J. Dhahri

Effect of Ru substitution on the physical properties of La0.6Pr0.1Sr0.3Mn1−xRuxO3 (x = 0.00, 0.05 and 0.15) perovskites

Morphology, magnetic, and magnetocaloric properties of La0.6Pr0.1Sr0.3Mn1−xRuxO3 (x = 0.00, 0.05 and 0.15) were experimentally investigated. A solid-state reaction method was used for the preparation of the samples. The microstructure of the samples was determined by scanning electron microscopy SEM. Field-cooled (FC) and zero-field-cooled (ZFC) thermomagnetic curves measured at low field and low temperatures exhibit a cluster spin state. A sensitive response to substituting Ru for Mn is observed in the magnetic and magnetocaloric properties. We found that Ru doping is not powerful enough to reduce the Curie temperature TC, however, it brings about cluster glass behaviors. A magneto-caloric effect has been calculated in terms of isothermal magnetic entropy change. The maximum entropy change |ΔSmaxM| reaches the highest values of 3.32 J kg−1 K−1, 3.11 J kg−1 K−1 and 2.57 J kg−1 K−1 in a magnetic field. However, the relative cooling power decreases with Ru content from 227.44 J kg−1 to 214.14 J kg−1 and then to 188.68 J kg−1 for x = 0.00, 0.05 and 0.15 compositions, respectively.

The effect of a cation radii on physical properties of doped manganites

Abstract The effect of divalent substitution into lanthanide sites in La0.7Sr0.3-xCa x MnO3 oxides has been studied. In this system the average size (rA) of the A site cations is systematically varied while keeping the Mn3+ /Mn4+ ratio fixed at 7/3. Polycrystalline samples of La0.7Sr0.3-xCaxMnO3 were prepared using solid-state reaction. X-ray diffraction patterns show that these compounds exist in a single-phase having a perovslute-like structure at room temperature. The paramagnetic to ferron∼agnetic transition temperature TC increases from 260 to 370 K when (rA) varies from 1.205 to 1.244 A. The ionic radii variance [sgrave]2 increases with the increase of (rA). The double-exchange energy is sensitive to the average ionic radius of the A site (rA) and to the variance [sgrave]2 of the A cation radii distribution. The structural, magnetic and electrical properties are correlated with the change of Mn-O-Mn bond angle. The variation of the activation energy Ea in our samples can be related to the existence ...

Critical parameters near the phase transition temperature in La0.67-xDyxPb0.33MnO3

La0.67–xDyxPb0.33MnO3 (x=0.00 and x=0.10) were elaborated by the solid state method and checked by X-ray diffraction. Close to magnetic temperature transition, the order transition and the critical behavior were investigated by dc magnetization measurements versus x composition. The critical properties were investigated through various techniques such as modified Arrott plot (MAP), Kouvel-Fisher (KF) method and critical isotherm (CI) analysis based on the data of static magnetic measurements recorded around the Curie temperature TC. The values of critical exponents (β and γ) estimated were found to lie between those predicted for a 3D-Ising model for x=0.00 and those of 3D-Heisenberg model for x=0.10. The reliability of the critical exponents values was confirmed by the Widom scaling relation and the universal scaling hypothesis. The change in the universality class should be due to the increase of the Dy content.

Magnetic, magnetocaloric and critical behavior investigation of La0.7Ca0.1Pb0.2Mn1−x−yAlxSnyO3 (x, y = 0.0, 0.05 and 0.075) prepared by a sol–gel method

A systematic study on the magnetic, magnetocaloric and critical behavior properties of polycrystalline La0.7Ca0.1Pb0.2Mn1−x−yAlxSnyO3 prepared via a sol–gel method are studied. These compounds present a single magnetic transition from a ferromagnetic (FM) to paramagnetic (PM) phase with decreasing temperature. The critical exponents are estimated using various techniques, such as a modified Arrott plot, the Kouvel–Fisher method and critical isotherm analysis based on the data of static magnetic measurements recorded around the Curie temperature, TC. The estimated critical exponent values are found to be consistent and comparable to those predicted by the 3D-Ising model for x, y = 0.0 and by the mean field model for x, y = 0.05 and 0.075. We have confirmed the obtained critical exponents with the single scaling equation: M(μ0H,e) = eβf ± (μ0H/eβ+γ), where e = (T − TC)/TC is the reduced temperature. We have investigated the validity and usefulness of theoretical modeling in our compound La0.7Ca0.1Pb0.2Mn1−x−yAlxSnyO3 based on the mean-field analysis of the magnetic entropy change (−ΔSM) versus the magnetization data. For comparison, the MSp has been also deduced from the classical extrapolation of the Arrott plot. We obtain an excellent agreement between the spontaneous magnetization determined from the entropy change (−ΔSM vs. M2) and the Arrot curves (μ0H/M vs. M2), confirming the validity of the magnetic entropy change approach in order to estimate the spontaneous magnetization MSp in a ferromagnetic system.

The effects of vacancies in oxygen sites on physical properties of the lanthanum manganites

Abstract The non-stoichiometric solution Ln1−xSrxMnO3-δ□δ was prepared by the classic ceramic method. The physical properties as crystallographic, magnetic and electrical properties were studied. A structural phase transition from rhombohedral to orthorhombic was observed at a concentration of Mn4+ between 10% and 15% per Mn atom in the unit formula. The magnetic properties are very sensitive to the presence of vacancies at the oxygen sites. The non-stoichiometric, samples change from metallic to insulating behaviour depending on their vacancy concentration. In the semiconductor phase, the activation energy value changes with the structural phase, increasing in the rhombohedral phase and decreasing in the orthorhombic phase.

