Influence of non-magnetic and magnetic ions on the MagnetoCaloric properties of La0.7Sr0.3Mn0.9M0.1O3 doped in the Mn sites by M=Cr, Sn, Ti

International audienceWe have studied the MagnetoCaloric Effect (MCE) in La0.7Sr0.3Mn0.9M0.1O3, M=Cr, Sn and Ti, prepared by a conventional solid state reaction. The temperature dependence of magnetization reveals that all compositions exhibit a ferromagnetic (FM) to paramagnetic (PM) transition at TC temperatures of 369, 326, 228 and 210 K, respectively for La0.7Sr0.3MnO3 (LSMO), La0.7Sr0.3Mn0.9Cr0.1O3 (LSMO-Cr), La0.7Sr0.3Mn0.9Sn0.1O3 (LSMO-Sn), and La0.7Sr0.3Mn0.9Ti0.1O3 (LSMO-Ti). Using Arrott plots, the phase transition from FM to PM is found to be of second order. The maximum magnetic entropy change (−ΔSM), at the applied magnetic field of 2 T, is found to be 1.27, 1.76, 0.47 and 1.45 J kg−1 K−1, respectively for LSMO, LSMO-Cr, LSMO-Sn and LSMO-Ti. The relative cooling power (RCP) for LSMO-Cr, LSMO-Sn and LSMO-Ti is in the order of 50%, 26% and 71%, respectively, compared to gadolinium (Gd). As a result, the LSMO-Cr and LSMO-Ti compounds can be considered as promising materials in magnetic refrigeration technology

Electrical conductivity and complex impedance analysis of La0.7-xNdxSr0.3Mn0.7Ti0.3O3 (x≤0.30) perovskite

International audiencePolycrystalline samples La0.7-xNdxSr0.3Mn0.7Ti0.3O3 (x= 0.10; 0.20 and 0.30) were prepared by a high-temperature solide-state reaction technique. The X-ray diffraction shows that all the samples crystallize in the orthorhombic structure, Pbnm space group, with presence of a minor unreacted Nd2O3. The electrical response was studied by impedance complex spectroscopy over a broad frequency range (40-100 MHz) at room temperature. The values of ac conductivity for all samples were fitted by the Jonscher law σ ( ω ) = σ dc + A ω s . For x= 0.10 and 0.20, hopping occurs between neighboring sites, whereas for x=0.30 the hopping process occurs through longer distance. Complex impedance plots exhibit semicircular arcs described by an electrical equivalent circuit, which indicates that the Nd-doped compounds obey a non-Debye relaxation proces

Electrical Resistivity Behavior and VRH Transport Mechanism in Semiconducting La0.6Sr0.4Mn1−2x Fe x Cr x O3 (0.10≤x≤0.25) Manganites

International audienceThe transport properties and conduction mechanism in La0.6Sr0.4Mn1−2x Fe x Cr x O3 (0≤x≤0.3) have been investigated. The undoped samples show metal-semiconductor transition with a peak of resistivity at a temperature T P , whereas for all doped compounds the semiconducting behavior persists in the whole temperature range. The insertion of Cr3+ and Fe3+ ions leads to the increase of resistivity because the simultaneous substitution of Fe3+ and Cr3+ for Mn3+ reduces the number of available hopping sites for the Mn e g↑ electron and suppresses the double-exchange mechanism. It was found that the transport mechanism for substituted samples is dominated by the variable range hopping of small polarons between localized states in a model where the various parameters estimated from Mott's relation obey the variable range hopping (VRH) mechanism

Electrical and magnetic properties of La0.67Ba0.33Mn1−x (Me) x O3 perovskite manganites: case of manganese substituted by trivalent (Me = Cr) and tetravalent (Me = Ti) elements

