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Bellouz R.,University of Monastir | Oumezzine M.,University of Monastir | Hlil E.K.,CNRS Neel Institute | Dhahri E.,Laboratoire Of Physique Applique
Journal of Magnetism and Magnetic Materials | Year: 2014

We have studied the effect of Cr substitution on magnetic and magnetocaloric properties in nanocrystalline La0.65Eu0.05Sr0.3Mn1-xCrxO3 (x=0.05, 0.1 and 0.15). The materials were prepared using the Pechini sol-gel method. All the studied samples were crystallized into a single phase rhombohedral structure with R-3C space group. Magnetic measurements indicate that the ferromagnetic double exchange interaction is weakened with increasing Cr concentration, resulting in a shift in TC from 338 K to 278 K as x varied between 0.05 and 0.15. Detailed analyzes in the vicinity of the ferromagnetic (FM)-paramagnetic (PM) phase-transition temperature prove the samples undergoing a second-order phase transition. The magnetocaloric effect is calculated from the measurement of initial isothermal magnetization versus magnetic field at various temperatures. The maximum magnetic entropy change |ʶMmax| is found to decrease with increasing of Cr content from 4.04 J/Kg K for x=0.05-0.78 J/KgK for x=0.15 upon 5 T applied field change. The relative cooling power (RCP) of La0.65Eu0.05Sr0.3Mn1-xCrxO3 series is nearly 54% of pure Gd, which will be an interesting system for application in room temperature refrigeration. © 2014 Elsevier B.V. All rights reserved. Source


Pousset J.,University of Salento | Millithaler J.-F.,University of Salento | Reggiani L.,University of Salento | Ziad P.,Laboratoire Of Physique Applique | And 5 more authors.
Journal of Applied Physics | Year: 2010

We report a microscopic investigation of the spectrum of voltage fluctuations in nanometric n -Si layers. Theory makes use of a Monte Carlo simulator self-consistently coupled with a two-dimensional Poisson solver. We consider layers of variable thickness W in the range of 2-100 nm and variable length L in the range of 10-1000 nm embedded in an external dielectric medium. Calculations are performed at T=300 K for different doping levels and in the presence of an applied voltage of increasing strength. The spectra are found to exhibit peaks centered on the terahertz region. For W≥100 nm and carrier densities of 5× 1017 and 5× 1018 cm -3, the frequency peaks agree with the value of the three dimensional plasma frequency. For W≤100 nm, the results exhibit a plasma frequency that depends on L, thus implying that the oscillation mode is dispersive. The corresponding frequency covers a wide range of values of 0.2-10 THz and is in agreement with the values of the two-dimensional plasma frequency predicted by existing analytical models. At sufficiently high voltages, the two-dimensional plasma peak is washed out and we observe the onset of a peak in the subterahertz region which is associated with transit time instabilities induced by current saturation conditions. © 2010 American Institute of Physics. Source


Dhahri N.,University of Monastir | Dhahri A.,University of Monastir | Dhahri J.,University of Monastir | Hlil E.-K.,CNRS Neel Institute | Dhahri E.,Laboratoire Of Physique Applique
Journal of Magnetism and Magnetic Materials | Year: 2013

In this paper we report the structural, magnetic and electrical properties of a series of nanocrystalline La 0.67-xEu xSr 0.33MnO 3 (0≤x≤0.3) materials which were prepared by the solid-state reaction method in air. The X-ray powder diffraction has shown that all our synthesized samples are a single phase and have crystallized in the hexagonal symmetry with R3̄C space group. The scanning electron microscopy has shown smooth and densified structures, clean and pure images. Electric and magnetic measurements show that all our samples had exhibited a ferromagnetic to paramagnetic transition and a metallic to semiconductor one when temperature increases. The substitution of La 3+ by Eu 3+ leads to a continuously simultaneous decrease of the Curie temperature T C (from 350 K for x=0.0-258 K for x=0.3) and the metal-semi-conductor transition temperature T p (from 310 K for x=0.0-224 K for x=0.3). The electrical resistivity data were analyzed using different theoretical models and it has been concluded that at low temperatures (ferromagnetic metallic region) the resistivity may originate from grain/domain boundary, electron-electron scattering and two-magnon scattering effects. While in the paramagnetic insulating regime, the variation of resistivity with temperature may be explained by adiabatic small polaron Hopping mechanism and variable-range hopping mechanisms. The values of activation energies were found decreasing, while the density of states at the Fermi-level, N (E F) was increasing with decreasing 〈r A〉. A suitable explanation for the observed behavior is given. © 2012 Elsevier B.V. Source


Bourguiba F.,University of Monastir | Dhahri A.,Laboratoire Of Physique Applique | Rhouma F.I.H.,University of Monastir | Rhouma F.I.H.,Tunis el Manar University | And 4 more authors.
Journal of Alloys and Compounds | Year: 2016

The dielectric properties of iron and tungsten co-substituted BaTiO3 perovskite ceramic with a compositional formula: BaTi0.5(Fe0.33W0.17)O3, prepared by the traditional solid-state route, were systematically characterized. The temperature dependence of the dielectric properties was investigated in the frequency range 1 kHz to 1 MHz. Three dielectric relaxations were observed in the present ceramic which originated from phase transitions from a cubic paraelectric to a tetragonal ferroelectric at the Curie temperature (TC), then to an orthorhombic ferroelectric (at TT-O), and finally to a rhombohedral ferroelectric (at TO-R) similar to those of pure BaTiO3. The dielectric relaxation behavior was analyzed with various models. The best way to characterize the degree of the dielectric relaxation for relaxor ferroelectrics was established using the experimental data. It was shown that the temperature dependence of the static dielectric constant can be well described by an exponential function, while the temperature dependence of the relaxation time was described by a power function. © 2016 Elsevier B.V. Source


Dhahri N.,University of Monastir | Dhahri A.,University of Monastir | Cherif K.,University of Monastir | Dhahri J.,University of Monastir | And 2 more authors.
Journal of Alloys and Compounds | Year: 2010

The structural, magnetic and electrical properties of La0.67Pb0.33Mn1-xCoxO3 (0 ≤ x ≤ 0.30) perovskites prepared by the sol-gel method were investigated. The rhombohedral distorted perovskite structure with space group R over(3, ̄) c is observed in this series of materials while the average size and Co content are varied simultaneously. The Rietveld refinement of the structure shows that the substitution of Mn by Co in the Mn site changes the structure parameters such as the (Mn,Co)-O bond length and (Mn,Co)-O-(Mn,Co) angles. An increase in the (Mn,Co)-O bond length with increasing Co content leads to a decrease in the paramagnetic-ferromagnetic transition temperature TC and the metal-semiconductor transition temperature Tp. Source

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