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The morphological, structural, magnetic and magnetocaloric properties of perovskite manganite oxides La0.8−x□xNa0.2−x□xMnO3 are studied with x = 0 and 0.1. The X-ray diffraction (XRD) data were analyzed by Rietveld refinement technique which reveals that the samples crystallized in a rhombohedral structure with R 3 ¯ c space group. As for the magnetic measurements, it is found that the two samples undergo a transition from the ferromagnetic state (FM) to paramagnetic state (PM) with the increase of temperature. The transition temperature, called the Curie temperature (TC), decreases from 324 K for the parent compound (x = 0) to the range of room temperature (304 K) for x = 0.1. Moreover, from magnetization measurements versus magnetic applied field, we studied the magnetocaloric effect (MCE) by the determination of the magnetic entropy change ΔSM. The maximum magnetic entropy changes Δ S M Max increases from 2.91 J/kg.K for x = 0–3.23 J/kg.K for x = 0.1 under an applied magnetic field of 2 T. The Relative Cooling Power (RCP) values were found to reach 76.13 J/K and 70.36 J/K for x = 0 and 0.1, respectively, for the same applied magnetic field (μ0H = 2 T). Thanks to the mentioned values obtained around the Curie temperatures, these materials are suggested to be good candidates as magnetic refrigerants for magnetic refrigeration in the above-room and room temperature range. Besides, a mixture of these two materials may form a composite refrigerant working in a larger temperature range limited by their TC.

International audience; Polycrystalline compounds of La0.75Ba0.25−xSrxFeO3 containing different amounts of Sr (x = 0.00, 0.05, 0.10 and 0.15) were synthesized by the sol–gel method. The samples were characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), FTIR and Raman spectroscopy, UV–Vis spectroscopy and I–V electrical measurements. XRD analysis reveals that all samples possess an orthorhombic structure with Pnma space group. The Lattice parameter values are found to decrease with the increase of Sr contents. The surface morphology and microstructure were analyzed using SEM images. It is found that all samples have similar microstructure morphology, but differ in the range of grain sizes. Raman spectroscopy measurements exhibit slight shift in Raman modes with Sr-doping. A shift of characteristic peaks for Fe–O bond in FTIR spectra were observed with the increase of Sr content. Using the Tauc plot methodology, the optical band gap values have been evaluated from the UV–Vis measurements. These values increase with Sr content. Finally, room temperature electrical resistivity decreases with Sr content increasing. The obtained results show that the structural, optical and electrical properties have low strongly affected by the strontium content.

In the present study, the structural, morphological, electrical, and dielectric properties of Ni0.6Zn0.4Fe1.5Al0.5O4 annealed at 600 °C, 900 °C, and 1200 °C were investigated. The X-ray diffraction patterns confirmed the presence of the single-phase cubic spinel structure with the Fdm space group. The SEM images of Ni0.6Zn0.4Fe1.5Al0.5O4 nanoparticles demonstrated that these samples (Ni900 and Ni1200) were nano-sized and that the increase in annealing temperature enhanced the agglomeration rate. It was found that the electrical conductivity of the system improved on increasing the temperature over the whole explored range for the two low annealing temperatures, while this improvement declined after 500 K in the case of the highest annealing temperature. For such a sample, a metallic behavior was seen. The sample annealed at 1200 °C possessed the highest conductivity and the lowest activation energy. The impedance measurements were in good agreement with the conductivity plots and confirmed the emergence of a grain boundary effect with the increase in annealing temperature. For the sample annealed at the highest temperature, Z′ decreased rapidly with frequency. This sample exhibited the lowest defect density than the other samples. Consequently, its electrical conductivity increased. A Nyquist diagram was used to examine the contribution of the grains and grain boundary to conduction and to model each sample by an equivalent electrical circuit. The dielectric behavior of the investigated samples was correlated to the polarization effect.

Nanoparticles of Li0.5ZnFe1.5O4 (LiZn2Fe3O8) with the spinel structure were prepared by a sol–gel auto-combustion method at two different annealing temperatures. X-ray diffractograms and Rietveld refinement confirmed the formation of the spinel structure. The morphology was analyzed by electron microscopy, which showed that the grains were composed of different crystallites. Elastic properties were determined from infrared spectroscopy. It was found that the elastic parameters increased with the increase in annealing temperatures. The band gap depends on the annealing temperature and it decreased on increasing the particle size. The conductivity of the specimen annealed at 500 °C followed either the Jonscher's model or Drude's model depending on the temperature range. This conductivity decreased when the annealing temperature was raised by 600 °C. AC conductivity was found to be controlled by the hopping model. A single relaxation phenomenon was evidenced for each sample from impedance analysis. The Nyquist diagram proved that the samples were simultaneously capacitive and resistive and also supported the presence of multiple relaxation times.

