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M'nassri R.,University of Sfax | M'nassri R.,Kairouan University | Cheikhrouhou A.,University of Sfax | Cheikhrouhou A.,CNRS Neel Institute
Journal of the Korean Physical Society | Year: 2014

The magnetocaloric proprieties of Ba2Fe1-xCrxMoO6 (cubic, Fm3m) have been investigated. With the help of the phenomenological model, the temperature dependences of the magnetization for Ba2Fe1-xCrxMoO6 (0 ≤ x ≤ 1) in a 1 T magnetic field were simulated. The behavior of the temperature-dependent magnetocaloric effect was investigated in the vicinity of magnetic phase transitions. The magnetic entropy change and the specific heat were obtained. The values of the maximum magnetic entropy change, the full width at half maximum, and the relative cooling power for a change in the magnetic field of 1 T were calculated. The values and the shapes of -ΔSM and -ΔCp strongly depended on the Cr concentration. As x was increased, the maximum values of -ΔSM and -ΔCp decreased and the peaks of -ΔSM and -ΔCp became broader. The magnetocaloric effect of this material is large and tunable, suggesting a possible technical application of the material at moderate magnetic fields at temperatures near room temperature. For Ba2Fe1-xCrxMoO6, the magnetic entropy change was shown to follow a master curve behavior. © 2014 The Korean Physical Society.


Harrabi A.,Kairouan University | Rahal B.,University of Sfax
Annales Henri Poincare | Year: 2016

In this paper, we study the solutions of the triharmonic Lane–Emden equation (Formula presented.)As in Dávila et al. (Adv. Math. 258:240–285, 2014) and Farina (J. Math. Pures Appl. 87:537–561, 2007), we prove various Liouville-type theorems for smooth solutions under the assumption that they are stable or stable outside a compact set of (Formula presented.). Again, following Dávila et al. (Adv. Math. 258:240–285, 2014), Hajlaoui et al. (On stable solutions of biharmonic prob- lem with polynomial growth. arXiv:1211.2223v2, 2012) and Wei and Ye (Math. Ann. 356:1599–1612, 2013), we first establish the standard integral estimates via stability property to derive the nonexistence results in the subcritical case by means of the Pohozaev identity. The supercritical case needs more involved analysis, motivated by the monotonicity formula established in Blatt (Monotonicity formulas for extrinsic triharmonic maps and the tri- harmonic Lane–Emden equation, 2014) (see also Luo et al., On the Triharmonic Lane–Emden Equation. arXiv:1607.04719, 2016), we then reduce the nonexistence of nontrivial entire solutions to that of nontrivial homogeneous solutions similarly to Dávila et al. (Adv. Math. 258:240–285, 2014). Through this approach, we give a complete classification of stable solutions and those which are stable outside a compact set of (Formula presented.) possibly unbounded and sign-changing. Inspired by Karageorgis (Nonlinearity 22:1653–1661, 2009), our analysis reveals a new critical exponent called the sixth-order Joseph–Lundgren exponent noted (Formula presented.). Lastly, we give the explicit expression of (Formula presented.). Our approach is less complicated and more transparent compared to Gazzola and Grunau (Math. Ann. 334:905–936, 2006) and Gazzola and Grunau (Polyharmonic boundary value problems. A monograph on positivity preserving and nonlinear higher order elliptic equations in bounded domains. Springer, New York, 2009) in terms of finding the explicit value of the fourth-Joseph–Lundgren exponent, (Formula presented.). © 2016 Springer International Publishing


M'Nassri R.,Kairouan University | M'Nassri R.,University of Monastir | Chniba-Boudjada N.,CNRS Neel Institute | Cheikhrouhou A.,University of Sfax
Journal of Alloys and Compounds | Year: 2015

The structure, critical exponents and magnetocaloric effect (MCE) of Pr0.4Eu0.2Sr0.4MnO3 were investigated in detail. A solid state reaction method was used in the preparation phase. Phase purity, structure, size, and crystallinity were investigated using XRD and SEM. The Reitveld refinement of XRD pattern shows that the sample adopts an orthorhombic structure with Pnma space group. Analyzing temperature and field dependences of magnetization around the ferromagnetic-paramagnetic transition reveals the sample undergoing the second-order magnetic phase transition with the critical parameters TC ∼ 238 K, β = 0.310(3), γ = 1.264(1) and δ = 4.761(9).The exponents are close to 3D-Ising values. This reflects an existence of ferromagnetic short-range order in our sample. With these values one can scale the magnetization below and above TC following a single equation of state. However, it is noteworthy that the scaling relations are obeyed indicating renormalization of interactions around the TC. Temperature variation in effective exponents βeff and γeff resemble with those for disordered ferromagnet. In the vicinity of TC, the magnetic entropy change ΔS reached maximum values of 2.80 and 5.32 J/kg K under magnetic field variations of 2 and 5 T, respectively. The field dependence of the magnetic entropy changes are also analyzed, which show power law dependence ΔSMmax≈aμ0Hn at transition temperature. The critical exponent analysis related to magnetocaloric effect is described. Moreover, the temperature dependence of the exponent n for a different magnetic field is also studied. The values of n obey to the Curie Weiss law above the transition temperature. In particularly, n can be related to the critical exponents β, γ and δ at the magnetic transition. We used the scaling hypotheses to scale the magnetic entropy change and heat capacity changes to a single universal curve respectively for Pr0.4Eu0.2Sr0.4MnO3 sample. © 2015 Elsevier B.V. All rights reserved.


