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Books by Moez Bejar

Research paper thumbnail of Dielectric relaxation of the Ca 2 MnO 4− system

Dielectric properties were measured in the frequency range from 40 Hz to 2 MHz and temperatures b... more Dielectric properties were measured in the frequency range from 40 Hz to 2 MHz and temperatures between 100 and 360 K using impedance spectroscopy technique. A single relaxation process was observed for ı = 0.00, but a second relaxation behavior can be identified for intermediate values of ı. For both processes the Cole–Cole relaxation model can be used to explain the results, corresponding to a distribution of relaxation times.

Papers by Moez Bejar

Research paper thumbnail of Rehabilitation Strategies after Spinal Cord Injury: Inquiry into the Mechanisms of Success and Failure

Journal of Neurotrauma, 2017

Research paper thumbnail of Effect of the oxygen deficiency on the physical properties of Ca2MnO4−δ compounds

Journal of Alloys and Compounds, 2011

ABSTRACT In this work, structural and magnetic studies have been investigated to explore the infl... more ABSTRACT In this work, structural and magnetic studies have been investigated to explore the influence of the oxygen-deficiency (δ) on the physical properties of Ca2MnO4−δ (δ=0 to 0.5) compounds. The samples cationic stoichiometry has been studied using energy dispersive X-ray analysis (EDAX). EDAX spectra, taken from the surface of the synthesized powders, have showed a nominal composition near the desired one. The structural study, by X-ray diffraction analysis, has confirmed the single-phase composition of all samples. The Rietveld refinement technique of the X-ray patterns, has revealed that samples crystallize in the tetragonal system with I41/acd space group for δ=0 and in the orthorhombic one with Bbcm space group for δ>0.The applied magnetic field (μ0H) and the temperature (T) dependences of the magnetization (M) have been studied using a vibrating sample magnetometer (VSM). From magnetization measurements versus temperature under the field-cooled (FC) and zero-field-cooled (ZFC) modes, we have deduced the presence of a spin-glass behavior characterized by a distinctive separation of the FC and ZFC curves. This behavior was found to become more important, as increasing the oxygen-deficiency δ, which leads to the appearance of magnetic frustration phenomenon.

Research paper thumbnail of Ab initio LSDA+U Study of Optical Properties of RVO4 (R = Eu, Ho, Lu) Compounds

Materials Research

A first principles investigation at the pressure 7 GPa of the optical properties of RVO 4 (R= Eu,... more A first principles investigation at the pressure 7 GPa of the optical properties of RVO 4 (R= Eu, Ho, Lu) orthovanadates in the framework of the density functional theory (DFT) using the linearizedaugmented plane-wave method is reported in order to predict new optical materials fo r continuouswave lasers. The electronic structure of all orthovanadates is studied in zircon-type structure. DFT+U (Hubbard parameter found to be 8eV) calculations predict an antiferromagnetic and nonmagnetic insulating ground states-at ambient conditions-for (EuVO 4 , HoVO 4) and LuVO 4 , respectively. The results show that these vanadates can be good candidates for laser-host materials, and indicate the possibility of material design to optimize the laser-host materials. The rare-earth ion-doped crystals could enhance the laser performances and improve the isolation characteristic of the optical isolators.

Research paper thumbnail of Mössbauer and magnetic studies of (La0.8Ca0.2)1-xBixFeO3 perovskites

Research paper thumbnail of Theoretical investigation of the magnetocaloric effect on La0.7(Ba, Sr)0.3Mn0.9Ga0.1O3 compound at room temperature

Journal of Magnetism and Magnetic Materials

ABSTRACT Based on a phenomenological model, the magnetic and magnetocaloric properties of La0.7(B... more ABSTRACT Based on a phenomenological model, the magnetic and magnetocaloric properties of La0.7(Ba, Sr)0.3Mn0.9Ga0.1O3 oxide have been studied. Indeed, the magnetic measurements have demonstrated that the sample exhibits a ferromagnetic-paramagnetic transition at room temperature. The value of the magnetocaloric effect such as magnetic entropy change, full width at half-maximum, relative cooling power and magnetic specific heat change has been determined from the calculation of magnetization as a function of temperature under different external magnetic fields. The maximum magnetic entropy change ( ) and the relative cooling power (RCP) are, respectively, 0.57 J kg−1 K−1 and 28.68 J kg−1 for a 10 kOe field change at 300 K, which are the characteristics of a good magnetocaloric material. Hence, the La0.7(Ba, Sr)0.3Mn0.9Ga0.1O3 compound can be considered as a promising material in magnetic refrigeration technology.

