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Because of their significant magnetocaloric properties, Ca and Pb-doped LaMnO3 can be employed in magnetic refrigeration systems. The partial substitution effect of Ca2+ by Pb2+ cations on the structural, magnetocaloric properties, and critical behavior of mixed valence perovskites La0.8Ca0.2−xPbxMnO3 (0 ≤ x ≤ 0.15), synthesized by the flash combustion route, was studied in this work. The presence of structural transformations of the prepared ceramics from an orthorhombic structure (Pbnm space group) for samples with x ≤ 0.05 to a rhombohedral system ( R 3 - c space group) for x ≥ 0.1 was detected by X-ray diffraction analysis, which is related to the A-site disorder. The field-cooled (FC) magnetization indicated a second-order ferromagnetic-paramagnetic transition for the investigated samples when the temperature was increased. Due to the change in the Mn3+/Mn4+ ratio, substituting Ca2+ with larger Pb2+ ions increases the Curie temperature (TC) by increasing Pb doping content from 90 K for x = 0 to 205 K for x = 0.15. The magnetization isotherms of all samples showing a second-order phase transition were studied, where an applied magnetic field of 5 T, the value of maximum magnetic entropy variation / Δ S M max / for the sample with x = 0.15 was estimated to be 2.73 Jkg−1 K−1. In addition, it has a significant relative cooling power of about 276 Jkg−1 under the same applied magnetic field strength. The critical behavior of the samples was investigated close to TC. The exponent values were identical to the mean-field method, indicating a large disorder of spin magnetic moments in the perovskite compounds. [ABSTRACT FROM AUTHOR]

Tunable manufacturing using flexible building blocks to bring functionalities while providing long-term stabilization for small-sized nanomaterials occupies a forefront position in nanoscience and nanotechnology. Iron oxide nanoparticles are among the most promising nano-objects, owing to their inherent magnetic properties. The propensity of iron oxide to aggregate constitutes a serious drawback for many of their potential properties that need to keep their size at the nanoscale. Hosting iron oxide in porous supports, intercalation in layered nanostructures or shelling with tunable partners are common approaches used for their stabilization. We herein report the controlled growth of iron oxide using cleverly crafted sol-gel transformable siloxane precursors. These flexible building blocks provide an entry to iron oxide encapsulated in hydrophobic silica, denoted as Fe3O4@PMSiO2, while the use of commercially available TEOS afforded Fe3O4@SiO2. Owing to the presence of PDMS-like segments, the grown Fe3O4 exhibits distinctive features in terms of the crystal size, restricted growth and dispersion in organic solvents compared to native iron oxide and those grown in conventional silica supports. Upon calcination at 500 °C, the size of the crystal expands by 25.4 nm in the case of native Fe3O4 reaching 35.9 nm. Comparatively, marginal expansion was observed using our siloxanes, with the size of those grown in Fe3O4@OMSiO2 and Fe3O4@PMSiO2 being restricted to 14.3 nm and 11.7 nm, because of the stabilization brought by the siloxane layers. Furthermore, Fe3O4@OMSiO2 and Fe3O4@PMSiO2 are fully soluble in apolar heptane and hexane, which convincingly substantiate the hydrophobic nature of the resulting mixed oxide materials. Highlights: The growth of iron oxide nanoparticles in the presence of sol-gel processable PDMS-like silica precursor. The bulkiness of the used PDMS-like precursor restricts the expansion of iron oxide nanoparticles. The presence of a hydrophobic segment allows for dispersing iron oxide in non-polar solvents. [ABSTRACT FROM AUTHOR]