International audienceThe effects of non-magnetic Ti4+ substitution on the structural, electrical and magnetic properties of La0.67Ba0.33Mn1−x Ti x O3 (0≤x≤0.1) are investigated and compared to those existing in La0.67Ba0.33Mn1−x Cr x O3 (magnetic Cr3+). The structural refinement by the Rietveld method revealed that Ti-doped samples crystallize in the cubic lattice with space group Pm3¯m , while samples with Cr crystallize in the hexagonal setting of the rhombohedral R3¯C space group for identical contents of dopant. The most relevant structural features are an increase of the lattice parameters, of the cell volume and of the inter-ionic distances with increasing Ti doping level. Both series of samples show a decrease of the paramagnetic-ferromagnetic transition temperature when the amount of chromium or titanium increases. Transport measurements show that when increasing the metal doping, the resistivity increases whereas the metallic behavior of the parent compound La0.67Ba0.33MnO3 is destroyed. For a substitution higher than 5 at.% of Ti and 10 at.% of Cr, the samples exhibit a semiconducting behavior in the whole range of temperature, for which the electronic transport can be explained by variable range hopping and/or small polaron hopping models

Primary resistance to clarithromycin, metronidazole and amoxicillin of Helicobacter pylori isolated from Tunisian patients with peptic ulcers and gastritis: a prospective multicentre study

<p>Abstract</p> <p>Background</p> <p>The frequency of primary resistance to antibiotics in H. pylori isolates is increasing worldwide. In Tunisia, there are limited data regarding the pattern of H. pylori antibiotic primary resistance.</p> <p>Aim</p> <p>To evaluate the primary resistance of H. pylori to clarithromycin, metronidazole and amoxicillin and to detect the mutations involved in clarithromycin resistance.</p> <p>Materials and methods</p> <p>273 strains isolated from adults and children were enrolled. The primary resistance to clarithromycin, metronidazole and amoxicillin was evaluated by means of E-test minimal inhibitory concentration (MIC). The real-time PCR using Scorpion primers was performed in all cases to assess clarithromycin primary resistance and point mutations involved.</p> <p>Results</p> <p>No resistance to amoxicillin was detected. For adults, resistance to clarithromycin and metronidazole was found respectively in 14.6% and 56.8%, and respectively in 18.8% and 25% in children. Overall, the rates of global primary resistance to clarithromycin and metronidazole in Tunisia were respectively determined in 15.4% and 51.3%.</p> <p>By the use of Scorpion PCR, the A2143G was the most frequent point mutation observed (88.1%), followed by the A2142G (11.9%); the A2142C was not found and 18 of 42 patients (42.8%) were infected by both the resistant and the susceptible genotype.</p> <p>The association of clarithromycin resistance with gender was not statistically significant, but metronidazole resistant strains were isolated more frequently in females (67.8%) than in males (32.2%) and the difference was significant. As for gastroduodenal diseases, the difference between strains isolated from patients with peptic ulceration and those with non peptic ulceration was not statistically significant. When about the distribution of resistant strains to clarithromycin and metronidazole between the three Tunisian cities (Tunis, Menzel Bourguiba and Mahdia), the difference was not statistically significant.</p> <p>Conclusion</p> <p>Local data regarding the primary resistance of H. pylori to clarithromycin, metronidazole and amoxicillin and the main genetic mutation involved in clarithromycin resistance in vivo (A2143G) are necessary to prove a clear need for a periodic evaluation of antibiotic consumption and new therapeutic strategies in Tunisia in order to avoid the emergence of resistant strains.</p

Structural Magnetic and Electrical Properties in Nd-Doped Manganites (La0.70−x Nd x )Sr0.30Mn0.70Ti0.30O3 with 0 ≤ x ≤ 0.30

International audienceThe structural, magnetic and electrical properties of (La0.70−xNdx)Sr0.30Mn0.70Ti0.30 O 3 perovskites (0 ≤ x ≤ 0.30) prepared by the usual ceramic procedure were investigated. Structural Rietveld refinements of the X-ray powder diffraction revealed that for x = 0 the compound crystallizes in a rhombohedral (R 3¯ c) perovskite structure, while for x = 0.10, 0.20 and 0.30 the structure becomes orthorhombic (Pbnm). The field-cooled, zero-field-cooled and M(H) measurements lead to conclude that the samples with x≥0.10 behave like spin-glass systems. The temperature dependence of the resistivity exhibits two transitions at two temperatures T p1 and T p2, with the same width for the two peaks. T P1 results from the competition between double-exchange and super-exchange mechanisms inside the grain (intrinsic effects) whereas T P2 arises due to the grain boundary effects (extrinsic effect). The resistivity of this system seems to be related to the competition between antiferromagnetic insulating (AFI) phases, ferromagnetic insulating (FMI) phases and ferromagnetic metal (FMM) phases, which is induced by Nd doping effect

