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.     

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.     

Magnetic properties,magnetocaloric effect,and critical behaviors in Co1−xCrxFe2O4

This research work focuses on the magnetic properties, nature of the magnetic phase transition, magnetocaloric effect, and critical scaling of magnetization of various Co_1−xCr_xFe₂O₄ (x = 0, 0.125, 0.25, 0.375, and 0.5). The tunability of the magnetic moment, exchange interactions, magnetocrystalline anisotropy constant, and microwave frequency using Cr³⁺ content has been found. The nature of the magnetic phase transitions for all the Cr³⁺ concentrations exhibits as second order which has been confirmed from the analysis of critical scaling, universal curve scaling, and scaling analysis of the magnetocaloric effect. The critical exponent analysis for all samples was performed from the modified Arrott-, and Kouvel–Fisher-plots. These critical analyses suggest that x = 0.125, 0.250, and 0.375 samples show reliable results in the magnetocaloric effect with relative cooling power (RCP) values in the range of 128–145 J kg⁻¹. On the other hand, x = 0.00, and 0.500 samples exhibit inconsistent RCP values. The universal curve scaling also confirms the reliability of the magnetocaloric effect of the investigated samples.

Magnetic entropy change as a function of temperature for various Co_1−xCr_xFe₂O₄ at an applied magnetic field of 5 T.     

Ti surface doping of LiNi0.5Mn1.5O4−δ positive electrodes for lithium ion batteries

The particle surface of LiNi_0.5Mn_1.5O_4−δ (LNMO), a Li-ion battery cathode material, has been modified by Ti cation doping through a hydrolysis–condensation reaction followed by annealing in oxygen. The effect of different annealing temperatures (500–850 °C) on the Ti distribution and electrochemical performance of the surface modified LNMO was investigated. Ti cations diffuse from the preformed amorphous ‘TiO_x’ layer into the LNMO surface during annealing at 500 °C. This results in a 2–4 nm thick Ti-rich spinel surface having lower Mn and Ni content compared to the core of the LNMO particles, which was observed with scanning transmission electron microscopy coupled with compositional EDX mapping. An increase in the annealing temperature promotes the formation of a Ti bulk doped LiNi_(0.5−w)Mn_(1.5+w)−tTi_tO₄ phase and Ti-rich LiNi_0.5Mn_1.5−yTi_yO₄ segregates above 750 °C. Fourier-transform infrared spectrometry indicates increasing Ni–Mn ordering with annealing temperature, for both bare and surface modified LNMO. Ti surface modified LNMO annealed at 500 °C shows a superior cyclic stability, coulombic efficiency and rate performance compared to bare LNMO annealed at 500 °C when cycled at 3.4–4.9 V vs. Li/Li⁺. The improvements are probably due to suppressed Ni and Mn dissolution with Ti surface doping.

LiNi_0.5Mn_1.5O_4−δ surface is doped with Ti ion maintaining the spinel structure at 500 °C, higher annealing temperatures cause Ti diffusion from surface towards the core.     

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.     

Structural,optical and dielectric properties of Cu1.5Mn1.5O4 spinel nanoparticles

In this study, a Cu_1.5Mn_1.5O₄ spinel was successfully synthesized by a sol–gel method at 500 °C for 5 h and characterized by different techniques. X-ray diffraction (XRD), Fourier transformation infrared (FTIR) spectroscopy and Raman spectroscopic analyses confirmed the formation of a spinel cubic structure with the Fd$\bar{3}$m space group. The SEM proves that the grain size of our compound is of the order of 48 nm. Crystallite sizes determined from three estimates are closer to the grain size obtained from the SEM, indicating the single domain nature of the sample. The optical properties of UV-visible spectroscopy for our sample showed that the gap value is equal to 3.82 eV, making our compound a good candidate for optoelectronic applications. For electrical properties, impedance spectroscopy was performed at a frequency range of 40 ≤ frequency ≤ 10⁶ Hz. This suggested hoping conduction due to three theoretical models. The latter can be attributed to the correlated barrier hopping (CBH) model in region I, overlapping large polaron tunneling (OLPT) in region II and non-overlapping small polaron tunneling (NSPT) mechanism in region III. One dielectric relaxation is detected from the dielectric impedance and modulus, attributed to grain contributions. This behavior was confirmed by both Nyquist and Argand''s plots of dielectric impedance at different measuring temperatures.