Research paper thumbnail of Prediction of magnetocaloric effect in La0.6Ca0.4−xSrxMnO3 compounds for x=0, 0.05 and 0.4 with phenomenological model

Ceramics International

ABSTRACT

Research paper thumbnail of Physical properties and ethanol sensing of perovskite La0.8Pb0.2Fe1−xMgxO3 compounds

Journal of Alloys and Compounds

ABSTRACT The nanocrystalline La0.8Pb0.2Fe1-xMgxO3 (x = 0.00, 0.05, 0.10 and 0.15) powders were pr... more ABSTRACT The nanocrystalline La0.8Pb0.2Fe1-xMgxO3 (x = 0.00, 0.05, 0.10 and 0.15) powders were prepared by sol-gel method using the citric acid route. The compounds were investigated with regard to their composition and microstructure, using XRD, Raman spectroscopy and FTIR methods. These compounds were found to exhibit an orthorhombic symmetry with the Pnma space group. The substitution of the Fe ion by the Mg one was found to increase the resistance of these compounds, where the resistance depended on both electrovalence compensation and oxygen vacancy compensation. The La0.8Pb0.2Fe1-xMgxO3 sensors showed p-type semiconducting properties within the temperature range of 100 - 400 °C. The electric results revealed that x = 0.15 sample presents high sensitivity and good selectivity to ethanol gas.

Research paper thumbnail of Theoretical investigation of the magnetocaloric effect of La0.7(Ba, sr)0.3MnO3 compound at room temperature with a second-order magnetic phase transition

Ceramics International

ABSTRACT In this paper, the prediction of magnetic and magnetocaloric properties of La0.7(Sr,Ba)0... more ABSTRACT 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.

Research paper thumbnail of Magnetic, Electrical Properties and Spin-Glass Effect of Substitution of Ca for Pr in Ca2-xPrxMnO4 Compounds

The Open Surface Science Journal

Ca 2-x Pr x MnO 4 (x = 0.000 to 0.200) manganites were synthesized by a sol-gel method and then c... more Ca 2-x Pr x MnO 4 (x = 0.000 to 0.200) manganites were synthesized by a sol-gel method and then characterized by X-ray diffraction (XRD) at room temperature. Rietveld refinement technique shows that all samples can be indexed in orthorhombic system except for Ca 2 MnO 4 (x = 0.000) which can be indexed in the tetragonal system. Magnetic and electrical measurements show that magnetization and resistivity have a large dependence of the fractional composition x and temperature. The applied magnetic field has also a large influence on magnetization level. Spin-glass behavior was detected at low temperature for the samples with x 0.175. For all x-values, all compounds exhibit semiconductor behavior and the ferromagnetic (FM) transition temperature T C (Curie temperature) is around 105 K, whatever the applied field.

Research paper thumbnail of Cartographie de la vitesse de recombinaison en surface et de la durée de vie des porteurs dans les semi-conducteurs III-V et le silicium par imagerie de photoluminescence à température ambiante

Research paper thumbnail of Study of critical behavior of perovskite La0.8Ca0.2−xPbxFeO3 (x=0.0, 0.1 and 0.2) compounds

Journal of Alloys and Compounds, 2015

ABSTRACT The critical properties of ferrite perovskite La0.8Ca0.2-xPbxFeO3 (x = 0.0, 0.1 and 0.2)... more ABSTRACT The critical properties of ferrite perovskite La0.8Ca0.2-xPbxFeO3 (x = 0.0, 0.1 and 0.2) compounds at the ferromagnetic–paramagnetic (FM-PM) 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 (TC, H). The critical exponent values for all samples are in a close agreement with those predicted for the mean-field theory. The reason for this behavior may be long-range interactions between spins in this system.

