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[en] The compound Fe2Mn2O7 was prepared using ceramic method Fe2O3 and MnO2 as a precursor. It was crystallized in pyrochlore oxides type monoclinic structure in the P2/M space group. Concerning the magnetic study, they showed that the compound undergoes a ferromagnetic (FM) transition near 68 K. As for the Arrott’s plots, they revealed a second-order phase transition. The variation of the applied magnetic field revealed a magnetocaloric effect that is manifested by a maximum variation of the magnetic entropy of 1 J/kgK for a field change of 5 T, with a cooling power of 63J/kg.
[en] The La2Mn2O7 − δ compound was prepared using ceramic method. The X-ray diffraction study revealed that this compound crystallized in pyrochlore oxide–type monoclinic structure with the P2/M space group. Concerning the magnetization measurements, they showed a paramagnetic (PM) behavior at high temperature and a ferromagnetic (FM) transition near 167 K. As for the magnetic measurements, they demonstrated the possible existence of a Griffiths-like phase, which was observed for the first time, to our knowledge, in a pyrochlore oxide-type A2B2O7. This originality can be due to the competition between the AFM and FM interactions of the Mn ions, induced by oxygen deficiency. The critical isotherm analyses indicated that the 3D-Heisenberg was the most acceptable model in the case of our compound.
[en] 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 theory (β = 0.5, γ = 1, and δ = 3). Above all, the temperature dependence of electrical resistivity shows a metal–insulator transition at Tρ. The electrical resistivity decrease when we apply a magnetic field giving a magnetoresistance effect in the order of 60% at room temperature.
[en] We discuss the effect of the nanometric grain size on the behavior of the electrical and magnetoresistive response of La0.8K0.2−x□xMnO3−δ (x = 0 and 0.1) nanocrystalline samples that were prepared by a sol–gel method. The results from transport and magneto-transport measurements evidence a robust dependence on the nanometric grain size. The temperature dependence of the resistivity was evaluated using different transport models. The results reveal a field-dependent minimum of the resistivity in the low-temperature region, which can be described in terms of intergranular spin-polarized tunneling. Remarkably, a considerable increase of the magnetoresistance (MR) with the decrease of nanoparticle size was found, which might open a new way for the search for potential candidates for magnetoresistive devices. Besides, the magnetic field dependence of the MR was also analyzed, and a distinct drop of MR at low fields was noticed. This behavior was primarily explained by the spin-polarized tunneling transport of conduction electrons across grain boundaries.
[en] H-T magnetic phase diagrams of the Ho0.43Y2.57Fe5O12 garnet, due to spin-reorientation transitions, have been determined in the low temperature range (2-30 K) by magnetization measurement under high static magnetic fields (23 T) on  and  oriented single crystals. It is shown that a very good agreement between computed and observed phase diagrams can be achieved when the free energy is calculated by direct diagonalization of a Hamiltonian including the crystal field (CF) and the exchange interactions considered in the mean-field formalism
[en] Magnetic and magneto-transport properties of Pr0.8−xBixSr0.2MnO3 (x = 0, 0.05 and 0.1) compounds were investigated. All samples exhibited a second-order paramagnetic-ferromagnetic transition with the decrease in temperature. The Curie temperature decreased from 210 to 140 K with the Bi-doping content. The electrical resistivity of the parent sample was described by the phenomenological percolation model, which is based on the phase segregation. The metallic character observed in the pristine sample was reduced, and then entirely suppressed by 10% of Bi. The resistivity data for doped samples were described using Mott’s variable range hopping model at high temperatures and by electron-electron interaction and inelastic scattering mechanisms at low temperatures. A Colossal magnetoresistance (MR) was observed in the studied samples. The parent sample illustrated the predominance of intrinsic MR but in doped compounds, MR curves revealed the coexistence of intrinsic and extrinsic contributions. Bi-doping was found to enhance the extrinsic MR in Pr0.8−xBixSr0.2MnO3 (0 ≤ x ≤ 0.1) samples. (paper)
[en] The effect of the B content on the microstructural, structural, and magnetic properties of partially amorphous Fe92−xNb8Bx (x = 5, 10, 15, and 20) alloys has been investigated by means of scanning electron microscopy, X-ray diffraction, high and low-temperature extraction-type magnetometers. The XRD results reveal the formation of a nanocomposite structure where nanocrystalline bcc α-Fe and Fe2B phases are embedded into an amorphous matrix. The FeB boride is observed for higher boron contents (x = 15 and 20), and the crystallite sizes are in the range of 7–24 nm. As the B content increases, the amorphous phase-relative proportion and coercivity increase, whereas the saturation magnetization decreases. An important magnetic hardening occurs by lowering the temperature from 400 to 5 K for x = 20% B. The variation of the Curie temperature can be attributed to the heterogeneity of the amorphous matrix.
[en] Magnetocaloric effect on SrFe12O19 ceramic have been studied using Monte Carlo simulation. The thermal magnetization, dM/dT, magnetic entropy, and the specific heat of SrFe12O19 ceramic are obtained for several magnetic fields. The temperatures dependence of the magnetic entropy and of the adiabatic temperature for a several magnetic field have been obtained. The field dependence of relative cooling power (RCP) of SrFe12O19 ceramic has been determined for a several magnetic fields. The magnetic hysteresis cycle of SrFe12O19 ceramic has been obtained for a several temperatures. The obtained values are close to the experimental values. The transition paramagnetic to ferromagnetic is found at the Curie temperature. The second phase transition is also obtained around the Curie temperature.
[en] In this paper, we have investigated the influence of substitution of Nd by Sr on the structural, magnetic, and magnetocaloric properties of Nd1 − xSrxMnO3 (x = 0.3 and x = 0.4). These compounds were prepared using the solid state method. The structural, morphological, and magnetic properties of our system were characterized by XRD, MEB techniques, and a magnetometer for magnetic characterization. The Rietveld refinement has revealed the coexistence of both Pnma orthorhombic and R-3c rhombohedral phases. The magnetic data indicate that the compounds exhibit a continuous paramagnetic (PM) to ferromagnetic (FM) phase transition. In addition, the critical behavior in both compounds associated with the magnetic phase transition has been studied by the magnetization isotherms. The critical exponents are deduced using various techniques such as the modified Arrott plot, Kouvel–Fisher plot, and critical isotherm technique. Finally, the results obtained of the magnetocaloric effect are in agreement with the reported values of previous researches. These results show that our samples can be used as a magnet refrigerant at room temperature.