Structural, magnetic and magnetocaloric properties of La_1.95Ca_0.05BMnO₆ (B = Ni and Co) double perovskite

We have synthesized La_1.95Ca_0.05BMnO₆ (B = Ni and Co) double perovskite using sol–gel method. The structural properties were investigated by X-ray diffraction. Ritveld analysis show that ours samples crystallize in the monoclinic system with P2₁/n space group. Refinement of X-ray results reveal that the cell parameters, the unit cell volume and the crystallite size increase with the ionic radius of B-site. In addition, a paramagnetic to ferromagnetic (PM–FM) phase transition is observed at 275 and 201 K for La_1.95Ca_0.05NiMnO₆ and La_1.95Ca_0.05CoMnO₆, respectively. The magnetization measurements, Arrott analysis and Landau theory suggest ferromagnetic phase transition with second order for all the synthesized samples. The magnetic entropy change was evaluated from the measurement of isothermal magnetization. The maximum values of magnetic entropy change are found to be \({-\varDelta S}_{M}^{max}\)= 1.18 and 0.91 J/Kg K for La_1.95Ca_0.05NiMnO₆ and La_1.95Ca_0.05CoMnO₆, respectively, under µ₀H = 5 T. We have also described the universal master curve for ΔS_M, which is the evolution of (ΔS_M/\({\varDelta S}_{M}^{max}\)) versus rescaled temperature θ = (T − T_C)/(T_r − T_C), where T_r is the reference temperature. Interestingly, all the ΔS_M (T, H) data points are collapsed into a universal curve in the whole temperature range.