In this study, a Cu_1.5Mn_1.5O₄ spinel was successfully synthesized by a sol–gel method at 500 °C for 5 h and characterized by different techniques.     

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.     

Bismuth trioxide-tailored sintering temperature,microstructure and NTCR characteristics of Mn1.1Co1.5Fe0.4O4 ceramics

Mn_1.1Co_1.5Fe_0.4O₄ ceramics with tailored sintering temperature, microstructure, and NTCR characteristics were prepared using Bi₂O₃ sintering additive by a solid-state reaction route. Densification and morphological characterization indicate that bismuth trioxide can play a critical role in the sintering process. The results reveal that the sintering temperature can be decreased significantly from 1200 °C to 1050 °C by using the appropriate content of Bi₂O₃ additive. The resistivity decreases first and then increases with increasing Bi₂O₃ content. The obtained B_25/50 value and ρ₂₅ ranges were 3647–3697 K, and 800–1075 Ω cm, respectively. Oxygen sorption theory can be used to illustrate the optimal thermal stability (ΔR/R₀ = 0.10%). Complex impedance analysis further elucidates that grain boundaries make a dominant contribution to the total resistance. The mechanisms of grain boundary conduction and relaxation behavior are systematically analyzed. These findings open up a window for the further advancement of NTC ceramics at lower sintering temperature.

The addition of bismuth trioxide hugely decreased the sintering temperature and improved aging stability.     

Cu(i) substituted wurtzite ZnO: a novel room temperature lead free ferroelectric and high-κ giant dielectric

Semiconducting wurtzite ZnO, with the highest incipient piezoelectricity is an attractive alternative choice with doping transition metal ions in the host lattice to develop novel binary ferroelectric materials that can be easily fabricated in any device architecture. Up to 8% Cu⁺ ion substitution on Zn²⁺ sites in the ZnO lattice was achieved by careful selection of raw material and adaptation of a low temperature sol–gel synthesis route for the preparation of bulk material. Phase purity and substitution of Cu⁺ ions in the ZnO lattice along with oxide-ion vacancy formation was confirmed using Powder X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive X-ray analysis (EDX), X-ray Photoelectron Spectroscopy (XPS) and Magnetic property measurement system (MPMS) studies. A giant dielectric constant (∼6300) was observed at 600 °C for Zn_0.95Cu_0.05O_1−δ pellets at 100 kHz frequency. Bulk Zn_0.95Cu_0.05O_1−δ also exhibits ferroelectricity at room temperature with remnant polarization P_r and V_c equal to 9.60 × 10⁻³ μC cm⁻² and 3.83 × 10² V cm⁻¹ respectively.

Cu⁺ ion substituted ZnO, Zn_1−xCu_xO_1−δ have shown high dielectric constant (∼6300) at 600 °C at 100 kHz frequency and ferroelectricity at room temperature than for bulk Zn_0.95Cu_0.05O_1−δ samples.     

Effectively enhanced structural stability and electrochemical properties of LiNi0.5Mn1.5O4 cathode materials via poly-(3,4-ethylenedioxythiophene)-in situ coated for high voltage Li-ion batteries

