E.K. Hlil

Influence of Ca-deficiency on the magneto-transport properties in La0.8Ca0.2MnO3 perovskite and estimation of magnetic entropy change

La(0.8)Ca(0.2 - x)□(x)MnO(3) (x = 0.00, 0.10, and 0.20) perovskite was prepared by the conventional solid-state reaction and annealed at 1473 K. X-ray diffraction and scanning electron microscopy shown the existence of a secondary phase attributed to the unreacted Mn(3)O(4) oxide. The magneto transport properties have been investigated based on the temperature dependence of the resistivity ρ(T) measurements under several applied magnetic fields. We note that the La(0.8)Ca(0.2)MnO(3) (x = 0.00) sample has a classical metal-insulator transition at T(ρ). But we have observed that the lacunars samples (x = 0.10 and 0.20) include a metallic and insulator behavior simultaneously below T(ρ) and the resistivity is dominated by tunneling through the barriers associated with the insulating phase. In other words, the calcium deficiency favors the enhancement of the insulator behavior. The electrical resistivity is fitted with the phenomenological percolation model, which is based on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions. Furthermore, we found that the estimated results are in good agreement with experimental data. Above all, the resistivity dependence on the temperature and magnetic field data is used to deduce the magnetic entropy change. We have found that these magnetic entropy change values are similar to those calculated in our previous work from the magnetic measurements. Finally, we have found an excellent estimation of the magnetic entropy change based on the Landau theory.

Nonmagnetic thin layers of Ni3N

From structural analysis by x-ray diffraction, Rutherford backscattering spectroscopy, and physical investigations (magnetization and resistance), reactive sputtering assisted by microwave multidipolar plasma appears to be a powerful means for tailoring the stoichiometry of transition metal nitrides. Reactive sputtered nitride Ni layers were elaborated while keeping the Ar partial pressure constant (∼1.5 μbar) and varying the N2 partial pressure in the 0.1–0.5 μbar range. A mixing of Ni2N and Ni3N and a single-phase Ni3N are stabilized in thin layers for high and low p(N2), respectively. For the first time, an optimization of the crystallization of Ni3N pure layers is obtained for intermediate p(N2) (0.23 μbar). Both nitrided phases Ni2N and Ni3N, elaborated with appropriate deposition parameters, do not present ferromagnetic behavior in contrast to experimental results in the literature. Theoretical results using the full-potential linearized augmented plane wave method confirm the nonferromagnetic behav...

Enhanced relative cooling power in Ga-doped La0.7(Sr,Ca)0.3MnO3 with ferromagnetic-like canted state

The effect of diamagnetic gallium “Ga” substitution on the microstructural, magnetic and magnetocaloric properties in perovskite, La0.7(Sr,Ca)0.3MnO3, has been investigated. The La0.7(Sr,Ca)0.3Mn1−xGaxO3 compounds undergo a rhombohedral-to-orthorhombic structural transition at x = 0.15. The partial substitution of Mn by Ga leads to a monotonic decrease in the Curie temperature (TC) of the compounds from 335 K for x = 0.00 to 138 K for x = 0.20. The ferromagnetic transition was broadened when introducing Ga content. Magnetic entropy (ΔSM) curves show a peak around the ferromagnetic–paramagnetic Curie temperature, which decreases from 5.16 J kg−1 K−1 for x = 0.00 to 2.08 J kg−1 K−1 for x = 0.20, with an applied magnetic field of μ0ΔH = 5 T. Because of the relatively easy possibility of tuning the Curie temperature and the interesting values of relative cooling power (RCP), these compounds can be considered as suitable candidates for room-temperature magnetic refrigeration. Moreover, our results suggest that the magnetocaloric effect (MCE) is an effective analysis to gain a basic understanding about the magnetic ordering.

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.

Effect of strontium deficiency on the structural, magnetic and magnetocaloric properties of La0.65Eu0.05Sr0.3−xMnO3 (0 ≤ x ≤ 0.15) perovskites

We have investigated the effect of nominal strontium deficiency on the structure, magnetic and magnetocaloric properties of La0.65Eu0.05Sr0.3−xMnO3 (x = 0, 0.10 and 0.15) perovskites. The nanocrystalline samples were prepared by the sol–gel-based Pechini method. Rietveld refinement of the X-ray diffraction patterns, shows the formation of single-phase compositions with rhombohedral symmetry (space group Rc, no. 167). Raman spectra at room temperature reveal a gradual change in phonon modes with increasing nominal strontium deficiency. All the samples undergo paramagnetic–ferromagnetic (PM–FM) transition. The Curie temperature decreases linearly with increasing x and changes from 355 K for x = 0 to 280 K for x = 0.15. Arrott plot analyses and a universal curve method were applied for studying the order of the magnetic transition in this system, found to be of second order. As strontium deficiency content increased further, peak entropy values were seen to decline. However, a simultaneous broadening of the ΔSmaxM peaks led to enhanced relative cooling power (RCP) in the Sr-deficiency samples of up to 22% over that of La0.65Eu0.05Sr0.3MnO3. La0.65Eu0.05Sr0.15MnO3 exhibits the largest RCP value of 283 J kg−1 at 280 K among the compounds investigated up to 5 T applied field. Through these results, La0.65Eu0.05Sr0.3−xMnO3 materials are strongly suggested for use as active refrigerants for magnetic refrigeration technology near and above room temperature.