Magnetic properties were studied just above the ferromagnetic–paramagnetic (FM–PM) phase transition of (Nd1−xGdx)0.55Sr0.45MnO3 with x = 0, 0.1, 0.3 and 0.5. The low-field inverse susceptibility (χ−1) of Nd0.55Sr0.45MnO3 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 and Magnetocaloric (MC) properties of the La2/3Sr1/3MnO3 (L2/3S1/3MO) nanoparticles have been examined using magnetization measurements and Monte Carlo Simulations (MCS). Good accordance between both measured and calculated results have been obtained. The sensitivity of the magnetic and MC properties to the nanoparticle size, temperature, and external magnetic field is discussed. From the theoretical point of view, the Curie temperature (TC) has been obtained close to 360 K for the length size L ≥ 20, with fourth nearest neighbors. The maximum entropy ( Δ S m m a x ) and relative cooling power (RCP) have been computed and their dependence on the applied magnetic field have been established and found in accordance with the experiment data as well.

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The magnetic properties of antiferromagnetic Mn3ZnN anti-perovskite nanoparticles have been studied using Monte Carlo simulations within the Ising model. The considered Hamiltonian includes the exchange interactions between Mn–Mn and the external magnetic field. The thermal magnetizations and magnetic susceptibilities of Mn3ZnN anti-perovskite nanoparticles are computed for different sizes. The Neel temperature is deduced. The exchange interaction between Mn and Mn atoms is obtained. Beyond the coupling value 2 K, the magnetization of system becomes independent of effect temperature. In addition, the variation of magnetizations with external magnetic field and crystal filed are also established for different temperatures. Mn3ZnN anti-perovskite nanoparticles exhibits the superparamagnetic behaviour for different temperatures value.

In this research paper, our central focus is upon structural, optic, magnetic, magnetocaloric, and critical phenomena of La0.57Nd0.1Sr0.18Ag0.15MnO3 (LNSMAO) compound. This compound is crystallized in the rhombohedric system, with a $$R\mathop 3\limits^{ - } c$$ space group. The band gap energy of this powder was 2.5 eV. Therefore, the LNSAMO sample proved to be not only a photocatalysis material but also a photovoltaic application. The FTIR spectra confirmed the formation of the structure of orthorhombic perovskite. The second-order transition of the magnetic phase in the LNSMAO sample was also reported by the positive slopes of the Arrott plot. Beneath 5 T, the values of magnetic entropy change and relative cooling power, respectively, amounted to about 5.08 J kg−1 K−1 and 146.7 J/Kg. In perspective, this sample stands for an agreeable candidate for magnetic refrigeration. By analyzing the dependence on the field of the magnetic entropy change (ΔSM) data as well as the relative cooling power (RCP), it was possible to assess the values of the critical exponents. Moreover, the critical exponents of LNSMAO compound were estimated based on multiple techniques like Kouvel-Fisher (KF), modified Arrott plot (MAP) method, and critical isotherm analysis (CIA). The reliability of these critical behaviors was verified by the Widom scaling relation and the universal scaling hypothesis. Our results demonstrate an excellent correlation between magnetocaloric effect (MCE) properties in manganese systems and critical behavior.

We have undertaken a systematic study of critical behavior in La 0.67 Sr 0.33 MnO 3 nanoparticles, sintered at different temperatures ( L6 , L8 , L10 and L12 sintered at 600 °C, 800 °C, 1000 °C, 1200 °C respectively), by magnetization measurements. The critical exponents are estimated by various techniques such as the Modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique. Compared to standard models, the critical exponents are close to those expected by the Mean-field model (with β = 0.5 γ = 1, and δ = 3) for ( L6 , L8 , and L10 ) samples and by the (3D) Heisenberg model (β = 0.365, γ = 1.336 and δ = 4.80) for L12 sample. We conclude that the reduction of grain size strongly influences the universality class.

Structural and magnetic properties of La0.6Ca0.4−xSrxMnO3 manganites have been investigated for x = 0.0, 0.1 and 0.4. The Rietveld analysis of X-ray diffraction data indicates that, with increasing Sr content, a structural phase transition from rhombohedral to orthorhombic is occurred. Structural parameters are calculated and found to be dependent upon the composition. The nanometric size of our samples estimated using Debye–Scherrer equation is confirmed by SEM images. Magnetic measurements according to the zero-field cooled and field cooled procedures are performed. The observed irreversibility between the two modes is assigned to the presence of magnetic anisotropy which has been explained as arising from the surface effects. The blocking temperature (TB) is estimated and found to increase as the particle size increases. Above TB, the materials show a superparamagnetic (SPM) behavior where the coercivity and remanence magnetizations are almost zero that favors their application in the biomedical field. Studying the thermal variation of dc susceptibility confirms the SPM nature of the studied compounds. The good fit of the magnetization vs. magnetic field curves with Langevin function supports our findings. From this simulation, the saturation magnetization is calculated and found to increase as particle size increases which is explained in terms of Core–shell model. With the reduction of particle size to the nanometric scale, single domain structure and SPM regime structure are detected.