Spinel LiNi_0.5Mn_1.5O₄ shows promise as a potential candidate for Li-ion batteries due to its high energy density and high rate performance. However, LiNi_0.5Mn_1.5O₄ (LNMO) spinel oxides usually deliver poor cycle life because of the increasing impedance and gradually dissolving Mn resulting in the destruction of crystal structure. Here, a conductive polymer poly-(3,4-ethylenedioxythiophene) (PEDOT) surface modified strategy is introduced to settle the above challenges. The main purpose is to construct a uniform and dense shell film on the surface of LiNi_0.5Mn_1.5O₄ (Industrial Grade), which is prepared by a simple chemical in situ oxidative polymerization method. The Mn dissolving from the lattice during the long-term cycling is well inhibited as the polymer shell protects LiNi_0.5Mn_1.5O₄ from direct exposure to the highly active electrolyte. As expected, the 3 wt% poly-(3,4-ethylenedioxythiophene) coated sample reveals long cycle life with acceptable capacity of 114.5 mA h g⁻¹ and high capacity retention of 91.6% after 200 cycles, compared to 70.9 mA h g⁻¹ and 56.5%, respectively, for the bare LiNi_0.5Mn_1.5O₄ sample. Furthermore, the coated sample demonstrates a higher capacity of 110 mA h g⁻¹ and 63 mA h g⁻¹ at 5C and 10C rate respectively. The improved performance is believed to be attributed to the formation of high conductivity and stable interface structure between electrolyte and LNMO, which is beneficial to suppress the destruction of crystalline structure due to the Mn dissolution and undesired side-reaction between electrolyte and LiNi_0.5Mn_1.5O₄ in long cycle, and improve simultaneously the conductivity and interface stability of LiNi_0.5Mn_1.5O₄ for high voltage lithium-ion batteries.

PEDOT coating on LNMO surface effectively improves it''s the crystal structure stability and electrochemical properties.     

Study of the structural,electronic, magnetic and magnetocaloric properties of La0.5Ca0.5Mn0.9V0.1O3 sample: first-principles calculation (DFT–MFT)

This paper presents a correlation between experimental and theoretical approaches to study the structural, electronic, magnetic, and magnetocaloric properties of La_0.5Ca_0.5Mn_0.9V_0.1O₃. The studied compound crystallizes in the Pbnm orthorhombic space group. The calculated DOS using the DFT + U method proves that La_0.5Ca_0.5Mn_0.9V_0.1O₃ sample exhibits semi-metallic behavior, which is preferred in spintronic applications. The calculated PDOS proves that the high hydration among Mn 3d, V 3d and O 2p at the Fermi energy level is responsible for the FM behavior of La_0.5Ca_0.5Mn_0.9V_0.1O₃. The magnetic moment has been calculated using DFT results by estimating the valence electron population. The optical properties show high light absorption in the UV region. By using the Bean–Rodbell method, the magnetic phase shows a second-order transition where η = 0.85, and the exchange parameter λ is found to be 1.19 T g⁻¹ emu⁻¹. Based on the mean-field theory, the saturation magnetization (M₀), the Landé factor (g), and the total angular momentum (J) were determined. These parameters were used to simulate magnetization as a function of the magnetic field at different temperatures as well as the variation of the magnetic entropy change ΔS_M (T).

This paper presents a correlation between experimental and theoretical approaches to study the structural, electronic, magnetic, and magnetocaloric properties of La_0.5Ca_0.5Mn_0.9V_0.1O₃.     

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.     

Impact of synthesis route on structural,magnetic, magnetocaloric and critical behavior of Nd0.6Sr0.4MnO3 manganite

Nd_0.6Sr_0.4MnO₃ polycrystalline manganite was synthesized by two different methods: the auto-combustion reaction (NSMO-AC) and the sol–gel method (NSMO-SG). The structural, magnetic, magnetocaloric and critical behavior of the samples were examined. Rietveld refinements of the XRD patterns revealed that both compounds are pure single phase indexed to the orthorhombic system adopting the Pnma space group. The nanometric size estimated using the Williamson–Hall method was confirmed by TEM micrographs. Magnetic measurements as a function of temperature indicated that both samples underwent a second order ferromagnetic (FM)–paramagnetic (PM) phase transition at Curie temperature (T_C). The relative cooling power was observed to be around 95.271 J kg⁻¹ for NSMO-AC and 202.054 J kg⁻¹ for NSMO-SG at μ₀H = 5 T, indicating that these materials are potential candidates for magnetic refrigeration application close to room temperature. The critical behavior was estimated using diverse techniques based on the isothermal magnetization data recorded around the critical temperature T_C. The calculated values are fully satisfactory to the requirements of the scaling theory, implying their reliability. The estimated critical exponents matched well with the values anticipated for the mean-field model and the 3D Ising model for NSMO-AC and NSMO-SG, respectively, showing that the magnetic interactions depended on the process of elaboration.

The relative cooling power is around 95.271 J kg⁻¹ for NSMO-AC and 202.054 J kg⁻¹ for NSMO-SG at μ₀H = 5 T, making these materials potential candidates for magnetic refrigeration application near room temperature.     