Research paper thumbnail of The effect of the annealing temperature on the structural and magnetic properties of the manganites compounds

Journal of Alloys and Compounds

Research paper thumbnail of Effect of Ga-doping on the Magnetic and Magnetocaloric Properties of (LaCaSr)(MnGa)O 3 Compound

Research paper thumbnail of Effect Ga-doping on the Magneticaloric Properties of (LaSrGa)(MnGa)O3 compound

Research paper thumbnail of Magnetic, Raman and Mössbauer properties of double-doping LaFeO3 perovskite oxides

Materials Chemistry and Physics, 2015

ABSTRACT The La0.8Ca0.2-xPbxFeO3 (x = 0.00, 0.05, 0.10, 0.15 and 0.20) compounds were prepared by... more ABSTRACT The La0.8Ca0.2-xPbxFeO3 (x = 0.00, 0.05, 0.10, 0.15 and 0.20) compounds were prepared by the sol-gel method using the citric acid route. The structural study revealed that all samples crystallized in the Pnma orthorhombic structure with the apparition of Ca2Fe2O5 and Fe3O4 secondary phases for samples with x >= 0.05, confirmed by the Raman spectroscopy study. The fitted Mossbauer spectra exposed, for x = 0.00 sample, the presence of a one sextuplet, related to the Fe3+ ion in the tetrahedral site, and a doublet. However, for x > 0.00 samples, the fit results showed the apparition of other sextuplets related to the Fe3O4 phase. The percentage of this latest phase was found to increase and to reach a maximum for the x = 0.10 sample and to decrease after for x = 0.15 and 0.20 samples. The variation of the magnetization (M) as a function of the temperature (T), under an applied magnetic field of 0.05 T, showed the presence of a ferromagnetic-paramagnetic transition. The magnetic study exposed that the magnetization decreases first for x <= 0.10 samples and then increases for the two other samples. This behavior was related to the competition between the Fe3+-Fe3+ tetrahedral ferromagnetic interactions and the octahedral antiferromagnetic ones between Fe3+ and Fe2+ ions.

Research paper thumbnail of Blue-green photoluminescence in BaZrO3−δ powders

Chemical Physics Letters, 2014

ABSTRACT BaZrO3-delta (delta = 0.0, 0.1 and 0.2) powders were prepared by the sal-gel process. XR... more ABSTRACT BaZrO3-delta (delta = 0.0, 0.1 and 0.2) powders were prepared by the sal-gel process. XRD patterns revealed that all samples present a major cubic structure as well as a ZrO2 secondary phase for delta = 0.1 and 0.2 samples, which is confirmed by the Raman study. Besides, blue-green photoluminescence (PL) emission at room temperature was observed. This emission is directly related to the decrease of singly ionized Vo(center dot) and the increase of the doubly ionized Vo(center dot) clusters, leading to the formation of intermediary energy levels in the band gap where an electron and a hole can recombine in the structure.

Research paper thumbnail of Shine red and yellow photoluminescence in GdAlO3-δ powders

Journal of Alloys and Compounds, 2015

ABSTRACT GdAlO3-δ (δ = 0.0, 0.1 and 0.2) compounds were prepared by the conventional solid-state ... more ABSTRACT GdAlO3-δ (δ = 0.0, 0.1 and 0.2) compounds were prepared by the conventional solid-state method. XRD patterns revealed that all samples present a major orthorhombic structure. An intense red and a yellow photoluminescence (PL) emissions were observed at room temperature for δ = 0.0 and δ ≠ 0.0 samples, respectively. The red emission was related to the singly ionized oxygen vacancies VO•. The production of doubly ionized oxygen vacancies VO•• observed directly from δ = 0.1 and 0.2 samples. The presence of singly and doubly ionized clusters was found to play an important role in the formation of hole-electron pairs and to give rise to the PL emission. The presence of singly ionized oxygen vacancies was confirmed by the EPR study.

Research paper thumbnail of Effect of the substitution of calcium by potassium on the dielectric properties in La0.7Ca0.3-xKxMnO3 compounds

ICTON-MW 2008 - International Conference on Transparent Optical Networks "Mediterranean Winter" 2008 - Conference Proceedings, 2008

... de Physique Appliquée, Faculté des Sciences de Sfax, BP 1171, 3000 Sfax, Tunisia 2Physics Dep... more ... de Physique Appliquée, Faculté des Sciences de Sfax, BP 1171, 3000 Sfax, Tunisia 2Physics Department (UA), Campus Universitario, Aveiro 3810-193, Portugal Laboratoire de cristallographie CNRS, Institut Néel, Département MCMF BP. ... [11] S. Brahma, RNP Choudhary and ...