In the current research work, a phenomenological model is applied to describe the magnetocaloric effect (MCE) of the two phases 0.75 La0.6Ca0.4MnO3/0.25 La0.6Sr0.4MnO3 composite system. Based on this model, the values of the magnetocaloric properties are predicted from the calculation of magnetization as a function of temperature under different external magnetic fields. A significant MCE is obtained over a large range of temperature compared to those observed in the mother compounds, La0.6Ca0.4MnO3 and La0.6Sr0.4MnO3, making of this material considered as a promising candidate for magnetic refrigeration applications in moderate magnetic fields near room temperature. The results are then compared to those obtained experimentally in our previous work. The excellent agreement observed between both data proves the validity of the adopted model in the estimation of the MCE properties of material under investigation.

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This research paper presents a theoretical work on the magnetocaloric properties of (SC.4-2) composite obtained by mixing citric-gel La0.6Ca0.4MnO3 (S0C1) and La0.6Sr0.4MnO3 (S1C0), with mole fractions [0.875 (S0C1)/0.125 (S1C0)]. This mixture was then fritted at 900 °C. The magnetization of the composite goes in good agreement with the following relationship $$M(\mathrm{S}\mathrm{C}.4{\text{-}}2)=0.865\times M(\mathrm{S}0\mathrm{C}1)+0.135 \times M(\mathrm{S}1\mathrm{C}0)$$, where (0.865, 0.135) are the corresponding weight fractions to mole fractions (0.875, 0.125) of parent compounds [(S0C1) (S1C0)]. Resting upon this equality, the magnetic entropy change and the specific heat of composite were predicted at a constant field and pressure. The variation of the magnetic entropy $$\left|{\Delta S}_{M}\right|$$ and the heat capacity $${\Delta C}_{P,H}$$ as a function of temperature of the two parent compounds (S0C1) and (S1C0), with a phenomenological model, were obtained in our previous research work. The values of the maximum magnetic entropy change $${\left|\left({\Delta S}_{M}\right)\right|}_{\mathrm{m}\mathrm{a}\mathrm{x}}$$, full width at half-maximum $${\updelta} T_{\mathrm{F}\mathrm{W}\mathrm{H}\mathrm{M}}$$ and relative cooling power (RCP), at several magnetic field variations, were determined. In addition to the S0C1 mother compound, the SC.4-2 composite displays the highest value of RCP, providing an estimate of the quantity of the heat transfer between the hot (Thot) and cold (Tcold) ends during one refrigeration cycle. At a later stage, the study of the dependence on temperature of the magnetic entropy of (x) S0C1/(1 − x) S1C0 composites reveals that the optimum composition stands for x = 0.4. Indeed, it gives comparable contributions of two parent compounds, leading to a practically uniform variation of entropy over a wide temperature range.

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Manganites with Dy0.5(Sr1-xCax)0.5MnO3 formula (x = 0; x = 0.05; x = 0.1; x = 0.2 and x = 0.3) were prepared via the well-known sol-gel method. They were indexed to the orthorhombic Pnma space group. Zero field cooled (ZFC) and field cooled (FC) magnetization measurements display antiferromagnetic (AFM) order among Mn spins at TN(Mn) which decreases from around 65 K (for x = 0) to about 35 K (for x = 0.3). Dy–spins order antiferromagnetically at very low temperature for samples with x > 0. Magnetization does not saturate up to field of 5 T. AC magnetic susceptibility was checked and investigated to show the absence of spin glasses in these systems.

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International audience; Nanoscale phosphors of Yb3+/Er3+/Tm3+-doped GdPO4 with different morphologies were synthesized using the solvothermal method with annealing at 750 °C and 1000 °C. A high concentration of Yb3+(35%mol) ions was successfully doped in the powders. Crystal structures were characterized through powder XRD measurement and Fourier transform infrared spectroscopy. SEM images have shown that the morphologies of the samples can be controlled by adjusting the pH value during their synthesis. Upconversion luminescence was investigated under 980 nm diode laser excitation. The upconversion processes were assessed on the basis of the power dependence of upconversion emission intensity. The light colour was investigated with respect to the 1931 CIE diagram. The samples show good emission tunability by monitoring the annealing temperature and the diode laser pump intensity. By properly adjusting the excitation power, white upconversion luminescence can be attained. Yb3+/Er3+/Tm3+-doped GdPO4 nanophosphors appear, which have potential applications in the field of upconversion-based white lighting devices.

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The perovskite La 0.8 Ca 0.1 Pb 0.1 Fe 1- x Co x O 3 ( x = 0.00, 0.05, 0.10, 0.15 and 0.20) compounds were synthesized by the sol-gel method using the citric acid route. Structural, morphological, electrical and gas-sensing properties of these compounds were investigated by various techniques, such as XRD, MEB and electric resistance measurements (from 30 to 350 °C). These compounds were found to exhibit a single phase and crystallized in the orthorhombic symmetry with a Pnma space group. Otherwise, we found that the substitution of iron (Fe) ion by cobalt (Co) one does not change the principal phase. The La 0.8 Ca 0.1 Pb 0.1 Fe 1- x Co x O 3 sensor showed interesting gas sensing performances towards ammonia (NH 3 ) gas. On the other hand, the La 0.8 Ca 0.1 Pb 0.1 Fe 0.95 Co 0.05 O 3 ( x = 0.05) sensor exhibited an early selectivity to ammonia at 270 °C including the ability to detect low concentrations of NH 3 (10 ppm ).

