in good agreement with those obtained from the classical extrapolation of Arrott curves (μ₀H/M vs. M²). The magnetic entropy change can be effectively used in studying the critical behavior and the spontaneous magnetization in manganites system.

A detailed investigation was conducted on the magnetocaloric properties of La_0.6Ca_0.3Sr_0.1MnO₃ and its potential application in cooling fields.     

Evaluation of thin film fuel cells with Zr-rich BaZrxCe0.8−xY0.2O3−δ electrolytes (x ≥ 0.4) fabricated by a single-step reactive sintering method

This paper reports a survey of power generation characteristics of anode-supported thin film fuel cells with Zr-rich BaZr_xCe_0.8−xY_0.2O_3−δ (x = 0.4, 0.6, 0.7, and 0.8) proton-conducting electrolytes, which were fabricated by single step co-firing with Zn(NO₃)₂ additives at a relatively low temperature (1400 °C). The grain sizes significantly increased to several μm for x = 0.4 and 0.6, whereas the grain sizes remained in the sub-μm ranges for x = 0.7 and 0.8, which resulted in large gaps of the fuel cell performances at x over and below 0.6. The cells for x = 0.4 and 0.6 exhibited efficient power generation, yielding peak powers of 279 and 336 mW cm⁻² at 600 °C, respectively, which were higher than those of the corresponding cells previously reported. However, the performances abruptly deteriorated with the increasing x to more than 0.7 because the electrolyte films were highly resistive due to the coarse-grained microstructures. Impedance spectroscopy for the dense sintered BaZr_xCe_0.8−xY_0.2O_3−δ discs confirmed that the total proton conductivity of BaZr_0.6Ce_0.2Y_0.2O_3−δ was higher than that of BaZr_0.4Ce_0.4Y_0.2O_3−δ at temperatures above 500 °C despite relatively small grain sizes. In addition, BaZr_0.6Ce_0.2Y_0.2O_3−δ cells could gain a stable current throughout a continuous run for a few days under CO₂-containing fuel supply, which was due to high fraction of thermodynamically stable BaZrO₃ matrices. It was demonstrated that BaZr_0.6Ce_0.2Y_0.2O_3−δ is a promising electrolyte for proton-conducting ceramic fuel cells with excellent proton conductivity and CO₂ tolerance at intermediate temperatures.

This paper reports on the survey of power generation characteristics of anode-supported thin film fuel cells with Zr-rich side BaZr_xCe_0.8–xY_0.2O_3–δ (x = 0.4, 0.6, 0.7, and 0.8) proton conducting electrolytes.     

Observation of enhanced magnetic entropy change near room temperature in Sr-site deficient La0.67Sr0.33MnO3 manganite

The effect of Sr-site deficiency on the structural, magnetic and magnetic entropy change of La_0.67Sr_0.33−yMnO_3−δ (y = 0.18 and 0.27) compounds was investigated. The compounds were prepared by the conventional solid-state route and powder X-ray diffraction technique along with Rietveld refinement was carried out to confirm the structure and phase purity. Lattice parameters and unit cell volumes are found to increase with the increase in Sr-deficiency due to the electrostatic repulsion from the neighbouring oxygen ions. A mixed valence state of Mn²⁺/Mn³⁺/Mn⁴⁺ was confirmed using the X-ray photoelectron spectroscopy technique and it was observed that the change of state from Mn³⁺ + Mn³⁺ pairs to Mn²⁺ + Mn⁴⁺ pairs is different for both the studied compounds. A second order ferromagnetic–paramagnetic transition with an enhancement in magnetization in comparison to the pristine compound (La_0.67Sr_0.33MnO₃) was observed due to multiple double exchange interactions. The La_0.67Sr_0.15□_0.18MnO_3−δ compound exhibits a magnetic entropy change (ΔS_M) of 4.61 J kg⁻¹ K⁻¹ at 310 K, and the La_0.67Sr_0.06□_0.27MnO_3−δ compound exhibits a ΔS_M of 4.11 J kg⁻¹ K⁻¹ at 276 K under a field of 50 kOe. In our previous work, we reported a large value of ΔS_M but at higher temperatures, around 350 K. However, in the present case, we have achieved a near room temperature (310 K) MCE with a significant ΔS_M value (4.61 J kg⁻¹ K⁻¹) which is larger than that reported for numerous perovskite manganites. Thus, the studied material could be a potential candidate for room temperature magnetic refrigeration applications.

The effect of Sr-site deficiency on the structural, magnetic and magnetic entropy change of La_0.67Sr_0.33−yMnO_3−δ (y = 0.18 and 0.27) compounds was investigated.     