Research paper thumbnail of Dielectric spectroscopy of Ca2MnO4-δ ceramics using equivalent circuit analysis

Physica Status Solidi (C) Current Topics in Solid State Physics, 2013

ABSTRACT In the last years a growing interest in the study of layered perovskites which acquire a... more ABSTRACT In the last years a growing interest in the study of layered perovskites which acquire a wide variety of interesting and useful physical properties such as colossal magnetoresistance, superconductivity, ferroelectricity is quite visible. From those properties, the magnetocaloric effect (MCE) offers an interesting alternative technology for refrigeration. On other hand, their possible use in spintronic devices has also attracted much attention.In this work, the effect of the creation of vacancies in oxygen sites on the structure of Ca2MnO4-δ Ruddlesden-Popper (RP) compounds, prepared by sol-gel method, was studied mainly through their structural, morphologic and dielectric properties. The cationic stoichiometry of the samples was studied using energy dispersive X-ray analysis (EDAX). EDAX spectra showed a nominal composition close to the desired one. The structural study, by X-ray diffraction analysis, confirmed the single-phase composition of all the samples. The dielectric studies, the temperature range between 80 K and 360 K, showed dielectric relaxation phenomena, which were related to the Mn4+/Mn3+ ratio. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Research paper thumbnail of Dielectric relaxation of the Ca 2 MnO 4− system

Dielectric properties were measured in the frequency range from 40 Hz to 2 MHz and temperatures b... more Dielectric properties were measured in the frequency range from 40 Hz to 2 MHz and temperatures between 100 and 360 K using impedance spectroscopy technique. A single relaxation process was observed for ı = 0.00, but a second relaxation behavior can be identified for intermediate values of ı. For both processes the Cole–Cole relaxation model can be used to explain the results, corresponding to a distribution of relaxation times.

Research paper thumbnail of Rehabilitation Strategies after Spinal Cord Injury: Inquiry into the Mechanisms of Success and Failure

Journal of Neurotrauma, 2017

Research paper thumbnail of Effect of the oxygen deficiency on the physical properties of Ca2MnO4−δ compounds

Journal of Alloys and Compounds, 2011

ABSTRACT In this work, structural and magnetic studies have been investigated to explore the infl... more ABSTRACT In this work, structural and magnetic studies have been investigated to explore the influence of the oxygen-deficiency (δ) on the physical properties of Ca2MnO4−δ (δ=0 to 0.5) compounds. The samples cationic stoichiometry has been studied using energy dispersive X-ray analysis (EDAX). EDAX spectra, taken from the surface of the synthesized powders, have showed a nominal composition near the desired one. The structural study, by X-ray diffraction analysis, has confirmed the single-phase composition of all samples. The Rietveld refinement technique of the X-ray patterns, has revealed that samples crystallize in the tetragonal system with I41/acd space group for δ=0 and in the orthorhombic one with Bbcm space group for δ>0.The applied magnetic field (μ0H) and the temperature (T) dependences of the magnetization (M) have been studied using a vibrating sample magnetometer (VSM). From magnetization measurements versus temperature under the field-cooled (FC) and zero-field-cooled (ZFC) modes, we have deduced the presence of a spin-glass behavior characterized by a distinctive separation of the FC and ZFC curves. This behavior was found to become more important, as increasing the oxygen-deficiency δ, which leads to the appearance of magnetic frustration phenomenon.