With a view to investigate the influence of nanosize on the magnetocaloric properties of the La0.67Sr0.33MnO3 nanoparticles, a systematic investigation of their temperature-dependant magnetic entropy change has been undertaken. The maximum of magnetic entropy change - Δ S M max and the relative cooling power (RCP) decrease with decreasing particle size. As a result the magnetocaloric effect was greatly changed by the particle size. Indeed, the relative cooling power can be tuned dramatically by particle size. Moreover, the nature of phase transition has been determined using Banerjee’s criterion plots and confirmed by the construction of the universal curve of the magnetic entropy change.

This paper presents the effects of oxygen deficiency on the structural, magnetic and magnetocaloric properties of La 0.8 Na 0.2 MnO 3 − δ where δ =0 and 0.05. The Polycrystalline La 0.8 Na 0.2 MnO 3 − δ samples were synthesized by a new method. The introduction of the oxygen deficiency in our samples leads to an increase of Mn 3+ content according to the electronic formula La 0 . 3 + N a 0.2 + M n 0.6 + 2 δ 3 + M n 0.4 − 2 δ 4 + O 3 − δ . The X-ray diffraction analysis shows a phase transition from a rhombohedral system with R 3 ¯ c space group for δ =0.00 to an orthorhombic with P n m a space group for δ =0.05. It also induces a great variation in the magnetic properties. The magnetization versus temperature study has shown that all samples exhibit a magnetic transition from ferromagnetic (FM) to paramagnetic (PM) phase with the increase in temperature. However, it can be clearly seen that the oxygen deficiency induces an increase in the magnetization and a decrease in the Curie temperature T C . Besides, the magnetocaloric effect (MCE) as well as the Relative Cooling Power (RCP) has been estimated. As an important result, the values of MCE and RCP in our oxygen deficiency manganites are reported to be close to those found in gadolinium that is considered as magnetocaloric reference material.

The critical parameters provide important information concerning the interaction mechanisms near the paramagnetic (PM) to ferromagnetic (FM) transition. In this paper, we present a detailed study for the critical behavior of AMn0.825Ga0.175O3 (A = La0.75Ca0.08Sr0.17) sample using the isothermal magnetization data. The critical exponents are estimated by various techniques such as the modified Arrott plot, Kouvel–Fisher plot and critical isotherm. Evaluations of critical exponents yield β = 0.365 ± 0.05, γ = 1.218 ± 0.02 and δ = 4.22 ± 0.02 that are consistent with both predictions for the three-dimensional-Heisenberg model and those reported for materials with the FM transition. The field's dependence on the magnetic entropy change is also analyzed. It shows a power law dependence such as ΔSM=α Hn. Finally, the second-order magnetic transition phase is proved by the construction of the universal curve's magnetic entropy change.

The La0.9Sr0.1MnO3 (LSMO) compound was prepared by the citrate-gel method and annealed at different temperatures (H6, H8, H10 and H12). X-ray diffraction (XRD), transmission electron microscopy (TEM) and magnetic measurements were used to investigate the particle size effects on the physical properties. All samples were found to be single phase and crystallize in rhombohedral symmetry with R3¯c space group except for the H12 sample, whose refinement has revealed the coexistence of both R3¯c rhombohedral and Pbnm orthorhombic phases. It was also found that the reduction of grain size intensively affects the magnetic properties of these compounds. The variation in the magnetic properties as a function of the particle size may be explained in terms of a core–shell model. The reduction of the Curie temperature with increasing grain size is explained as the strain effect of grain induced by the distortion at grain boundaries and the orthorhombic strain caused by the strong JT coupling. The temperature dependence of magnetization based on a thermal distribution of spin waves and fit results to a power law Tα shows a deviation from Bloch low and α deviates from 1.50 to 2.88 as the particle size decreases from 94.9 to 44.98 nm. The experimental magnetization measurements at 5 K were interpreted in the framework of a random magnetic anisotropy model. From the analysis of the approach to saturation magnetization some fundamental parameters have been extracted, quantifying the anisotropy forces.

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International audience; Red, green, and blue emissions through up conversion and energy transfer processes were investigated in triply doped YP5O14: Yb3+, Er3+, Tm3+ ultraphosphates upon excitation at 980 nm as a function of the Yb3+, Er3+ and Tm3+ concentrations. Highly crystalline tri-doped samples showing a monoclinic form with C2/c space group symmetry were synthesized by a flux method leading to the elaboration of single crystals which were crushed for physical-chemical mainly by X-ray powder diffraction and optical up conversion characterization.The blue, green and red emissions intensity can be adjusted by the Yb3+, Er3+ and Tm3+ amounts. The Er3+ red and green up conversion emissions were examined in the case of the Yb3+/Er3+ co-doped YP5O14 materials. Two- and three-photons processes are observed for the red and green emissions of Er3+ respectively. Three visible red, green and blue up conversion emissions corresponding to Er3+4F9/2→4I15/2 (red), Tm3+1G4→3F4; 3F2,3→3H6 (red), Er3+2H11/2, 4S3/2 →4I15/2 (green) and Tm3+1G4 → 3H6 (blue) emission bands were simultaneously observed in triply doped YP5O14: Yb3+, Er3+, Tm3+ ultraphosphates due to Yb–Er and Yb–Tm up conversion mechanisms. Moreover, a three-photon process is observed for the red emission in the tri-doped materials instead of a 2-photons mechanism observed for samples without Tm, due to a Er3+→ Tm3+ energy transfer involving the Er3+4F9/2 and the Tm3+3F2,3 states.Despite the presence of large phonons energy due to the phosphate material, around 1200 cm−1, Er3+ and Tm3+ visible emissions due to up conversion phenomena can be subsequently combined in order to obtain a white light and find new applications in display devices.

