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Polycrystalline compounds La0.8Na0.2-x...xMnO3 were prepared by the solid-state reaction with 0.00=x=0.15. Structural, magnetic and electrical measurements were investigated. The XRD data have been analyzed by Rietveld refinement technique which reveals that all samples are crystallized in a rhombohedral structure with R...c space group. Magnetic measurement versus temperature shows that all samples exhibit a magnetic transition from ferromagnetic (FM) to paramagnetic (PM) phase when increasing temperature. The Curie temperature (TC) decrease from 340K for x=0.00 samples to 260K for x=0.15 one. Hysteresis cycles confirm the ferromagnetic character at low temperature with a decrease of the remanent magnetization and the coercive field when the vacancy rate increases. Moreover, the temperature dependence of electrical resistivity shows a metal-insulator transition at Tρ for all samples. In addition, Tρ decreases with vacancy content in accordance with TC. Thus, the conduction mechanism was explained by the adiabatic small polaron hopping (ASPH) in the insulating region and by the competition between the small-polaron and spin-wave scattering and the electron-magnon scattering mechanisms. Finally, the minimum of resistivity at very low temperature range is explained by the Kondo-like scattering model. [ABSTRACT FROM AUTHOR]

Abstract: La0.8−x □ x Ca0.2MnO3 (x=0, 0.1 and 0.20) samples were prepared by the conventional solid-state reaction and annealed at 1200K. X-ray diffraction and scanning electron microscopy were used to check the crystalline structure and the samples morphology. Resistivity measurements versus temperature under different magnetic fields are performed to investigate the electronic transport as well as the magnetoresistance properties. A classical metal–insulator transition at T MI is observed for all samples. As other results, the increase of the lanthanum deficiency content induces the decrease of the resistivity values. In addition, a resistivity minimum has been observed for all samples in the range of temperatures from 5 to 50K, which is strongly influenced by an applied magnetic field. Therefore, the electrical resistivity is fitted with the phenomenological percolation model which is based on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions. Magnetoresistance measurements show high values which reach 76% for x=0.1 sample. [Copyright &y& Elsevier]

Highlights: [•] Structural transition from orthorhombic to rhombohedral structure in La0.8Ca0.2−x□ x MnO3 compounds. [•] Enhancement of TC for the rhombohedral samples. [•] A large magnetocaloric effect which give the possibility of application in the magnetic refrigeration area. [•] High values of the magnetoresistance in all samples. [ABSTRACT FROM AUTHOR]

Abstract: The critical behavior associated with the magnetic phase transition has been investigated by magnetization isotherms in La0.8Ca0.2MnO3 and La0.8Ca0.1□0.1MnO3 (□ 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 La0.8Ca0.2MnO3 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 =181.676K). Moreover, the estimated critical exponents of La0.8Ca0.1□0.1MnO3 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 =178.182K). 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. [Copyright &y& Elsevier]

Abstract: The La0.8Ca0.2−x □ x MnO3, (x =0–0.20) compounds were prepared by the solid-state reaction. X-ray diffraction (XRD) and magnetic measurements were used to investigate the calcium-vacancy effect on the physical properties. The XRD data have been analyzed by Rietveld refinement technique. Samples with x =0, 0.05 and 0.10 are found to be single phase and crystallize in orthorhombic symmetry with Pnma space group. Moreover, for x =0.15 and 0.20 samples, the refinement has revealed the coexistence of both Pnma orthorhombic and rhombohedral phases. The magnetic study has showed that the magnetization exhibits maximum at x =0.15 and minimum at x =0.2. Such behavior was interpreted in terms of orthorhombic distortion contribution, which is related to the orthorhombic structure, leading to magnetization decrease. In addition, maximum magnetic entropy change (ΔS M) for x =0 sample was found to reach ∼4.1J/kgK under an applied magnetic field of 5T, which is very sufficient for potential application in magnetic refrigeration. For the same applied magnetic field (μ 0 H =5T), the Relative Cooling Power (RCP) values are found to vary between 270 and 300J/kg. As consequence, the La0.8Ca0.2−x □ x MnO3compounds can be used as a composite for magnetic refrigeration in the [175–183] K temperature range. [Copyright &y& Elsevier]