The report attempts to study some spinel such as cobalt (Co–F), nickel (Ni–F), copper (Cu–F) and magnesium (Mg–F) nanoferrites, which are synthesized by coprecipitation route and annealed at 850 °C for 2 h. Powder X-ray diffraction patterns depict the formation of single cubic spinel structure (Fd-3m). Thereby, lattice parameter is ranging between 8.3420 and 8.4009 Å, while average crystallite size is found in 20–38 nm. Transmission electron and scanning electron microscopies exhibit that the nanoparticles are agglomerated and revealing mixture of spherical and irregular shapes. The fit of Co–F, Ni–F and Cu–F room 57Fe Mössbauer spectra was established with two sextets, which reflects that the Fe3+ is occupied two sub-lattices of spinel system. A weak percentage of quadruple doublet, 2%, was observed in both Ni–F and Cu–F spectra. Exceptionally, the Mg–F room 57Fe Mössbauer spectrum indicates broadening lines due to disordering of iron magnetic moments because of small nanoparticle size (20 nm), it was fitted with hyperfine field distribution, Mg–F spectrum was also collected at 6 K reflecting a ferrimagnetic ordering according to Néel's theory. M(T) curves indicate that the blocking temperature is higher than 300 K, except the Mg–F sample shows a TB of ~226 K. Besides, the M(H) loops at 5 K exhibit a soft behavior and the magnetic parameters like Ms, Mr, µe, Mr/Ms ration, Hc and Ka have determined for the studied spinel ferrites. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the structural, magnetic, and magnetocaloric properties of La0.62Nd0.05Ba0.33Mn1-xCrxO3 manganites elaborated by sol gel process. X-ray diffraction analyses confirm that all elaborated samples are crystallized on the rhombohedral perovskite structure with R-3c space group. Magnetic measurements reveal the presence of a magnetic transition from paramagnetic (PM) to ferromagnetic (FM) phase in the vicinity of Curie temperature TC. However, as Cr concentration increases, we witness a decrease in TC from 285 K for x = 0 to 250 K for x = 0.20. This behavior can be explained by the double exchange interaction of Mn3+-O-Mn4+. The maximum entropy change (- Δ S m max ) at µ0H = 5 T is found to be 2.63, 1.88, and 1.76 J.kg−1.K−1 for x = 0, 0.10, and 0.20, respectively. However, our research underlines the enhancement of relative cooling power (RCP) by Cr doping, from 310 to 340 J.kg−1 as Cr concentration increases from x = 0 to 0.20. This finding suggests the usefulness of our samples as magnetic refrigeration material around room temperature. Furthermore, the analysis of the magnetocaloric effect (MCE) using the Landau theory shows a good agreement with the experimental results with a soft difference, indicating the importance of Jahn–Teller interactions on manganites perovskite. [ABSTRACT FROM AUTHOR]

A novel series of cesium-modified POM (Cs-POMs) compounds (CsxH4−xPW11VO40, x = 0.5, 1.5, 2.5, 3.5 and 4) was synthesized by self-assembly method. The characterization techniques including scanning electron micrograph (SEM), X-ray diffraction (XRD), infrared (FT-IR), diffuse reflectance UV–Vis (UV–Vis/DRS), and dielectric were used to reveal the effect of cesium substitution on structural, optical and dielectric properties. The relationship between Cs+ content, optical and dielectric behavior of CsxH4−xPW11VO40 complexes was established. XRD results showed a structural transition from triclinic (P-1) to cubic (Pn-3m) when the H4PW11VO40 proton was substituted by more than 0.5 of Cs+ content. Moreover, FT-IR indicated that the primary Keggin structure of Cs-POMs was preserved even at high Cs contents. The higher Cs+ substitution gradually decreased the band-gap energy to reach 2.49 eV for Cs4PW11VO40, which revealed its optical property. Furthermore, the addition of a high amount of Cs into H4PW11VO40 improved the dielectric constant at low frequency. [ABSTRACT FROM AUTHOR]