The influence of quenched disorder on the structural properties and on low-temperature field-dependent magnetization, in terms of the spin-wave theory, of (La1−x Nd x )2/3(Ca1−y Sr y )1/3MnO3 with J 1 (x=0,y=0); J 2 (x=0.05,y=0.04); J 3 (x=0.25,y=0.20); J 4 (x=0.3,y=0.24); J 5 (x=0.98,y=0.8) is investigated by characterizing a series of samples with the same A-site cational mean radius and the same average valence of the Mn ion but different A-site ionic radii variance. Our results demonstrate that moderate-small chemical disorder does not affect the crystallographic structure, but enhance the random local distortion of MnO6 octhaedra. Magnetization measurements show that the Curie temperature (T C ) decreases with disorder. The evolution of the saturation magnetization with the disorder cannot be deduced due to the possible contribution of the Nd spins to the total magnetic moment. The thermal evolution of magnetization in the ferromagnetic phase at low temperature varies as T 3/2, in accordance with Bloch’s law. The spin wave stiffness constant D obtained from the Bloch constant is found to decrease with the increase of disorder. We found that for the range of the study disorder, the magnetic behavior corresponds to a conventional ferromagnet, where T C should be proportional to D.

In this article we focus on the optical properties of a luminescent two-dimensional organic–inorganic material (thiopheneethyl ammonium lead iodide) with the formula (C 4 H 3 SC 2 H 4 NH 3 ) 2 [PbI 4 ]. This compound exhibits a self-assembled nano-layered structure, forming a multi-quantum wells (QWs) structure. X-ray diffraction patterns revealed that thin films deposited by a spin coating method were highly oriented with the c -axis perpendicular to the substrate surface. The optical properties were investigated by, absorption, ellipsometry and photoluminescence spectroscopy. At room temperature a strong photoluminescence, due to exciton confined in the PbI quantum well, is found around 2.32 eV. The Stokes shift was determined and compared with those found for homologous materials. The exciton-phonon interaction was also investigated by studying the temperature dependence of photoluminescence. The exciton-phonon coupling constants for both acoustic and optical phonons were extracted using a phenomenological model are more than one order of magnitude larger than in conventional QWs.

The La 0.67 Sr 0.33 MnO 3 ( LSMO ) compound was prepared by the citrate-gel method and annealed at different temperatures (600 °C ( L6 ), 800 °C ( L8 ), 1000 °C ( L10 ) and 1200 °C ( L12 )). X-ray diffraction ( XRD ), transmission electron microscopy ( TEM ) and magnetic measurements were used to investigate the particle size effects on the physical properties. All samples were found to be single phase crystallizing in rhombohedral symmetry with R 3 ¯ c space group. It was also found that the reduction of grain size intensively affects the magnetic properties of these compounds. The variation in the magnetic properties as a function of the particle size may be explained in terms of core–shell model. For the L6 , L8 and L10 samples, the weaker effective magnetic moments and the deviation of the inverse susceptibility from the Curie–Weiss law were observed, indicating the possible existence of a Griffiths-like cluster phase. However, the latter was found to disappear for L12. The reduction of Griffiths phase may be related to the weaker FM interactions which were weakened by the size reduction, possibly due to the surface spin disorders. Otherwise, the competition between paramagnetic and ferromagnetic phases may strongly affect the magnetic properties that may result in the disappearance of the Griffiths phase.

The polycrystalline compounds of La 0.6 Ca 0.4 MnO 3 (S0C1) and La 0.6 Sr 0.4 MnO 3 (S1C0) were prepared using the citric–gel method. With the resultant nanoparticle powders having crystallite size of 22.51 nm from S0C1 and 27.39 nm from S1C0, the 0.875(S0C1)0.125(S1C0) composition was sintered at different temperatures: SC. 4-1 (at 700 °C), SC. 4-2 (at 900 °C), SC. 4-3 (at 1100 °C) and SC. 4-4 (at 1300 °C). XRD data were analyzed by Rietveld refinement technique. The two S0C1 and S1C0 mother compounds were found to crystallize in the rhombohedral and orthorhombic structure, respectively. The compounds sintered at 700 °C (SC. 4-1) and 900 °C (SC. 4-2) were found to present the two rhombohedral and orthorhombic phases corresponding to the mother compounds, which indicates that no interfacial reaction occurs. However, when augmenting the sintering temperature, the formation of the 0.875(S0C1)0.125(S1C0) new phase was observed. These observations were confirmed from the magnetization study, which revealed that the SC. 4-1 and SC. 4-2 compounds present two magnetic transitions temperature corresponding to the two mother compounds. For higher sintering temperature (1100 and 1300 °C), we have noted the presence of a new magnetic transition corresponding to the composition 0.875(S0C1)0.125(S1C0) compound. The variation of the M(T) curves were discussed in terms of the orthorhombic distortion σ 2 (Mn–O).