The electrical and magnetoresistance behavior of L a 0.8 N a 0.2 − x □ x M n O 3 (0.00≤ x ≤0.15) polycrystalline sample was investigated. Resistivity measurements versus temperature under different magnetic fields were performed to investigate the electronic transport and magnetoresistance properties. The abnormal transport properties were induced by the sodium deficiency content, characterized by the double metal–insulator transitions ( T P 1 and T P 2 ). The present results have been discussed and possible explanations given based on the related theory and previous reported results. The conduction mechanism was explained by the adiabatic small polaron hopping (ASPH) in the insulating region and by the competition between the small-polaron and spin-wave scattering in the metallic region. Moreover, a resistivity minimum has been observed for all samples in the temperature range from 2 to 70 K, which is strongly influenced by an applied magnetic field explained by a Kondo-like scattering mechanism. Therefore, the percolation theory is introduced to understand the transport mechanism in the whole temperature range. Hence, the estimated values of the resistivity was found to be in good agreement with the experimental data. Magnetoresistance measurements show high values reaching 60% for x =0.15 sample. [ABSTRACT FROM AUTHOR]

Abstract: Lanthanum-deficiency effect on the magnetic and magnetocaloric properties of La0.8−x □ x Ca0.2MnO3 (0.00≤x≤0.20) polycrystalline samples has been investigated using X-ray diffraction and magnetization measurements. The structural characterization has revealed that all samples are crystallized in the orthorhombic structure with Pnma space group. From magnetic measurement a first-order magnetic phase transition from ferromagnetic to paramagnetic state is observed at the Curie temperature (T C) which is found to increase from 236.5 to 247K when lanthanum-deficiency rate increases. Besides, both magnetocaloric effect (MCE) and relative cooling power (RCP) has been estimated. As important result, the values of MCE and RCP in our managanite with Lanthanum-deficiency are reported to be near to those found in the gadolinium considered as magnetocaloric material reference. Finally, the first-order magnetic phase transition has been confirmed by the construction of the universal curve of the magnetic entropy change. [Copyright &y& Elsevier]

The effect of strontium doping on the structural, magnetic and magnetocaloric properties of La 0.67 Ca 0.33- x Sr x Mn 0.98 Fe 0.02 O 3 samples for (x = 0.15; 0.2.and 0.3) has been investigated. Analysis using X-ray diffraction shows there is an orthorhombic structure with Pnma space group symmetry for x = 0.15 compound, whereas there is a rhombohedral structure exhibiting R 3 ‾ c space group symmetry found for x = 0.2, and x = 0.3 compounds. A second-order magnetic phase transition from ferromagnetic to paramagnetic state transition is evidenced for all samples, with Curie temperature (T C) values equal to 329 K, 330 K and 350 K and that respectively for x = 0.15, 0.2 and 0.3. Based on isothermal magnetization measurements versus applied magnetic field at several temperatures around T C , the magnetic entropy change was calculated, which is very close to that approximated by the Landau theory for. x = 0.15. Under an applied magnetic field of 5 T, one can see the maximum magnetic entropy change (− Δ S M m a x) values, which are around 4.89, 4.71 and 3.35 J.kg− 1.K−1 for x = 0.15, 0.2, and 0.3 respectively. It confirms that close to room temperature, these manganites are admirably potential functional materials, which can be applied as solid state magnetic refrigerants. [Display omitted] • Structural and magnetic properties have been studied in detail. • A large magnetocaloric effect is observed just above room temperature. • High values of Δ S M and relative cooling power are reported. • Second-order of ferromagnetic phase transition is confirmed. [ABSTRACT FROM AUTHOR]