Large magnetic entropy change and prediction of magnetoresistance using a magnetic field in La0.5Sm0.1Sr0.4Mn0.975In0.025O3

A detailed study of the structural, magnetic, magnetocaloric and electrical effect properties in polycrystalline manganite La_0.5Sm_0.1Sr_0.4Mn_0.975In_0.025O₃ is presented. The X-ray diffraction pattern is consistent with a rhombohedral structure with R$\bar{3}$c space group. Experimental results revealed that our compound prepared via a sol–gel method exhibits a continuous (second-order) ferromagnetic (FM) to paramagnetic (PM) phase transition around the Curie temperature (T_C = 300 K). In addition, the magnetic entropy change was found to reach 5.25 J kg⁻¹ K⁻¹ under an applied magnetic field of 5 T, corresponding to a relative cooling power (RCP) of 236 J kg⁻¹. We have fitted the experimental data of resistivity using a typical numerical method (Gauss function). The simulation values such as maximum resistivity (ρ_max) and metal–semiconductor transition temperature (T_M–Sc), calculated from this function, showed a perfect agreement with the experimental data. The shifts of these parameters as a function of magnetic field for our sample have been interpreted. The obtained values of β and γ, determined by analyzing the Arrott plots, are found to be T_C = 298.66 ± 0.64 K, β = 0.325 ± 0.001 and γ = 1.25 ± 0.01. The critical isotherm M (T_C, μ₀H) gives δ = 4.81 ± 0.01. These critical exponent values are found to be consistent and comparable to those predicted by the three-dimensional Ising model with short-range interaction. Thus, the Widom scaling law is fulfilled.

Rietveld refinement for the sample LSSMIO. Experimental data (the point symbols), calculated data (the solid lines), difference between them is shown at the bottom of the diagram and Bragg positions are marked by vertical bars.     

Enhancing the thermoelectric power factor of nanostructured ZnCo2O4 by Bi substitution

Bi_xZnCo_2−xO₄ (0 ≤ x ≤ 0.2) nanoparticles with different x values have been prepared by the sol–gel method; the structural, morphological, thermal and thermoelectric properties of the prepared nanomaterials are investigated. XRD analysis confirms that Bi is completely dissolved in the ZnCo₂O₄ lattice till the x values of ≤0.1 and the secondary phase of Bi₂O₃ is formed at higher x value (x > 0.1). The synthesized nanomaterials are densified and the thermoelectric properties are studied as a function of temperature. The electrical resistivity of the Bi_xZnCo_2−xO₄ decreased with x value and it fell to 4 × 10⁻² Ω m for the sample with x value ≤ 0.1. The Seebeck coefficient value increased with the increase of Bi substitution till the x value of 0.1 and decreased for the sample with higher Bi content (x ≤ 0.2) as the resistivity of the sample increased due to secondary phase formation. With the optimum Seebeck coefficient and electrical resistivity, Bi_0.1ZnCo_1.9O₄ shows the high-power factor (α²σ_{550 K}) of 2.3 μW K⁻² m⁻¹ and figure of merit of 9.5 × 10⁻⁴ at 668 K respectively, compared with other samples. The experimental results reveal that Bi substitution at the Co site is a promising approach to improve the thermoelectric properties of ZnCo₂O₄.

Nanostructuring and Bi substitution have considerably increased the thermoelectric power factor and ZT of Bi_xZnCo_2−xO₄; Bi_1.9ZnCo_1.9O₄ shows a higher power factor than that of other Bi substituted samples.     

Structural study and large magnetocaloric entropy change at room temperature of La1−x□xMnO3 compounds

In this study, our central focus is to investigate the magnetocaloric characteristics of a La_1−x□_xMnO₃ (x = 0.1, 0.2 and 0.3) series prepared by a sol–gel technique published in Prog. Mater. Sci., 93, 2018, 112–232. The crystallographic study revealed that our compounds crystallize in a rhombohedral structure with R$\bar{3}$c. Ferromagnetic (FM) and paramagnetic (PM) characters were detected from the variation in magnetization as a function of magnetic fields at different temperatures. The second order transition was verified from the Arrott plots (M²vs. (μ₀H/M)), where the slopes have a positive value. In order to verify the second order, we traced the variation of magnetization vs. temperature at different magnetic fields for x = 0.2. This revealed a ferromagnetic (FM)–paramagnetic (PM) transition when temperature increases. Relying on the indirect method while using the Maxwell formula, we determined the variation in the entropy (−ΔS_M) as a function of temperature for different magnetic fields for the three samples. We note that all the studied systems stand as good candidates for magnetic refrigeration with relative cooling power (RCP) values of around 131.4, 83.38 and 57.26 J kg⁻¹ with magnetic fields below 2 T, respectively. Subsequently, the magnetocaloric effect was investigated by a phenomenological model for x = 0.2. The extracted data confirm that this phenomenological model is appropriate for the prediction of magnetocaloric properties. The study also demonstrated that this La_0.8□_0.2MnO₃ system exhibits a universal behaviour.

In this study, our central focus is to investigate the magnetocaloric characteristics of a La_1−x□_xMnO₃ (x = 0.1, 0.2 and 0.3) series prepared by a sol–gel technique.     