Critical phenomena and estimation of the spontaneous magnetization from a mean field analysis of the magnetic entropy change in La0.7Ca0.1Pb0.2Mn0.95Al0.025Sn0.025O3

In the present work, we have studied the universal critical behavior in the perovskite-manganite compound La0.7Ca0.1Pb0.2Mn0.95Al0.025Sn0.025O3. Experimental results revealed that all samples exhibit a second-order magnetic phase transition. To study the critical behavior of the paramagnetic–ferromagnetic transition, various techniques such as modified Arrott plots, the Kouvel–Fisher method and critical isotherm analysis were used to determine the values of the Curie temperature TC, as well as the critical exponents β (corresponding to the spontaneous magnetization), γ (corresponding to the initial susceptibility) and δ (corresponding to the critical magnetization isotherm). The critical exponent values for our sample are consistent with the prediction of the mean field model (β = 0.46, γ = 1.0, and δ = 2.94 at an average TC = 302.12 K), indicating that the magnetic interactions are long-range. The reliability of the critical exponent values was confirmed by the Widom scaling relation (WSR) and the universal scaling hypothesis. Moreover, to estimate the spontaneous magnetization of La0.7Ca0.1Pb0.2Mn0.95Al0.025Sn0.025O3 perovskite, we used the magnetic entropy change (ΔSM), obtained from isothermal magnetization. The results obtained through this approach are compared to those obtained from classical analysis using Arrott curves. An excellent agreement is found between this approach and the one obtained from the extrapolation of the Arrott curves.

Correlation between magnetic and electric properties based on the critical behavior of resistivity and percolation model of La0.8Ba0.1Ca0.1MnO3 polycrystalline

We investigated the magneto-electrical properties of a La0.8Ba0.1Ca0.1MnO3 polycrystalline sample, prepared by the polymerization-complex sol–gel method. Comparison of experimental data with the theoretical models showed that in the metal-ferromagnetic region, the electrical behavior of the sample is quite well described by a theory based on electron–electron, electron–phonon and electron–magnon scattering and Kondo-like spin dependent scattering. For the high temperature paramagnetic insulating regime, the adiabatic small polaron hopping model was found to fit well with the experimental curves. The estimated critical exponents, obtained from resistivity, were (β = 0.516 ± 0.013, γ = 1.181 ± 0.005 and α = 0.004). They were very close to those predicted by the mean-field model. These results were in good agreement with the analysis of the critical exponents from magnetization measurements.

Large magnetocaloric effect and critical behavior in La0.7Ba0.2Ca0.1Mn1−xAlxO3

The structural, magnetic and magnetocaloric properties of La0.7Ba0.2Ca0.1Mn1−xAlxO3 (0 ≤ x ≤ 0.1) perovskite manganite oxides have been investigated. X-ray diffraction (XRD) analysis using Rietveld refinement has shown that all of the samples under investigation crystallize with a rhombohedric structure in the Rc space group (N°167). Paramagnetic (PM) to ferromagnetic (FM) transitions have been observed in all of the synthesized samples. In addition, the maximum magnetic entropy change (ΔSmaxM) for the x = 0 sample was found to reach ∼5.8 J kg−1 K−1 under an applied magnetic field of 5 T, which is sufficient for potential applications in magnetic refrigeration. For the same applied magnetic field (μ0H = 5 T), the relative cooling power (RCP) value is found to be 167 J kg−1. The critical properties of manganites La0.7Ba0.2Ca0.1Mn1−xAlxO3 (0 ≤ x ≤ 0.1) are investigated through various techniques, such as modified Arrott plots, Kouvel–Fisher methods and critical isotherm analyses around the paramagnetic–ferromagnetic phase transition (Tc). The values of critical exponents, derived from magnetic data using the above methods, yield 0.246 ≤ β ≤ 0.253, 1.01 ≤ γ ≤ 1.12 and 4.76 ≤ δ ≤ 4.9 with a Tc of 300–350 K. The exponent values are close to those expected for tricritical mean field model ferromagnets with short-range dipole–dipole interactions.

Electrical properties of La0.67Sr0.16Ca0.17MnO3 perovskite

La0.67Sr0.16Ca0.17MnO3 perovskite was prepared by the solid-state reaction technique. Preliminary room-temperature structural studies confirm the formation of the sample in the rhombohedral structure. The electrical response was studied using the impedance spectroscopy technique over a broad frequency range (250 Hz–3 MHz). The results indicate that the electrical properties of the material are strongly dependent on temperature and frequency. Impedance spectrum analysis shows that the material can be described as a grain and grain boundary medium and permits to estimate the grain boundary contribution. Electronic conduction is found to be dominated by a thermally activated hopping of small polaron at high temperatures.

Physical properties of the doped manganites Ln0.5M0.5Mno3 (Ln = Pr, La; M = Sr, Ca, Ba)

Abstract We present the structural, magnetic and electrical properties of some manganese oxides such as Ln0.5M0.5MnO3 (Ln = La, Pr and M = Ca, Sr, Ba) in which the average ionic radius of the A site cation is varied while keeping the Mn3+/Mn4+ ratio fixed to 1. All the studied samples have a perovskite structure with rhombohedral or orthorhombic distortion. This distortion was related to the A cation size. The lattice volume increases lineraly with ( r A) in all the range 1.179–1.3264 Å. The magnetic investigation shows that the magnetic transition temperature T t, and magnetisation saturation M s have similar behaviour by increasing and decreasing with (r A). The electrical measurements show conduction by small polarons attributed to a large canting angle 9 in all the range 4–300 K in the two ferromagnetic compounds La0.5Sr0.5MnO3 and La0.5Sr0.5MnO3.