Research paper thumbnail of Ab initio LSDA+U Study of Optical Properties of RVO4 (R = Eu, Ho, Lu) Compounds

Materials Research

A first principles investigation at the pressure 7 GPa of the optical properties of RVO 4 (R= Eu,... more A first principles investigation at the pressure 7 GPa of the optical properties of RVO 4 (R= Eu, Ho, Lu) orthovanadates in the framework of the density functional theory (DFT) using the linearizedaugmented plane-wave method is reported in order to predict new optical materials fo r continuouswave lasers. The electronic structure of all orthovanadates is studied in zircon-type structure. DFT+U (Hubbard parameter found to be 8eV) calculations predict an antiferromagnetic and nonmagnetic insulating ground states-at ambient conditions-for (EuVO 4 , HoVO 4) and LuVO 4 , respectively. The results show that these vanadates can be good candidates for laser-host materials, and indicate the possibility of material design to optimize the laser-host materials. The rare-earth ion-doped crystals could enhance the laser performances and improve the isolation characteristic of the optical isolators.

Research paper thumbnail of Mössbauer and magnetic studies of (La0.8Ca0.2)1-xBixFeO3 perovskites

Research paper thumbnail of Theoretical investigation of the magnetocaloric effect on La0.7(Ba, Sr)0.3Mn0.9Ga0.1O3 compound at room temperature

Journal of Magnetism and Magnetic Materials

ABSTRACT Based on a phenomenological model, the magnetic and magnetocaloric properties of La0.7(B... more ABSTRACT Based on a phenomenological model, the magnetic and magnetocaloric properties of La0.7(Ba, Sr)0.3Mn0.9Ga0.1O3 oxide have been studied. Indeed, the magnetic measurements have demonstrated that the sample exhibits a ferromagnetic-paramagnetic transition at room temperature. The value of the magnetocaloric effect such as magnetic entropy change, full width at half-maximum, relative cooling power and magnetic specific heat change has been determined from the calculation of magnetization as a function of temperature under different external magnetic fields. The maximum magnetic entropy change ( ) and the relative cooling power (RCP) are, respectively, 0.57 J kg−1 K−1 and 28.68 J kg−1 for a 10 kOe field change at 300 K, which are the characteristics of a good magnetocaloric material. Hence, the La0.7(Ba, Sr)0.3Mn0.9Ga0.1O3 compound can be considered as a promising material in magnetic refrigeration technology.

Research paper thumbnail of Prediction of magnetocaloric effect in La0.6Ca0.4−xSrxMnO3 compounds for x=0, 0.05 and 0.4 with phenomenological model

Ceramics International

ABSTRACT

Research paper thumbnail of Physical properties and ethanol sensing of perovskite La0.8Pb0.2Fe1−xMgxO3 compounds

Journal of Alloys and Compounds

ABSTRACT The nanocrystalline La0.8Pb0.2Fe1-xMgxO3 (x = 0.00, 0.05, 0.10 and 0.15) powders were pr... more ABSTRACT The nanocrystalline La0.8Pb0.2Fe1-xMgxO3 (x = 0.00, 0.05, 0.10 and 0.15) powders were prepared by sol-gel method using the citric acid route. The compounds were investigated with regard to their composition and microstructure, using XRD, Raman spectroscopy and FTIR methods. These compounds were found to exhibit an orthorhombic symmetry with the Pnma space group. The substitution of the Fe ion by the Mg one was found to increase the resistance of these compounds, where the resistance depended on both electrovalence compensation and oxygen vacancy compensation. The La0.8Pb0.2Fe1-xMgxO3 sensors showed p-type semiconducting properties within the temperature range of 100 - 400 °C. The electric results revealed that x = 0.15 sample presents high sensitivity and good selectivity to ethanol gas.

Research paper thumbnail of Theoretical investigation of the magnetocaloric effect of La0.7(Ba, sr)0.3MnO3 compound at room temperature with a second-order magnetic phase transition

Ceramics International

ABSTRACT In this paper, the prediction of magnetic and magnetocaloric properties of La0.7(Sr,Ba)0... more ABSTRACT 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.