M'Nassri R.,Kairouan University | Chniba Boudjada N.,CNRS Neel Institute | Cheikhrouhou A.,University of Sfax
Journal of Alloys and Compounds | Year: 2014

This article covers the detailed study on the magnetic and magnetocaloric properties of Pr0.5Eu0.1Sr0.4MnO3 samples. The materials were synthesized through the sol-gel using nitrate-citrate route method by sintering at four different temperatures starting from 600 to 1000 °C. Phase purity, structure, size, and crystallinity were investigated using XRD and SEM. The structure, magnetic properties and magnetocaloric effect of Pr0.5Eu0.1Sr0.4MnO3 ceramics with different particle sizes have been investigated. Regardless of the sintering temperature, the orthorhombic crystal structure with the Pnma space group is stable. XRD analysis showed that the particle size increases with the increase in sintering temperature. The Curie temperature shifted from 90 K to 270 K. Magnetization, magnetic entropy change (ΔS), and relative cooling power (RCP) also increased with size in the nanocrystalline samples. The magnetic entropy changes were about 0.89 J/kg K, 2.75 J/kg K, 3.34 J/kg K, and 3.82 J/kg K for the samples sintered at 600 °C, 800 °C, 900 °C and 1000 °C, respectively when a magnetic field of 5 T was applied. It is shown that for Pr0.5Eu0.1Sr0.4MnO3 the magnetic entropy change follows a master curve behavior. © 2014 Elsevier B.V.


Selmi A.,University of Sfax | M'Nassri R.,Kairouan University | Cheikhrouhou-Koubaa W.,University of Sfax | Chniba Boudjada N.,CNRS Neel Institute | Cheikhrouhou A.,University of Sfax
Journal of Alloys and Compounds | Year: 2014

Structural, magnetic and magnetocaloric properties of Pr0.7Ca0.3Mn0.95X0.05O3(X = Cr, Ni, Co and Fe) ceramics have been investigated by X-ray diffraction (XRD) and magnetic measurements. Powder samples have been elaborated using the solid state reaction method at high temperature. The Rietveld analysis of the powder X-ray diffraction shows that the samples crystallize in the orthorhombic structure with Pnma space group. Magnetic measurements show that all our materials exhibit a paramagnetic-ferromagnetic transition with decreasing temperature. The Arrott plots of ours materials reveal the occurrence of a second-order phase transition. The maximum values of magnetic entropy change |ΔSMmax| are 2.92, 2.96, 3.1, and 2.38 J kg-1K-1and the relative cooling power (RCP) values are 405.8, 378.2, 352.2 and 337.4 J kg-1for a magnetic-field change from 0 to 5 T for Cr, Ni, Co and Fe respectively. The large RCP found in our substituted samples will be interesting for magnetic refrigeration over a wide temperature range ∼130 K around its paramagnetic to ferromagnetic transition temperature. With the scaling laws of ΔSM, the experimental ΔSMcollapse onto a universal curve for several ceramics, where an average curve is obtained. With the phenomenological universal curve, the experimental ΔSMare well predicted for all materials. © 2014 Published by Elsevier B.V.


Khlifi M.,University of Sfax | Khlifi M.,Kairouan University | Dhahri E.,University of Sfax | Hlil E.K.,CNRS Neel Institute
Journal of Alloys and Compounds | Year: 2014

La0.8Ca0.2-x□xMnO 3 compounds were prepared by the solid-state reaction with 0.00 ≤ x ≤ 0.20, annealed at high temperature and post annealed at 800 C. Structural, magnetic and electrical measurements were performed to examine the effect of the calcium deficiency on the physical properties. The XRD data have been analyzed by Rietveld refinement technique. We have noticed that the calcium deficiency causes a structural transition form an orthorhombic structure with Pnma space group to a rhombohedral one with R3â€c space group, and an increase of the unit cell volume. Magnetization versus temperature study has shown that all samples exhibit a magnetic transition from ferromagnetic (FM) to paramagnetic (PM) phase when increasing temperature. All samples present a large magnetocaloric effect which reaches 8 J/K kg under magnetic field of 5 T for x = 0.00. Moreover, the temperature dependence of electrical resistivity shows a metal-insulator transition at Tρ for all samples. The electrical resistivity is fitted with the phenomenological percolation model. Hence, we found that the estimated values of the resistivity are in good agreement with experimental data. Above all, the magnetoresistance study showed a peak which has a great value around the insulator-metal transition temperature. © 2013 Elsevier B.V. All rights reserved.