Determination of the spontaneous magnetization by analysis of the magnetic entropy change in La0.40Nd0.30Sr0.30Mn0.70Cr0.30O3

International audienceThe magnetic phase transition and magnetic entropy change (−ΔSM) in the La0.40Nd0.30Sr0.30Mn0.70Cr0.30O3 perovskite were investigated by measuring the magnetization as a function of temperature. The ferromagnetic transition temperature TC and the critical exponents β and γ, determined by analyzing the Arrott plots, are found to be TC = 191 K, β = 0.433, γ = 1.053 and δ = 3.486. These values for the critical exponents are close to the mean-field values. In order to estimate the spontaneous magnetization Ms(T) at a given temperature, we use a process based on the analysis, in the mean-field theory, of the magnetic entropy change (−ΔSM) versus the magnetization data. An excellent agreement is found between the spontaneous magnetization determined from the entropy change ((−ΔSM) vs. M2) and the classical extrapolation from the Arrott curves (μ0H/M vs. M2), thus confirming that the magnetic entropy is a valid approach to estimate the spontaneous magnetization in this system and in other compounds as well

Large magnetocaloric effect in Ti-modified La0.70Sr0.30MnO3 perovskite

International audienceThe effect of Ti substitution on the magnetocaloric effect of the La0.70Sr0.30MnO3 perovskite was investigated. The magnetic entropy change (− ΔSM) was deduced by two methods: a Maxwell relation and the Landau theory. The magnetocaloric data displays a large value of the magnetic entropy change (− ΔSM) near the Curie temperature (TC = 210 K), which increases when increasing the applied magnetic field. A good agreement is found between the experimental (− ΔSM) and the one estimated by Landau theory. The relative cooling power values vary from 49 to 288 J kg− 1 upon variation of the applied magnetic field at 1 and 5 T. Under 5 T, the relative cooling power value for La0.70Sr0.30Mn0.90Ti0.10O3 is about 70% of the conventional refrigerant Gd material. As a result, the herein reported compound can be considered as a promising material in magnetic refrigeration technology

Large magnetic entropy change above 300 K in (La0.56Ce0.14)Sr0.3MnO3 perovskite

International audienceMagnetic field dependence of the magnetic entropy change (ΔSM) is the key for magnetic refrigeration. A large magnetic entropy change (ΔSM) associated with the ferromagnetic-paramagnetic transition in Ce-doped La0.7Sr0.3MnO3 material has been observed. It is shown for (La0.56Ce0.14)Sr0.3MnO3 composition at the Curie temperature of 357 K, that the maxima of the magnetic entropy changes View the MathML sourceΔSMmax upon variation of the applied magnetic field at 1 and 5 T are about 1.55 and 4.78 J kg−1 K−1, respectively. Due to the large (ΔSM) and high Curie temperature, the (La0.56Ce0.14)Sr0.3MnO3 perovskite is suggested to be used as potential magnetic refrigerants for magnetic refrigeration technology above room temperature

Structural and magnetic properties of (La0.70−xYx)Ba0.30Mn1−xFexO3 perovskites simultaneously doped on A and B sites (0.0 ≤ x ≤ 0.30)

International audienceWe present the structural and magnetic properties of polycrystalline samples of the solid solution (La0.70−xYx)Ba0.30Mn1−xFexO3 (x = 0.00, 0.10, 0.20 and 0.30). Samples have been prepared by a conventional solid-state reaction method in air. Rietveld refinements of the X-ray powder diffraction data show a structural transition from rhombohedral (View the MathML sourceR3¯c) to orthorhombic (Pbnm) symmetry when x ≥ 0.10. The ZFC, FC and M(H) measurements lead to conclude that the samples with x ≥ 0.10 behave like spin-glass systems. The substitution of Mn3+ ions by Fe3+ ions triggers antiferromagnetic interactions between the Fe3+ and Mn4+ spins. The values of the magnetization (M(H)) decrease when increasing the Fe content. On the basis of a simple model of Mn and Fe spins pointing in opposite directions, we found the experimental data close to the calculated values, confirming the antiferromagnetic alignment between Fe and Mn moments