International audience; Eu3+ doped GdPO4 orthophosphates with hexagonal and monoclinic crystal structures were prepared by a solvothermal process at 160 °C. Nanoparticles (NPs) and nanowires (NWs) were obtained using a mixture of water and glycerol as solvents. Luminescence properties of 5% Eu3+ doped GdPO4 NPs and NWs were investigated. As-obtained luminescent NWs:Eu and NPs:Eu powders were post-annealed at 750 °C since this temperature did not change neither their initial morphology nor their crystalline structure and led to luminescence efficiency intense enough to suit optical applications. A post-heating treatment of the samples at higher temperature (1000 °C) resulted in a change in both the crystalline structure from hexagonal to monoclinic form and the morphology. Excitation and emission spectra as well as luminescence decays were recorded on both post-annealed luminescent NW:Eu and NP:Eu powders. A comparative study of the optical results obtained for the different morphologies and structures was carried out by calculating the asymmetry ratio (A21) of Eu3+ from emission spectra. The symmetry of the local environment of Eu3+ activators has been described in the frame of Judd–Ofelt theory, using Ω2 and Ω4 intensity parameters derived from emission spectra data analysis, adopting a standard procedure. It was observed that Ω2 value for the NPs is different from that of the NWs indicating that the environment surrounding the active ion changes depending on the sample morphologies. The radiative lifetimes of the 5D0 excited state and the corresponding branching ratios for different emission transitions were also estimated and compared to the recorded luminescence decay rates at room temperature.

Structural, magnetic and magnetocaloric properties of manganites series with the AMn 1− x Ga x O 3 (A=La 0.75 Ca 0.08 Sr 0.17 and x =0, 0.05, 0.1 and 0.2) composition have been investigated to shed light on Ga-doping influence. Solid-state reaction method was used for preparation. From XRD study, all samples are found single phase and crystallize in the orthorhombic structure with the Pnma space group. The variation of the magnetization M vs. temperature T , under an applied magnetic field of 0.05 T, reveals a ferromagnetic–paramagnetic transition for all samples. The experimental results indicate that T C decreases from 336 to 135 K with increasing Ga substitution. Magnetocaloric effect (MCE) was estimated, in terms of isothermal magnetic entropy change (−Δ S M ), using the M ( T , μ 0 H ) data and employing the thermodynamic Maxwell equation. The maximum entropy change and Relative Cooling Power (RCP) show non-monotonic behaviors with increasing the concentration of Gallium. In fact, the maximum value of Δ S Mmax of AMn 1− x Ga x O 3 for x =0.00 and 0.2 samples is found to be, respectively, 2.87 and 1.17 J/kg/K under an applied magnetic field change of 2 T. For the same applied magnetic field ( μ 0 H =2 T), the RCP values are found to vary between 97.58 and 89 J/kg.

In the present work we analyze the magnetic entropy change ΔSM of La0.8Pb0.1MnO3 (LPMO) and La0.8Pb0.1Na0.1MnO3 (LPNMO). Using Arrott plots, it was found that the phase transition for both samples is of second-order. The presence of short-range magnetic order (SRMO) in the paramagnetic phase of these samples greatly depresses the magnetic entropy: the Δ S M max values are well below the values expected without considering the effect of SRMO. Moreover, the analysis of the MCE using Landau theory of phase transition shows that the contributions to the free energy from the presence of ferromagnetic clusters are strongly influencing the MCE by coupling with the order parameter around the Curie temperature.

Study of structural, magnetic and transport properties of the polycrystalline La 0.8 Pb 0.1 □ 0.1 MnO 3 (LPMO) and La 0.8 Pb 0.1 Na 0.1 MnO 3 (LPNMO) samples has been carried out. The X-ray diffraction shows that both samples crystallize in a rhombohedral structure with the R 3 ¯ c space group. Ferromagnetic and insulating states are found for the LPMO sample, whereas a ferromagnetic behavior below 247 K and insulator–metal transition at 180 K are observed for the LPNMO sample. The temperature dependence of magnetic susceptibility at higher temperature for both samples reveals the presence of the Griffiths phase above the Curie temperature. The thermal evolution of magnetization in the ferromagnetic phase at low temperature varies as T 3/2 , in accordance with Bloch’s law. The magnetization does not reach complete saturation up to a field of 6 T. Moreover, analysis of cell parameters indicates that the structure of LPMO is more distorted than LPNMO whereas additional results from the unit cell volume and the Curie temperature combined with previous results point to a lower average radius 〈 r A 〉 (the average ionic radius, related to the one-electron bandwidth) and higher variance σ r A 2 (the variance, measuring the quenched disorder) for LPMO reference to LPNMO. This result is in agreement with the enhancement of Griffiths phase characteristics and the low value of the spin stiffness constant D for LPMO. Finally, we show that the magnitude of the magnetic inhomogeneity depends on the random substitution in the A -site cations.