Research paper thumbnail of Magnetic, Electrical Properties and Spin-Glass Effect of Substitution of Ca for Pr in Ca2-xPrxMnO4 Compounds

The Open Surface Science Journal

Ca 2-x Pr x MnO 4 (x = 0.000 to 0.200) manganites were synthesized by a sol-gel method and then c... more Ca 2-x Pr x MnO 4 (x = 0.000 to 0.200) manganites were synthesized by a sol-gel method and then characterized by X-ray diffraction (XRD) at room temperature. Rietveld refinement technique shows that all samples can be indexed in orthorhombic system except for Ca 2 MnO 4 (x = 0.000) which can be indexed in the tetragonal system. Magnetic and electrical measurements show that magnetization and resistivity have a large dependence of the fractional composition x and temperature. The applied magnetic field has also a large influence on magnetization level. Spin-glass behavior was detected at low temperature for the samples with x 0.175. For all x-values, all compounds exhibit semiconductor behavior and the ferromagnetic (FM) transition temperature T C (Curie temperature) is around 105 K, whatever the applied field.

Research paper thumbnail of Cartographie de la vitesse de recombinaison en surface et de la durée de vie des porteurs dans les semi-conducteurs III-V et le silicium par imagerie de photoluminescence à température ambiante

Research paper thumbnail of Study of critical behavior of perovskite La0.8Ca0.2−xPbxFeO3 (x=0.0, 0.1 and 0.2) compounds

Journal of Alloys and Compounds, 2015

ABSTRACT The critical properties of ferrite perovskite La0.8Ca0.2-xPbxFeO3 (x = 0.0, 0.1 and 0.2)... more ABSTRACT The critical properties of ferrite perovskite La0.8Ca0.2-xPbxFeO3 (x = 0.0, 0.1 and 0.2) compounds at the ferromagnetic–paramagnetic (FM-PM) 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 (TC, H). The critical exponent values for all samples are in a close agreement with those predicted for the mean-field theory. The reason for this behavior may be long-range interactions between spins in this system.

Research paper thumbnail of The effect of the annealing temperature on the structural and magnetic properties of the manganites compounds

Journal of Alloys and Compounds

Research paper thumbnail of Effect of Ga-doping on the Magnetic and Magnetocaloric Properties of (LaCaSr)(MnGa)O 3 Compound

Research paper thumbnail of Effect Ga-doping on the Magneticaloric Properties of (LaSrGa)(MnGa)O3 compound

Research paper thumbnail of Magnetic, Raman and Mössbauer properties of double-doping LaFeO3 perovskite oxides

Materials Chemistry and Physics, 2015

ABSTRACT The La0.8Ca0.2-xPbxFeO3 (x = 0.00, 0.05, 0.10, 0.15 and 0.20) compounds were prepared by... more ABSTRACT The La0.8Ca0.2-xPbxFeO3 (x = 0.00, 0.05, 0.10, 0.15 and 0.20) compounds were prepared by the sol-gel method using the citric acid route. The structural study revealed that all samples crystallized in the Pnma orthorhombic structure with the apparition of Ca2Fe2O5 and Fe3O4 secondary phases for samples with x >= 0.05, confirmed by the Raman spectroscopy study. The fitted Mossbauer spectra exposed, for x = 0.00 sample, the presence of a one sextuplet, related to the Fe3+ ion in the tetrahedral site, and a doublet. However, for x > 0.00 samples, the fit results showed the apparition of other sextuplets related to the Fe3O4 phase. The percentage of this latest phase was found to increase and to reach a maximum for the x = 0.10 sample and to decrease after for x = 0.15 and 0.20 samples. The variation of the magnetization (M) as a function of the temperature (T), under an applied magnetic field of 0.05 T, showed the presence of a ferromagnetic-paramagnetic transition. The magnetic study exposed that the magnetization decreases first for x <= 0.10 samples and then increases for the two other samples. This behavior was related to the competition between the Fe3+-Fe3+ tetrahedral ferromagnetic interactions and the octahedral antiferromagnetic ones between Fe3+ and Fe2+ ions.

Research paper thumbnail of Blue-green photoluminescence in BaZrO3−δ powders

Chemical Physics Letters, 2014

ABSTRACT BaZrO3-delta (delta = 0.0, 0.1 and 0.2) powders were prepared by the sal-gel process. XR... more ABSTRACT BaZrO3-delta (delta = 0.0, 0.1 and 0.2) powders were prepared by the sal-gel process. XRD patterns revealed that all samples present a major cubic structure as well as a ZrO2 secondary phase for delta = 0.1 and 0.2 samples, which is confirmed by the Raman study. Besides, blue-green photoluminescence (PL) emission at room temperature was observed. This emission is directly related to the decrease of singly ionized Vo(center dot) and the increase of the doubly ionized Vo(center dot) clusters, leading to the formation of intermediary energy levels in the band gap where an electron and a hole can recombine in the structure.

