Search

In this paper, we report the magnetic properties of Ca vacancies in La0.65Ca0.35−x□xMnO3 (0 ≤ x ≤ 0.15) powder samples synthesized using the sol-gel reaction. Rietveld refinement of powder X-ray diffraction data reveals that all samples crystallize in the orthorhombic structure with Pbnm space group. Then, the magnetic properties of these compounds are discussed in detail, building on the magnetization and the susceptibility. The temperature dependence of magnetization reveals a magnetic transition from ferromagnetic to paramagnetic phase when increasing temperature. The inverse of magnetic susceptibility shows a downturn with decreasing temperature before the Curie temperature TC indicating the existence of Griffiths phase for the temperature range TC ≤ T ≤ TG, where TG is the Griffiths temperature, which proves the existence of ferromagnetic clusters in the paramagnetic domain. Furthermore, by investigating the magnetocaloric parameters, the field dependences of ΔSMMax and RCP can be expressed by the power laws, ΔSMmax ≈ a (µ0H)n and RCP ≈ b (µ0H)m, where a and b are coefficients, n and m are the field exponents, respectively. [ABSTRACT FROM AUTHOR]

Improvement of magnetocaloric properties with tunable Curie temperature near room temperature is revealed to be possible by the mixture of both individual phases La CaMnO (phase 1) and LaCaSrMnO (phase 2). This work demonstrates the outstanding agreement between the experimental results and the continuous curves predicted by numerical calculations in a two phases composite system (1 − x)La CaMnO/xLaCaSrMnO (0 ≤ x ≤ 1). Our samples were synthesized using the sol-gel process and the solid-state reaction for phase 1 and phase 2, respectively. For both samples, a clear ferromagnetic (FM)-paramagnetic (PM) transition at the Curie temperature (T) is observed. Such a T is found to be 274.7 and 308 K for La CaMnO and LaCaSrMnO, respectively. Besides, the magnetic entropy change and the Relative Cooling Power (RCP) values were estimated for the biphasic system (1 − x)La CaMnO/xLaCaSrMnO with 0 ≤ x ≤ 1. We revealed that the optimum magnetocaloric effect at µH = 2 T has been found for the composition with x = 0.65, which displays a table-like ΔS curve with a peak value slightly lowered and strongly expanded around room temperature when compared with those of the parent compounds. Therefore, the obtained results will draw considerable attention in the field of research regarding the development of magnetic refrigeration devices. [ABSTRACT FROM AUTHOR]

In this paper, we studied in detail the critical behavior of LaCaBaMnO 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 < T and the other for T > T, 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 β and γ resemble with those for disordered ferromagnet around Curie temperature. [ABSTRACT FROM AUTHOR]

Magnetic and magnetocaloric properties of La□CaMnO (≤ x ≤ 0.15) perovskite manganites were investigated. Our polycrystalline samples were elaborated using sol-gel method. The Rietveld refinement shows that all our samples are single phase and crystallize in the orthorhombic structure with Pbnm space group. Lanthanum deficiency leads to a decrease of the unit cell volume. The surface morphology was carried out using scanning electron microscopy (SEM). Magnetization measurements versus temperature in a magnetic applied field of 50 mT showed that all our deficient samples exhibit a paramagnetic-ferromagnetic transition with decreasing temperature. With increasing lanthanum deficiency, the Curie temperature T decreases from 275 K for x = 0 to 257 K and for x = 0.15. A large magnetic entropy change |Δ S|, deduced from isothermal magnetization curves, has been observed in our samples with a peak centred around their respective transition temperature. The maximum of the magnetic entropy change $ \vert {\Delta } S_{M}^{\text {Max}}\vert $ under a magnetic field change of 5 T is found to decrease from 5.11 J/kg/ K for x = 0 to 3.98 J/kg K and for x = 0.15. The relative cooling power decreases from 237.5J/kg for x = 0 to 217.4 J/kg and for x = 0.15. [ABSTRACT FROM AUTHOR]