In this paper, the prediction of magnetic and magnetocaloric properties of La0.7(Sr,Ba)0.3MnO3 (LSBMO) compound have been conducted using a phenomenological model. Indeed, the investigation of magnetic properties has revealed that the sample exhibits a paramagnetic to ferromagnetic phase transition. Interestingly, the Curie temperature (TC) was found to decrease from 322 to 300 K with the decrease of magnetic fields. Moreover, the magnetocaloric data displays a large value of the magnetic entropy change, 1.5 J kg−1 K−1 under an applied magnetic field change of 20 kOe, which increases with the increase of the applied magnetic field, making it a promising compound for ecologically-friendly magnetic refrigeration at room temperature. Finally, the construction of the universal curve of the magnetic entropy change proves that the studied manganite undergoes a second order magnetic phase transition.

International audience; Single crystal and thin films of a new organic–inorganic hybrid material (C6H14N)2[BiBr5] were synthesized by the slow evaporation method at room temperature and characterized by X-ray diffraction, Raman spectroscopy, optical absorption and photoluminescence measurements. The crystal structure was determined in the monoclinic system with P21/c space group. The structure is built up from BiBr6 octahedra sharing two cis-bromine atoms and forming infinite [BiBr5]n zig-zag chains surrounded by organic cations. Such a one-dimensional (1D) structure may be regarded as quantum wires system in which the [BiBr5]n inorganic chains act as semiconductor wires and the (C6H14N) organic cations act as insulator barriers. The cohesion of the structure is achieved by an extensive network of N–H…Br hydrogen bonds. The Raman and Infrared spectra where interpreted by analogy with the homologous materials and by calculation of normal mode frequencies using the density functional theory (DFT) method. The optimized geometry and the calculated frequencies are in good agreement with the experimental data.

The magnetocaloric properties of the polycrystalline manganites La0.7□0.1Ca0.2MnO3 based on magnetic and resistivity measurements were investigated. Firstly, the equation Δ S = - α ∫ 0 H δ Ln ( ρ ) δ T H dH relates magnetic order to the transport behavior of the compounds. The relation between ρ and M was found to maintain the form of ρ = ρ 0 exp ( - M / α ) for temperatures near and slightly below TC. This result reveals the important effect of magnetic polarons on the transport properties. It is obvious that magnetic disorder, characterized by ΔSM, affects the magnetic polarons, while the latter influence the electronic transport properties, which may be the underlying reason for the salient ΔSM–ρ relation. On the other hand, based on a phenomenological model, the value of the magnetocaloric effect has been determined from the calculation of magnetization as a function of temperature under different magnetic fields M (T, H). This study also provides an alternative method to determine magnetic entropy change on the basis of magnetic and resistivity measurements.

We present the effect of vacancy in Pr0.6Sr0.4MnO3via dc magnetisation measurements. Using various techniques such as modified Arrott plots, the Kouvel–Fisher method, and Widom scaling relationship the values of TC (ferromagnetic transition temperature), as well as the β, γ and δ (critical exponents) are estimated. Critical exponents for the stoichiometric sample and the strontium deficient sample match well with those predicted for the tricritical mean field model. The vacancy in Pr0.5□0.1Sr0.4MnO3 changes the universal class. The estimated critical exponents of the praseodymium deficient sample are close to those found out by the 3D-Ising model.

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The critical properties of perovskite manganite AMn 1− x Ga x O 3 (A=La 0.75 Ca 0.08 Sr 0.17 , x= 0, 0.05 and 0.1) at the ferromagnetic–paramagnetic transition have been analyzed. Experimental results revealed that all samples exhibit a second-order magnetic phase transition. The estimated critical exponents derived from the magnetic data using various techniques such as modified Arrott plot, Kouvel–Fisher method, and critical magnetization isotherms M ( T C , H ). The critical exponent values for the undoped compound were found to match well with those predicted for the three-dimensional Heisenberg model ( β =355±0.007, γ =1.326±0.002, δ =4.90±0.01). While on non-magnetic Ga substitution it tends towards mean-field with long-range interaction. The mean-field model might be due to the random dilution of the Mn sublattice by non-magnetic ion Ga 3+ and/or the development of the physical size of the clusters which enhance the dipole–dipole interaction.

The critical behavior associated with the magnetic phase transition has been investigated by magnetization isotherms in La 0.8 Ca 0.2 MnO 3 and La 0.8 Ca 0.1 □ 0.1 MnO 3 (□ is the calcium deficiency). The critical exponents are estimated by various techniques such as the Modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique. The critical exponents values for La 0.8 Ca 0.2 MnO 3 are very close to those found out by the 3D-Ising model ( β =0.328, γ =1.180, and δ =4.826 at an average T C =181.676 K). Moreover, the estimated critical exponents of La 0.8 Ca 0.1 □ 0.1 MnO 3 are consistent with the prediction of the 3D-Heisenberg model ( β =0.357, γ =1.167, and δ =4.802 at an average T C =178.182 K). We noted that the critical exponents γ are almost similar to the value of the mean-field theory which can be explained by the existence of a long-range dipole–dipole interaction. Following the Harris criterion, we deduced that the disorder in our case is relevant, which can be the cause of the change in the universality class.