Research paper thumbnail of Shine red and yellow photoluminescence in GdAlO3-δ powders

Journal of Alloys and Compounds, 2015

ABSTRACT GdAlO3-δ (δ = 0.0, 0.1 and 0.2) compounds were prepared by the conventional solid-state ... more ABSTRACT GdAlO3-δ (δ = 0.0, 0.1 and 0.2) compounds were prepared by the conventional solid-state method. XRD patterns revealed that all samples present a major orthorhombic structure. An intense red and a yellow photoluminescence (PL) emissions were observed at room temperature for δ = 0.0 and δ ≠ 0.0 samples, respectively. The red emission was related to the singly ionized oxygen vacancies VO•. The production of doubly ionized oxygen vacancies VO•• observed directly from δ = 0.1 and 0.2 samples. The presence of singly and doubly ionized clusters was found to play an important role in the formation of hole-electron pairs and to give rise to the PL emission. The presence of singly ionized oxygen vacancies was confirmed by the EPR study.

Research paper thumbnail of Effect of the substitution of calcium by potassium on the dielectric properties in La0.7Ca0.3-xKxMnO3 compounds

ICTON-MW 2008 - International Conference on Transparent Optical Networks "Mediterranean Winter" 2008 - Conference Proceedings, 2008

... de Physique Appliquée, Faculté des Sciences de Sfax, BP 1171, 3000 Sfax, Tunisia 2Physics Dep... more ... de Physique Appliquée, Faculté des Sciences de Sfax, BP 1171, 3000 Sfax, Tunisia 2Physics Department (UA), Campus Universitario, Aveiro 3810-193, Portugal Laboratoire de cristallographie CNRS, Institut Néel, Département MCMF BP. ... [11] S. Brahma, RNP Choudhary and ...

Research paper thumbnail of Dielectric spectroscopy of Ca2MnO4-δ ceramics using equivalent circuit analysis

Physica Status Solidi (C) Current Topics in Solid State Physics, 2013

ABSTRACT In the last years a growing interest in the study of layered perovskites which acquire a... more ABSTRACT In the last years a growing interest in the study of layered perovskites which acquire a wide variety of interesting and useful physical properties such as colossal magnetoresistance, superconductivity, ferroelectricity is quite visible. From those properties, the magnetocaloric effect (MCE) offers an interesting alternative technology for refrigeration. On other hand, their possible use in spintronic devices has also attracted much attention.In this work, the effect of the creation of vacancies in oxygen sites on the structure of Ca2MnO4-δ Ruddlesden-Popper (RP) compounds, prepared by sol-gel method, was studied mainly through their structural, morphologic and dielectric properties. The cationic stoichiometry of the samples was studied using energy dispersive X-ray analysis (EDAX). EDAX spectra showed a nominal composition close to the desired one. The structural study, by X-ray diffraction analysis, confirmed the single-phase composition of all the samples. The dielectric studies, the temperature range between 80 K and 360 K, showed dielectric relaxation phenomena, which were related to the Mn4+/Mn3+ ratio. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Research paper thumbnail of Magnetic Refrigeration: Application to the Electron Doped Manganites

NATO Science for Peace and Security Series B: Physics and Biophysics, 2008

ABSTRACT Magnetic refrigeration is a revolutionary, efficient, environmentally friend technology,... more ABSTRACT Magnetic refrigeration is a revolutionary, efficient, environmentally friend technology, which is on the threshold of commercialization. The magnetic rare earth materials are utilized as the magnetic refrigerants in most cooling devices. Recently, some data on the magnetocaloric effect in hole-doped manganites are reviewed. It is shown that the variation of interaction exchange energy, taking place under the effect of a magnetic field in the vicinity of the phase transformation, provides a significant contribution to the change of magnetic entropy. In this paper we report different results found for electron-doped manganites La1-xCexMnO3 (x = 0.3, 0.4 and 0.5). These manganites present a large magnetic entropy change induced by low magnetic change, which is beneficial for the household application of active magnetic refrigerant materials.