Herein, we reported a successful synthesis of spinel ferrites MFe2O4 (M = Ni, Co, and Mg) nanopowders through heat treatment of carbonate-intercalated layered double hydroxide (MFe–CO3–LDH) precursors at 600 and 1000 °C for 2 h. The physicochemical properties of as-synthesized LDHs and resulting calcined products were investigated using X-ray diffraction, thermogravimetric/differential thermal analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, magnetic measurements and Mössbauer spectrometry. The results reveal the formation of a single-phase spinel crystal structure of space group Fd-3m. The effect of calcination temperature on the structural and magnetic properties of ferrite nanoparticles was discussed in detail. The Ni2+, Co2+, and Mg2+ substitution on the structure resulted in significant changes in the crystal composition, surface morphology, particle size, and magnetic properties of spinel ferrites. Furthermore, the effects of calcination on the structural and magnetic phase transformation of iron-containing LDH precursors have been also studied by Mössbauer spectrometry at room temperature. The M(H) curves at 5 K revealed a soft magnetic behaviour for both Ni and Mg calcined products, while Co spinel ferrite shows wasp-waist magnetic nature which is discussed in present study. By comparing to reported works in literature, the calcination effect on the present LDH samples has relatively improved the saturation magnetization property. [ABSTRACT FROM AUTHOR]

The thermal, structural and magnetocaloric properties of La1−xLixMn1−yFeyO3 (x = 0.1, 0.2 and y = 0, 0.1) powders were investigated in this study for application in magnetic refrigeration systems, using thermogravimetric analysis, X-ray diffraction analysis and superconducting quantum interference device magnetometer. All samples were synthesized by flash combustion method using glycine as fuels and nitrates as precursors. The Li and Fe doped samples exhibited a rhombohedral structure and an R 3 ¯ c space group. The increase of Li concentration to 20%, leads to the formation of LiMn2O4 spinel phase as a secondary phase. The ferromagnetic–paramagnetic transition was inferred from the temperature dependence of the magnetization. The magnetic entropy change (−ΔSM) has been calculated using the magnetization isotherms data. However, the increase of La substitution by Li from 10 to 20% leads to a decrease of the Curie temperature TC from 215 to 75 K accompanied with a maximum magnetic entropy change / - Δ S M max / from 3.63 to 1.52 J.Kg−1 K−1 under a magnetic field change of 5 T. Moreover, by the 10% Fe doping, there is a clear trend of decreasing the Curie temperature, the magnetic entropy change and the relative cooling efficiency. Therefore, in the vicinity of TC, (−ΔSM) reached a maximum value of 1.54 J.Kg−1 K−1 for La0.9Li0.1Mn0.9Fe0.1O3 and 0.96 J.Kg−1 K−1 for La0.8Li0.2Mn0.9Fe0.1O3. According to the Arrott plots results, a second-order ferromagnetic–paramagnetic transition was found for all the doped samples. [ABSTRACT FROM AUTHOR]

We report the synthesis, structural, optical and dielectric properties of single-phase spinel of Ni-substituted Zn nanoparticles prepared by co-precipitation method. The annealed samples at 1000 °C were characterized by X-ray diffraction (XRD) technique, infrared spectrophotometer (FTIR), UV–VIS spectrophotometer and dielectric measurements. X-ray diffraction analysis shows that the increase of zinc content has been manifested on crystallite size and lattice parameter. Crystallite size and lattice parameter were systematically increased from 83 to 130 nm and 8.386 to 8.443 Å, for x varied from 0.5 to 0.9. The FTIR analysis show two absorption bands in the wave number range of 400–540 cm−1. The UV–VIS analysis shows that the estimated optical energy band gaps (Eg) of the samples, through Tauc plots, do not exceed 5 eV which confirms the semiconductor nature of our samples. The dielectric behavior was investigated as function of both frequency (1 kHz–1 MHz) and temperature (298–700 K), and the electrical response was investigated by impedance spectroscopy measurements. [ABSTRACT FROM AUTHOR]