Analysis based on scaling relations of critical behaviour at PM–FM phase transition and universal curve of magnetocaloric effect in selected Ag-doped manganites

The critical behaviour of Pr_0.5Sr_0.5−xAg_xMnO₃ (0 ≤ x ≤ 0.2) samples around the paramagnetic–ferromagnetic phase transition is studied based on isothermal magnetization measurements. The assessments based on Banerjee''s criteria reveal the samples undergoing a second-order magnetic phase transition. Various techniques such as modified Arrott plot, Kouvel–Fisher method, and critical isotherm analysis were used to determine the values of the ferromagnetic transition temperature T_C, as well as the critical exponents of β, γ and δ. The values of critical exponents, derived from the magnetization data using the Kouvel–Fisher method, are found to be (β = 0.43 ± 0.002, 0.363 ± 0.068 and 0.328 ± 0.012), (γ = 1.296 ± 0.007, 1.33 ± 0.0054 and 1.236 ± 0.012) for x = 0.0, 0.1 and 0.2, respectively. This implies that the Pr_0.5Sr_0.5−xAg_xMnO₃ with 0 ≤ x ≤ 0.2 systems does not belong to a single universality class and indicates that the presence of magnetic disorder in the system must be taken into account to fully describe the microscopic interaction of these manganites. With these values, magnetic-field dependences of magnetization at temperatures around T_C can be well described following a single equation of state for our samples. From magnetic entropy change (ΔS_M), it was possible to evaluate the critical exponents of the magnetic phase transitions. Their values are in good agreement with those obtained from the critical exponents using a modified Arrott plot (MAP). We used the scaling hypotheses to scale the magnetic entropy change and heat capacity changes to a universal curve respectively for Pr_0.5Sr_0.5−xAg_xMnO₃ samples.

The universal curves of magnetic entropy changes and heat capacity changes for Pr_0.5Sr_0.5−xAg_xMnO₃ (0 ≤ x ≤ 0.2) are obtained by using the critical exponents.     

Role of Al-doping with different sites upon the structure and electrochemical performance of spherical LiNi0.5Mn1.5O4 cathode materials for lithium-ion batteries

Al-doped spinel LiNi_0.5Mn_1.5O₄ materials with different sites and contents were synthesized by rapid precipitation combined with hydrothermal treatment and calcination. The roles of Al on structural stability and electrochemical performance were studied by utilizing a series of techniques. XRD patterns indicated lower ion diffusion and no impure phased in doped samples. FT-IR and CV results reveal that Al-doped materials possess a Fd$\bar{3}$m space group with increased disorder and increasing amounts of Mn³⁺. SEM and TEM equipped with EDS were used to characterize the regular morphology accompanied by a complete crystal structure and homogeneous distribution of elements. The Al content at the Ni, Mn, and Ni/Mn sites was optimized to be 5%, 3% and 5% (in total), respectively. The cycling stability was considerably enhanced at an ambient temperature (25 °C) and high temperature (55 °C). A typical Al dual-doped sample at Ni/Mn sites with 5% content delivered a reversible capacity of 113.5 mA h g⁻¹ after 200 cycles at 0.5C. The discharge capacity at 5, 10 and 20C was 127.3, 125.5 and 123.1 mA h g⁻¹, respectively. The discharge capacity remained at 126 mA h g⁻¹ after 50 cycles (55 °C, 0.5C). Subsequent EIS and analytical results of the cycled electrode showed improved structural stability with a lower resistance, stable cathode/electrolyte interface, and reduced dissolution of Mn. These data further demonstrated the feasibility and reliability of preparing high-performance spinel LiNi_0.5Mn_1.5O₄ cathode materials by doping with a suitable amount of Al.

Al-doped spinel LiNi_0.5Mn_1.5O₄ materials with different sites and contents were synthesized by rapid precipitation combined with hydrothermal treatment and calcination.     