Critical behavior near room temperature in La0.75Ca0.05Na0.20MnO3 sample

The critical behavior of La_0.75Ca_0.05Na_0.20MnO₃ was studied at around room temperature via magnetization measurements. From the relative slope, we deduced that the Tricritical Mean-Field model was the most suitable model. The estimated critical exponents were found to be β = 0.24 ± 0.004, γ = 0.98 ± 0.065 and δ = 5.08 at T_C = 300 K. These critical exponents satisfied the Widom scaling relation δ = 1 + γ/β, implying the reliability of our values. Based on the critical exponents, the magnetization–field–temperature (M–μ₀H–T) data around T_C collapsed into two curves, obeying the single scaling equation with ε = (T − T_C)/T_C being the reduced temperature. These results suggest short-range interaction in our sample.

A set of typical M²vs. μ₀H/M for La_0.75Ca_0.05Na_0.20MnO₃ sample.     

Copper-doped lanthanum manganite La0.65Ce0.05Sr0.3Mn1−xCuxO3 influence on structural,magnetic and magnetocaloric effects

Bulk nanocrystalline samples of La_0.65Ce_0.05Sr_0.3Mn_1−xCu_xO₃ (0 ≤ x ≤ 0.15) manganites are prepared by the sol–gel based Pechini method. The effect of the substitution for Mn with Cu upon the structural and magnetic properties has been investigated by means of X-ray diffraction (XRD), Raman spectroscopy and dc magnetization measurements. The structural parameters obtained using Rietveld refinement of XRD data showed perovskite structures with rhombohedral (R$\bar{3}$c) symmetry without any detectable impurity phase. Raman spectra at room temperature reveal a gradual change in phonon modes with increasing copper concentration. The analysis of the crystallographic data suggested a strong correlation between structure and magnetism, for instance a relationship between a distortion of the MnO₆ octahedron and the reduction in the Curie temperature, T_c. A paramagnetic to ferromagnetic phase transition at T_C is observed. The experimental results confirm that Mn-site substitution with Cu destroys the Mn³⁺–O²⁻–Mn⁴⁺ bridges and weakens the double exchange (DE) interaction between Mn³⁺ and Mn⁴⁺ ions, which shows an obvious suppression of the FM interaction in the La_0.65Ce_0.05Sr_0.3Mn_1−xCu_xO₃ matrix. The maximum magnetic entropy change −ΔS^max_M is found to decrease with increasing Cu content from 4.43 J kg⁻¹ K⁻¹ for x = 0 to 3.03 J kg⁻¹ K⁻¹ for x = 0.15 upon a 5 T applied field change.

Bulk nanocrystalline samples of La_0.65Ce_0.05Sr_0.3Mn_1−xCu_xO₃ (0 ≤ x ≤ 0.15) manganites are prepared by the sol–gel based Pechini method.     

Improved electrical transport properties of polycrystalline La0.8(Ca0.12Sr0.08)MnO3 ceramics by Ag2O doping

Polycrystalline La_0.8(Ca_0.12Sr_0.08)MnO₃:mol%Ag_x (LCSMO:Ag_x, x = 0, 0.1, 0.2, 0.3 and 0.4) ceramics were synthesized by the sol–gel technique. Structural, electrical and magnetic properties of the LCSMO:Ag_x ceramics were investigated in detail. X-ray diffraction (XRD) data analyses revealed that all the samples were crystalized in the orthorhombic structure with space group of Pnma. With the increase in Ag doping (x), the grain sizes of the LCSMO:Ag_x samples increased and the amount of grain boundaries (GBs) decreased accordingly. At the same time, the Mn–O bond distance and the Mn–O–Mn bond angles changed correspondingly, leading to the slight increase in the lattice constants (a, b and c) and slight expansion of cell volume (V). For the LCSMO:Ag_x sample with x = 0.3, the optimal values of temperature coefficient of resistivity (TCR) and magnetoresistance (MR) reached 16.22% K⁻¹ (265.1 K) and 42.07% K⁻¹ (270.48 K), respectively. In addition, the fitting analysis of ρ–T curves showed that the experimental data were consistent with the theoretical calculation data. In the T < T_MI (metal-insulator transition temperature) region, the electrical conduction mechanism of LCSMO:Ag_x was clarified by electron-magnon, electron–electron and electron-phonon scattering. In the T > T_MI region, the resistivity data were interpreted by using the adiabatic small-polaron hopping model. Furthermore, in the entire temperature range, the phenomenological equation called the percolation model was used to explain the resistivity data and the phase-separation mechanism of ferromagnetic metallic (FM) and paramagnetic insulating (PI) phases. All the obtained results indicated that the improvement in the electrical properties of the LCSMO:Ag_x samples was attributed to the doping of Ag, which changed the A-site (La, Ca and Sr ions) average ion radius, the Mn–O–Mn bond angles and the Mn–O bond distance. In addition, the grain size increased, which led to improvement in the Mn⁴⁺ ion concentration and the GBs connectivity in the LCSMO:Ag_x polycrystalline ceramics.

Polycrystalline La_0.8(Ca_0.12Sr_0.08)MnO₃:mol%Ag_x (LCSMO:Ag_x, x = 0, 0.1, 0.2, 0.3 and 0.4) ceramics were synthesized by the sol–gel technique.