Investigation of the critical properties near room temperature in La0.7Ca0.2Ba0.1MnO3 manganite

In this paper, we studied in detail the critical behavior of La0.7Ca0.2Ba0.1MnO3 manganite sample synthesized by the solid state reaction. The critical properties near the ferromagnetic–paramagnetic phase transition have been analyzed from the magnetic data, using various techniques such as the modified Arrott plots, the Kouvel fisher method, critical isotherm analysis and the field dependence of magnetocaloric results. Moreover, the validity of the critical exponents using the various methods has been confirmed by the scaling equation of state and all data fall on two distinct branches, one for T < TC and the other for T > TC, indicating that the critical exponents obtained in this work are accurate. The exponents determined in this study have revealed that the β value is in between the theoretically expected values for 3D-Ising and tricritical mean field model. While, the γ value is close to the mean field theory. The third exponent δ was determined independly from the critical isotherms satisfying the Widom scaling relation. Our systematic result reflects the coexistence of a long range and short range ferromagnetic order in our sample. The temperature variation in the effective exponents βeff and γeff resemble with those for disordered ferromagnet around Curie temperature.

Enhanced magnetoresistance induced by oxygen deficiency in La0.4Ca0.6MnO3-δ oxides

We report electrical features and magnetoresistance behavior of the oxygen deficient La0.4Ca0.6MnO3-δ perovskites (δ = 0, 0.15, and 0.2). These samples will be referred to as S0, S15, and S20, respectively. The dependence of electrical transport on temperature and magnetic field is systematically investigated between 2 K and 400 K in magnetic field ranging up to 5 T. The parent compound shows a stable charge ordering/antiferromagnetic state with a semiconductor-like behavior in all considered temperature range. The variable range hopping and thermally activated hopping models are found to fit well with the electrical resistivity data at low and high temperatures, respectively. Oxygen deficiency tends to weaken the charge ordering and induce ferromagnetism and metallicity at low temperature. Metal insulator transition appears at higher fields for lower oxygen deficit (S15 sample) and without field for the S20 sample. The resistivity data for S15 sample are discussed in the framework of the variable-range h...

Room temperature magnetocaloric effect, critical behavior, and magnetoresistance in Na-deficient manganite La0.8Na0.1MnO3

The La0.8Na0.1MnO3 oxide was prepared by the solid-state reaction and annealed in air. The X-ray diffraction data reveal that the sample is crystallized in a rhombohedral structure with R3¯c space group. Magnetic study shows a second-order magnetic phase transition from ferromagnetic to paramagnetic state at the Curie temperature TC = 295 K. In addition, the magnetizations as a function of temperature and the magnetic field is used to evaluate the magnetic entropy change ΔSM. Then, we have deduced that the La0.8Na0.1MnO3 oxide has a large magnetocaloric effect at room temperature. Such effect is given by the maximum of the magnetic entropy change ΔSMmax = 5.56, and by the Relative cooling power (RCP) factor which is equal to 235 under a magnetic field of 5 T. Moreover, the magnetic field dependence of the magnetic entropy change is used to determine the critical exponents β, γ, and δ which are found to be β = 0.495, γ = 1.083, and δ = 3.18. These values are consistent with the prediction of the mean field...

An efficient composite magnetocaloric material with a tunable temperature transition in K-deficient manganites

This paper reports on a magnetic material with high steady relative cooling power (RCP) over a temperature range from 325 to 275 K induced by potassium-deficiency in polycrystalline samples of La0.8K0.2−x□xMnO3 (x = 0, 0.1 and 0.2). The possibility to adjust the temperature transition close to room temperature exists by changing the potassium deficiency rate. All the samples were synthesized using the citrate-gel method. X-ray diffraction and magnetization measurements were conducted to examine the crystallographic structure and magnetocaloric properties. All the samples were found to be a single phase and crystallize in rhombohedral symmetry with Rc space group. From the magnetic measurements, a second-order magnetic phase transition from the ferromagnetic to paramagnetic state was noticed at the Curie temperature (TC), which was found to decrease from 325 to 275 K when the potassium-deficiency rate increased. Besides, the magnetocaloric effect (MCE) as well as the RCP has been estimated. The obtained results have confirmed that our manganite with potassium-deficiency has significant advantages for magnetic refrigeration. Finally, the critical behavior associated with the magnetic phase transition reveals that the estimated critical exponents are consistent with the prediction of the mean field theory (β = 0.5, γ = 1, and δ = 3) for the x = 0.00 and x = 0.10 samples, while the estimated critical exponents for x = 0.2 were consistent with the prediction of the 3D-Heisenberg model (β = 0.365, γ = 1.336 and δ = 4.8).

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.