Simulation of the d.c. critical current in superconducting sintered ceramics

The new superconducting high-Tc sintered ceramics can be described in some case as a lattice of interconnected rods, in other cases as a more or less random packing of parallelepiped crystallites ; their size is about a few microns. The d.c. critical current at zero voltage of such a material is not related to the critical current of the bulk material, but to its granular structure. Indeed, the critical current between two adjacent cells is governed by the critical current of the weak link between them ; this link behaves within some limits as a Josephson junction, the critical current of which is known. For our present problem, the system can be modeled as a lattice of Josephson junctions. We present here results for the d.c. critical current at zero voltage of lattices of identical Josephson junctions in two dimensions. The influence of the finiteness of size of the sample is examined. The relationship with normal conductivity simulations and percolation is discussed

Neutron diffraction and magnetocaloric effect studies of MnFe 1-x Co x P series of solid solutions

International audienceMnFe 1-x Co x P intermetallic series of solid solutions (0.4<x<0.6) have been studied by means of powder neutron diffraction in 10−320 K temperature range. Rietveld analysis pointed out that Co 2 P-type orthorhombic crystal structure (SG: Pnma) presents for all series. Helicoidal incommensurate antiferromagnetic structure with propagation vector q = [0, 0, q] were evidenced for all compounds at low temperature range. The q value decreases with cobalt content and the second order polynomial q(x) it was evidenced, that is found well correlated with this dependence. Magnetic moments values of µ Mn =3.34 µ B and µ (Fe,Co) =0.06 µ B were determined from neutron diffraction refinements for x=0.4 at 10 K. In addition, magnetic interactions in relations with electronic band structure calculations of MnFe 1-x Co x P were presented and discussed reference to previous published data. Finally, magnetocaloric properties for selected compounds of the MnFe 1-x Co x P and MnFe 0.45 Co 0.45 P 0.9 Ge 0.1 series of compounds are presented

Transitions dipolaires induites par collisions sur un faisceau de formaldéhyde. Effet d'un champ électrique

Using a H2CO molecular beam, a method is first described which permits the observation of rotational transitions ΔJ = ± n (n ≽ 1), induced by collisions with the molecules of a target gas. In the case H2CO → H 2CO, selection rules are defined and rotational resonance effects are observed on the differential cross sections for some ΔJ = — 1 transitions. It appears that a direct electric field E considerably modifies the H2CO cross sections σJ. Using the pair H 2CO → NH3, the study of σJ variations with E shows that the ΔJ = 0 transitions become forbidden, when E is intense. The experimental results are compared with the theoretical transition probabilities variations, due to E and calculated in the Born approximation. An experimental method is deduced which permits, by application of an electric field, to select AJ = 0 transitions on a molecular beam.Nous présentons d'abord une méthode expérimentale qui permet d'observer les transitions rotationnelles ΔJ = ± n (n ≽ 1) induites par collisions entre les molécules d'un faisceau de formaldéhyde et un gaz tampon. En prenant H2CO comme gaz tampon, nous précisons les règles de sélection et, sur les sections de collisions correspondant à plusieurs transitions ΔJ = — 1, nous mettons en évidence des effets de résonance rotationnelle. Nous constatons qu'un champ électrique continu modifie considérablement les sections de collisions du formaldéhyde. Sur le système dipolaire H2 CO-NH3, l'étude des variations caractéristiques de celles-ci en fonction du champ appliqué montre que les transitions ΔJ. = 0 de H2CO deviennent interdites en champ fort. Nous comparons les résultats expérimentaux aux variations de probabilité de transition dues au champ électrique et calculées dans l'approximation de Born. On déduit une méthode expérimentale permettant de trier par effet de champ les transitions ΔJ = 0 sur le faisceau moléculaire

Correlation of electronic structure and magnetic moment in Ga1−xMnxN : First-principles, mean field and high temperature series expansions calculations

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Enhancement of Magnetocaloric Effect in (La0.67Ca0.33MnO3)/(La0.7Ba0.3MnO3) Composite

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Estimation of the magnetic entropy change by means of Landau theory and phenomenological model in La 0.6 Ca 0.2 Sr 0.2 MnO 3 /Sb 2 O 3 ceramic composites

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Resurgence of ferromagnetic behavior coupled with Griffiths singularity in the electron doped La0.3Ca0.7MnO3 compound following the creation of oxygen deficiency

International audienceThe relative cooling power is around 95.271 J kg −1 for NSMO-AC and 202.054 J kg −1 for NSMO-SG at μ 0 H = 5 T, making these materials potential candidates for magnetic refrigeration application near room temperature

Large magnetocaloric effect in manganese perovskite La 0.67−x Bi x Ba 0.33 MnO 3 near room temperature

International audienceLa0.67−xBixBa0.33MnO3 (x = 0 and 0.05) ceramics were prepared via the sol–gel method. Structural, magnetic and magnetocaloric effects have been systematically studied. X-ray diffraction shows that all the compounds crystallize in the rhombohedral structure with the R[3 with combining macron]c space group. By analyzing the field and temperature dependence of magnetization, it is observed that both samples undergo a second order magnetic phase transition near TC. The value of TC decreases from 340 K to 306 K when increasing x from 0.00 to 0.05, respectively. The reported magnetic entropy change for both samples was considerably remarkable and equal to 5.8 J kg−1 K−1 for x = 0.00 and 7.3 J kg−1 K−1 for x = 0.05, respectively, for μ0H = 5 T, confirming that these materials are promising candidates for magnetic refrigeration applications. The mean-field theory was used to study the magnetocaloric effect within the thermodynamics of the model. Satisfactory agreement between experimental data and the mean-field theory has been found

Giant magnetoresistance and correlation between critical behavior and electrical properties in a new compound La0.6Gd0.1Sr0.3Mn0.8Si0.2O3 manganite

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Large magnetic entropy change and prediction of magnetoresistance using a magnetic field in La 0.5 Sm 0.1 Sr 0.4 Mn 0.975 In 0.025 O 3

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