The pyrochlore was synthesized via a ceramic method using two different oxides (Er2O3 and TiO2). The compound was found to crystallize in the cubic system with the space group (No. 227). A magnetic study, carried out at 2 and 300 K under an applied magnetic field mu (0) H=0.05 T, has revealed a complex magnetic structure at low temperature. The effective paramagnetic moment , deduced from (chi-chi (0))(-1)=f(T) curve, was found by assuming a zero moment on the transition metal atom Ti4+. The paramagnetic Curie-Weiss temperature theta (CW)=-21.54 K, the nearest neighbor interaction J (nn) =-2.30 K, the classical nearest neighbor J (cl)=-8.65 K and the dipolar D (nn) =3.76 K interactions' values have revealed an antiferromagnetic behavior for Er2Ti2O7 compound at low temperature. We have also studied the effects of the magnetic field splitting of rare-earth atom Er3+ in the compound Er2Ti2O7 curved Arrott plots.

Critical behavior in La0.7Pb0.05Na0.25MnO3 has been investigated by dc magnetization measurements. Magnetic data indicate that the compound exhibits a continuous (second-order) paramagnetic (PM) to ferromagnetic (FM) phase transition. Estimates of critical exponents yield δ = 4.80 ± 0.01 , γ = 1.296 ± 0.002 and β = 0.344 ± 0.007 (consistent with both the predictions for the three-dimensional-Heisenberg model and with those reported for materials when the FM transition is ascribed to the double exchange (DE) mechanism as a major origin) with T C = 334.54 ± 0.08 . The critical exponent γ is slightly inferior than predicted from the 3D Heisenberg model. Such a difference may be due, within the context of the quenched disorder, to the presence of some alterations of short-range magnetic order of FM clusters in the PM phase. The temperature variation in the effective exponent ( γ e f f ) is similar to those for disordered ferromagnets.

Geometric computations can fail because of inconsistencies due to floating-point inaccuracy. For instance, the computed intersection point between two curves does not lie on the curves: it is unavoidable when the intersection point coordinates are non rational, and thus not representable using floating-point arithmetic. A popular heuristic approach tests equalities and nullities up to a tolerance e. But transitivity of equality is lost: we can have A approx B and B approx C, but A not approx C (where A approx B means ||A - B|| < e for A,B two floating-point values). Interval arithmetic is another, self-validated, alternative; the difficulty is to limit the swell of the width of intervals with computations. Unfortunately interval arithmetic cannot decide equality nor nullity, even in cases where it is decidable by other means. A new approach, developed in this paper, consists in modifying the geometric problems and algorithms, to account for the undecidability of the equality test and unavoidable inaccuracy. In particular, all curves come with a non-zero thickness, so two curves (generically) cut in a region with non-zero area, an inner and outer representation of which is computable. This last approach no more assumes that an equality or nullity test is available. The question which arises is: which geometric problems can still be solved with this last approach, and which cannot? This paper begins with the description of some cases where every known arithmetic fails in practice. Then, for each arithmetic, some properties of the problems they can solve are given. We end this work by proposing the bases of a new approach which aims to fulfill the geometric computations requirements.

We have prepared new perovskite manganites La 0.7 Ca 0.3− x K x MnO 3 (0⩽ x ⩽0.15). These compounds are shown to undergo structural transition from orthorhombic to rhombohedral phase. When increasing x , the structural properties reveal the existence of a stronger disorder, for the orthorhombic structure, which leads to an increase of the Mn–O–Mn bond angle and a decrease of the Mn–O bond length. This behaviour contributes to the increase of the one-electron bandwidth W , which results in an increase of the Curie temperature T C . For the rhombohedral structure, the disorder is suppressed, so W and T C decrease. The Raman studies at 300 K indicate that the spectra of La 0.7 Ca 0.3− x K x MnO 3 are modified when x increases. The strong x dependence of the phonon linewidth suggests that considerable lattice disorder is already present which confirms the structural transition observed by X-ray diffraction patterns.

International audience; During both hot and cold seasons, masonry walls play an important role in the thermal performance between the interior and the exterior of occupied spaces. It is thus essential to analyze the thermal behavior at the hollow block’s level in order to better understand the temperature and heat flux distribution in its structure and potentially limit as much as possible the heat transfer through the block.In this scope, this paper offers an experimental and numerical in-depth analysis of heat transfer phenomena inside a hollow block using a dedicated experimental setup including a well-insulated reference box and several thermocouples and fluxmeters distributed at the boundaries and inside the hollow block. The block was then numerically 3D modelled and simulated using COMSOL Multiphysics under the same conditions, properties, and dimensions as the experimentally tested block. The comparison between the numerical and experimental results provides very satisfactory results with relative difference of less than 4% for the computed thermal resistance.

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