Characterization of the structural, magnetic and magnetocaloric properties of double perovskite La_1.95Sr_0.05BMnO₆ (B = Ni and Co)

An exhaustive study of structural, magnetic and magnetocaloric properties on La_1.95Sr_0.05BMnO₆ (B = Ni and Co) double perovskite were performed. The samples were prepared by the sol–gel method. The crystallographic structure was studied by the X-ray diffraction patterns and Rietveld refinement which revealed that all samples crystallize in a monoclinic structure with P2₁/n space group. The magnetic behaviors of these double perovskite have been studied in detail. For La_1.95Sr_0.05NiMnO₆, the M(T) curves exhibit double magnetization transition temperature at 68 K and 266 K which can be ascribed to Ni³⁺–O–Mn³⁺ and Ni²⁺–O–Mn⁴⁺ superexchange interaction, respectively. However, unique magnetic transition has been observed for the La_1.95Sr_0.05CoMnO₆ double perovskite at 210 K due to Co²⁺–O–Mn⁴⁺ superexchange interaction. A deep investigation based on the Landau Theory and Arrot analysis confirmed a second order ferromagnetic phase transition for both samples. Besides, the magnetocaloric behaviors of these new samples have been studied by analysis the magnetic entropy change. This latter reached maximum values of 1.01 and 1.35 J/kg/K for La_1.95Sr_0.05NiMnO₆ and La_1.95Sr_0.05CoMnO₆, respectively, under µ₀H = 5 T. Moreover, the relative cooling power values for La_1.95Sr_0.05NiMnO₆ and La_1.95Sr_0.05CoMnO₆ are found to be 94 J/kg and 116 J/kg, respectively, under µ₀H = 5 T. Based on the obtained ΔS_M data, we have also described the universal master curve for (ΔS_M/\(\Delta S_{M}^{{\hbox{max} }}\)) versus rescaled temperature to confirm the order magnetic phase transition. Interestingly, all the ΔS_M(T, H) data points are collapsed into a universal curve in the whole temperature range. The significant values of relative cooling power for both samples suggest that they might be an interesting candidate for exploring a new kind of magnetic refrigerants.