M'Nassri R.,University of Sfax | M'Nassri R.,Kairouan University | Cheikhrouhou A.,University of Sfax | Cheikhrouhou A.,CNRS Neel Institute
Journal of Superconductivity and Novel Magnetism | Year: 2014

Effects of oxygen deficiency on the thermomagnetic properties of La 2/3Ba1/3MnO3-δ polycrystalline perovskites have been predicted. By the help of the phenomenological model, the temperature dependences of the magnetization for La2/3Ba 1/3MnO3-δ with δ=0.0, 0.02, 0.05, 0.08, and 0.1 upon 1 T magnetic field were simulated. The behavior of the temperature dependent magnetocaloric effect, in the vicinity of magnetic phase transitions, was investigated. The magnetic entropy change, specific heat, and adiabatic temperature change for several δ were obtained. The values of maximum magnetic entropy change, full-width at half-maximum, and relative cooling power, in 1 T magnetic field variation, were calculated. As the oxygen content increases, the magnetocaloric effect of La2/3Ba1/3MnO 3-δ, decreases and shifts to room temperature. The results obtained show a strong dependence on the oxygen deficiency of the materials. The magnetocaloric effect of these materials is large and tunable, suggesting a possible technical application of the materials at moderate magnetic fields near room temperature. It is shown that for La2/3Ba1/3MnO 3-δ, the magnetic entropy change and adiabatic temperature change follows a master curve behavior. © 2013 Springer Science+Business Media New York.


Harrabi A.,Kairouan University
Advanced Nonlinear Studies | Year: 2014

We prove an existence result for a fourth order elliptic equation where the associated functional does not satisfy the Palais-Smale condition. We use some truncations argument and L8-norm estimates.


Mahjoub S.,University of Monastir | Baazaoui M.,University of Monastir | M'Nassri R.,Kairouan University | Rahmouni H.,Gabes University | And 2 more authors.
Journal of Alloys and Compounds | Year: 2014

The effects of partial substitution of manganese by iron on the physical properties of Pr0.6Ca0.1Sr0.3Mn 1-xFexO3 (0 ≤ x ≤ 0.075) were investigated experimentally. Our samples were synthesized using the Pechini sol-gel method. X-ray diffraction analysis shows that our compounds crystallize in the orthorhombic structure with the Pbnm space group. Magnetic measurements show that all our materials exhibit a paramagnetic-ferromagnetic transition with decreasing temperature. The M(T) curves at μ0H = 0.05 T indicate a monotonic drop in the Curie temperature (TC) from 270 K for x = 0.0-112 K for x = 0.075 as the Fe concentration is increased. The results of electrical resistivity versus temperature measurements show that all our samples exhibit a metallic behavior at low temperature. For higher values of temperature, metal-insulator transition is seen at temperature Tp. From isothermal magnetization versus magnetic applied field, we deduced the magnetic entropy change ΔSM. The maximum values of the magnetic entropy change (-ΔSMmax) at 5 T applied change in the magnetic field are 3.64 and 3.12 J/kg K for x = 0 and x = 0.075 samples respectively. Under an applied magnetic field of 5 T, the relative cooling power (RCP) values are found between 243 and 256 J/kg for undoped sample and x = 0.075 respectively. Our results are interesting enough and suggest that these materials may be suitable candidates as working substances in magnetic refrigeration. © 2014 Elsevier B.V. All rights reserved.


M'Nassri R.,Kairouan University
Journal of Superconductivity and Novel Magnetism | Year: 2014

In this paper, the field dependence of magnetocaloric properties of La 0.6Pr 0.4Fe 10.7Co 0.8Si 1.5 with second-order phase transition material is studied using a phenomenological model. The model parameters were determined from the magnetization data adjustment and were used to give better fits to magnetic transition and to calculate the magnetocaloric thermodynamic quantities. The entropy curves have been observed to behave as an asymmetrical broadening of ΔS M peak with increasing magnetic field. For larger fields, the peak shifts to higher temperatures, while the overall shape of the curve broadens over a wide temperature range. The values of maximum magnetic entropy change, full width at half maximum, and relative cooling power, at several magnetic field variations, were calculated. The maximum magnetic entropy changes of 3.957(5) and 14.197(4) J kg -1 K -1 and the relative cooling power (RCP) values of 95.420(3) and 392.729(2) J kg -1 are obtained for 1 and 5 T, respectively. The theoretical calculations are compared with the available experimental data. The critical exponents associated with ferromagnetic transition have been determined from magnetocaloric effect (MCE) methods. By using the field dependence of ΔS M max≈a(μ 0 H) n and the distance (T peak-T c)≈b(μ 0 H)1/Δ, we have investigated the critical behavior of La 0.6Pr 0.4Fe 10.7Co 0.8Si 1.5. From the analysis of the relationship between the local exponent n and the gap exponent Δ, we have calculated other exponents: β, γ, and δ. The large MCE, relatively high RCP, high magnetization, and low cost jointly make the present compound a promising candidate for magnetic refrigerant near room temperature. © 2014 Springer Science+Business Media New York.

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