Magnetic entropy table-like shape and enhancement of refrigerant capacity in La1.4Ca1.6Mn2O7–La1.3Eu0.1Ca1.6Mn2O7 composite

In this work, we have investigated the structural, magnetic and magnetocaloric properties of La_1.4Ca_1.6Mn₂O₇ (A) and La_1.3Eu_0.1Ca_1.6Mn₂O₇ (B) oxides. These compounds are synthesized by a solid-state reaction route and indexed with respect to Sr₃Ti₂O₇-type perovskite with the I4/mmm space group. The substitution of La by 10% Eu enhances the value of magnetization and reduces the Curie temperature (T_C). It is also shown that these compounds undergo a first-order ferromagnetic–paramagnetic phase transition around their respective T_C. The investigated samples show large magnetic entropy change (ΔS_M) produced by the sharp change of magnetization at their Curie temperatures. An asymmetric broadening of the maximum of ΔS_M with increasing field is observed in both samples. This behaviour is due to the presence of metamagnetic transition. The ΔS_M(T) is calculated for A_x/B_1−x composites with 0 ≤ x ≤ 1. The optimum ΔS_M(T) of the composite with x = 0.48 approaches a nearly constant value showing a table-like behaviour under 5 T. To test these calculations experimentally, the composite with nominal composition A_0.48/B_0.52 is prepared by mixing both individual samples A and B. Magnetic measurements show that the composite exhibits two successive magnetic transitions and possesses a large MCE characterized by two ΔS_M(T) peaks. A table-like magnetocaloric effect is observed and the result is found to be in good agreement with the calculations. The obtained ΔS_M(T) is ≈4.07 J kg⁻¹ K⁻¹ in a field change of 0–5 T in a wide temperature span over ΔT_FWHM ∼ 68.17 K, resulting in a large refrigerant capacity value of ≈232.85 J kg⁻¹. The MCE in the A_0.48/B_0.52 has demonstrated that the use of composite increases the efficiency of magnetic cooling with μ₀H = 5 T by 23.16%. The large ΔT_FWHM and RC values together with the table-like (−ΔS_M)^max feature suggest that the A_0.48/B_0.52 composite can meet the requirements of several magnetic cooling composites based on the Ericsson-cycle. In addition, we show that the magnetic field dependence of MCE enables a clear analysis of the order of phase transition. The exponent N presents a maximum of N > 2 for A, B and A_0.48/B_0.52 samples confirming a first-order paramagnetic–ferromagnetic transition according to the quantitative criterion. The negative slope observed in the Arrott plots of the three compounds corroborates this criterion.

In this work, we have investigated the structural, magnetic and magnetocaloric properties of La_1.4Ca_1.6Mn₂O₇ (A) and La_1.3Eu_0.1Ca_1.6Mn₂O₇ (B) oxides.     

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.     

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.     

A hydrothermal synthesis of Ru-doped LiMn1.5Ni0.5O4 cathode materials for enhanced electrochemical performance

An Ru-doped spinel-structured LiNi_0.5Mn_1.5O₄ (LNMO) cathode has been prepared via a simple hydrothermal synthesis method. The as-prepared cathode is characterized via Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), laser particle size distribution analysis, X-ray photoelectron spectroscopy (XPS) and electrochemistry performance tests. The FTIR spectroscopy and XRD analyses show that the Ru-doped LNMO has a good crystallinity with a disordered Fd$\bar{3}$m space group structure. The disordered structure in the cathode increased and the Li_xNi_1−xO impurity phase decreased when Ru addition increased. SEM shows that all samples are octahedral particles with homogeneous sizes distribution, and the particle size analysis shows that the Ru-doped samples have smaller particle size. XPS confirms the existence of Ru ions in the sample, and reveals that the Ru induce to part of Mn⁴⁺ transfers to Mn³⁺ in the LNMO. The electrochemical property indicated that the Ru-doped cathode exhibits better electrochemical properties in terms of discharge capacity, cycle stability and rate performance. At a current density of 50 mA g⁻¹, the discharge specific capacity of the Ru-4 sample is 140 mA h g⁻¹, which is much higher than that of the other samples. It can be seen from the rate capacity curves that the Ru-doped samples exhibit high discharge specific capacity, particularly at high current density.

An Ru-doped spinel-structured LiNi_0.5Mn_1.5O₄ (LNMO) cathode has been prepared via a simple hydrothermal synthesis method.     

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.