The aim of this research work was to study the structural and magnetic properties of iron oxide nanoparticles. The as-prepared sample was synthesized by a co-precipitation route and annealed at different temperatures. The annealed samples were investigated using different techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), vibrating sample magnetometry (VSM), and Mössbauer spectrometry (MS). The XRD results indicate the formation of three phases which have been identified as magnetite (Fe3O4), maghemite (γ-Fe2O3), and hematite (a-Fe2O3). The crystallite size was very similar for both magnetite and maghemite, and it was higher for hematite. The TEM observations showed that the particle shapes were affected by the annealing temperature (Tan). In addition, the SEM analysis revealed a wide distribution of the particle size. The magnetic measurements enabled the determination of a blocking temperature for both Fe3O4 and γ-Fe2O3 as 210 and 240 K, respectively. The Morin transition temperature was determined in the case of α-Fe2O3 from the magnetization and the MS measurements. The synthesized iron oxide nanoparticles can be good candidates for hyperthermia applications. [ABSTRACT FROM AUTHOR]

Nanoparticles Ni1-xZnxFe2O4 (0.4 ≤ x ≤ 0.7) samples were synthesized through co-precipitation method, and the effect of zinc concentration on the structural, magnetic, and magnetocaloric properties was investigated. X-ray diffraction patterns reveal the formation of single-phase spinel. The average crystallite size, estimated using Scherrer's formula, was found to be ranging from 83 to 116.6 nm. The Curie temperature was found to decrease from 619 to 350 K with increasing x from 0.4 and 0.7. The Arrott plots reveal a second-order ferromagnetic transition in all samples. Magnetic entropy change (-ΔSm) allowing estimation of the magnetocaloric effect was determined based on the thermodynamic Maxwell's relation. The (-ΔSm) was found to slightly increase with increasing x from 0.4 to 0.5 and decreasing with increasing x. The relative cooling power increases linearly with zinc content, up to x = 0.5. [ABSTRACT FROM AUTHOR]

The objective of this work was to study the influence of annealing temperature on the structural changes and magnetic properties of the Ni0.5Zn0.5Fe2O4 spinel-type nanoparticles. The nanomaterial was prepared by the chemical co-precipitation method and studied by thermal analysis (TG–DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM), magnetic measurements and 57Fe Mössbauer spectrometry. XRD has revealed that the as-prepared sample shows poor crystallization with less defined diffraction lines. As the annealing temperature increases, the diffraction peaks become intense and well defined, reflecting perfect crystallization of the sample. The estimated crystallite size varies from 25 to 83 nm. TEM observations give information on the morphology and confirm the XRD results. To quantify the proportions of the iron atoms in the tetrahedral and octahedral sites, in-field Mössbauer spectrometry measurements were carried out at low temperature. Saturation magnetization (Ms) and the average hyperfine magnetic field B hf increase gradually with annealing temperature. For the sample annealed at 1000 °C, the magnetic entropy change Δ S M max and relative cooling power, measured under field change of 2T, are 0.67 J kg−1 K−1 and 112.5 J kg−1, respectively. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the structural, magnetic, and magnetocaloric properties of La0.62Nd0.05Ba0.33Mn1-xCrxO3 manganites elaborated by sol gel process. X-ray diffraction analyses confirm that all elaborated samples are crystallized on the rhombohedral perovskite structure with R-3c space group. Magnetic measurements reveal the presence of a magnetic transition from paramagnetic (PM) to ferromagnetic (FM) phase in the vicinity of Curie temperature TC. However, as Cr concentration increases, we witness a decrease in TC from 285 K for x = 0 to 250 K for x = 0.20. This behavior can be explained by the double exchange interaction of Mn3+-O-Mn4+. The maximum entropy change (- Δ S m max ) at µ0H = 5 T is found to be 2.63, 1.88, and 1.76 J.kg−1.K−1 for x = 0, 0.10, and 0.20, respectively. However, our research underlines the enhancement of relative cooling power (RCP) by Cr doping, from 310 to 340 J.kg−1 as Cr concentration increases from x = 0 to 0.20. This finding suggests the usefulness of our samples as magnetic refrigeration material around room temperature. Furthermore, the analysis of the magnetocaloric effect (MCE) using the Landau theory shows a good agreement with the experimental results with a soft difference, indicating the importance of Jahn–Teller interactions on manganites perovskite. [ABSTRACT FROM AUTHOR]