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1,267 results on '"Dhahri E"'

525. Influence of Ca-deficiency on the magneto-transport properties in La0.8Ca0.2MnO3 perovskite and estimation of magnetic entropy change.

Author

Khlifi, M., Bejar, M., Dhahri, E., Lachkar, P., and Hlil, E. K.

Subjects
PEROVSKITE, OXIDE minerals, SCANNING electron microscopy, ELECTRON microscopy, ENTROPY
Abstract

La0.8Ca0.2 - x[larger_open_square]xMnO3 (x = 0.00, 0.10, and 0.20) perovskite was prepared by the conventional solid-state reaction and annealed at 1473 K. X-ray diffraction and scanning electron microscopy shown the existence of a secondary phase attributed to the unreacted Mn3O4 oxide. The magneto transport properties have been investigated based on the temperature dependence of the resistivity ρ(T) measurements under several applied magnetic fields. We note that the La0.8Ca0.2MnO3 (x = 0.00) sample has a classical metal-insulator transition at Tρ. But we have observed that the lacunars samples (x = 0.10 and 0.20) include a metallic and insulator behavior simultaneously below Tρ and the resistivity is dominated by tunneling through the barriers associated with the insulating phase. In other words, the calcium deficiency favors the enhancement of the insulator behavior. 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. Furthermore, we found that the estimated results are in good agreement with experimental data. Above all, the resistivity dependence on the temperature and magnetic field data is used to deduce the magnetic entropy change. We have found that these magnetic entropy change values are similar to those calculated in our previous work from the magnetic measurements. Finally, we have found an excellent estimation of the magnetic entropy change based on the Landau theory. [ABSTRACT FROM AUTHOR]

Published
2012
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526. Optical and theoretical study of NaCr(P2O7): a look through the Neuhauser model and Racah theory.

Author

Souissi, H., Kammoun, S., Dhahri, E., and López-Lago, E.

Subjects
*POTENTIAL energy surfaces, *ENERGY levels (Quantum mechanics), *MODEL theory, *ABSORPTION spectra, *LIGHT absorption, *SPIN-orbit interactions
Abstract

A sample of NaCr(P2O7) was synthesized using the solid-state reaction method. X-ray diffraction and Rietveld refinement confirmed the formation of a monoclinic structure with the P21/a space group. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis identified the morphology and homogeneity of the chemical composition. The optical absorption spectrum revealed a direct optical band gap of 2.9 eV and an Urbach energy of 0.44 eV. Notably, the absorption spectrum showed an interference dip on the lowest energy band 4T2g(4F), attributed to spin--orbit coupling between states ²Eg(²G) and 4T2g(4F). This phenomenon was analyzed using the Neuhauser model based on coupled potential energy surfaces. From this analysis, the electronic structure of Cr3+ (3d3) ions in the NaCr(P2O7) sample was determined, enabling a reliable calculation of Racah and crystal-field parameters. The results showed good agreement between experimental and theoretical energy levels. The study demonstrates the impact of spin--orbit interactions using the coupled potential energy surface model. [ABSTRACT FROM AUTHOR]

Published
2024
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550. Enhancement of the Magnetotransport Behavior in a Phase-Separated LaAgCaMnO3Polycrystalline: Unraveling the Role of a Multi-Double-Exchange Mechanism

Author

Smari, M., Hamdi, R., Slimani, S., Bajorek, A., Peddis, D., Walha, I., Koneva, U., Dhahri, E., and Haik, Y.

Abstract

Manganese-based perovskite oxides have attracted much attention since the phenomenon of colossal magnetoresistance (CMR) was revived, where the magnetoresistance (MR) can attend higher orders of magnitude larger than that typically found in other types of materials. While trying to understand the mechanism behind CMR, morphostructural, magnetoelectrical, and theoretical studies have been performed on the La0.4Ag0.2Ca0.4MnO3(LACMO) sample. An orthorhombic-shaped single-phase LACMO sample with size distribution DSEM= 2.5 μm was synthesized using a conventional sol–gel chemical method. The mixed valence states of different chemical elements were revealed by X-ray photoemission spectroscopy. The results of thermal dependence of magnetization were obtained according to the zero field cooled–field cooled cooling–field cooled warming protocols using a superconducting quantum interference device magnetometer. The phase separation (PS) phenomenon was observed because of the competition between the superexchange and the double-exchange (DE) mechanisms within the same material. Field dependence of magnetization at varying temperatures and in cooling–warming processes has also been reported and discussed in this paper. The MR value was found to be 99% under an applied field of 2 T. In essence, the reported CMR arises from the metal–insulator phase transition Tρ= 64 K, which accompanies the transition from an insulator state to a metallic state. Not only the large CMR value was noted in our sample, but also the high obtained value of the temperature coefficient of resistivity of 69% at 123 K in zero field has also been reported. The obtained CMR values are the result of PS and multi-DE phenomenon. The latter proved to be directly correlated with the improved magnetotransport properties because an additional hopping mechanism is provided by an interstitially filled Mn2+ion, which is electronically involved in the conduction mechanism, showing an unconventional behavior in the transport properties of manganite.

Published
2020
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