Investigation of production, structural, magnetic and magnetocaloric properties of manganite composite compounds exhibiting transition around room temperature

In this study, the compounds La₀.₇Ca₀.₂₇Na₀.₀₃MnO₃, La₀.₇Sr₀.₂Na₀.₁MnO₃, and La₀.₆₇Ca₀.₂₇Sr₀.₀₆MnO₃, identified as the primary phases, were initially synthesized. Subsequently, Bi and Ni were doped into the Mn sites to obtain the series La₀.₇Ca₀.₂₇Na₀.₀₃Mn₁₋ₓBiₓO₃ (x = 0.005, 0.01, 0.02), La₀.₇Ca₀.₂₇Na₀.₀₃Mn₁₋ₓNiₓO₃ (x = 0.0, 0.005, 0.01, 0.02), and La₀.₇Sr₀.₂Na₀.₁Mn₁₋ₓBiₓO₃ (x = 0.02, 0.04, 0.06). Finally, two samples with transition temperatures slightly above and below room temperature were combined in different mass ratios to produce composite compounds with the compositions (x)La₀.₆₇Ca₀.₂₇Sr₀.₀₆MnO₃/(1–x)La₀.₇Ca₀.₂₇Na₀.₀₃MnO₃ (x = 0.75, 0.50, 0.25). Structural characterization of the sol–gel synthesized compounds was performed using X-ray diffraction (XRD), followed by Rietveld refinement using the FullProf software. The morphology of the sample surfaces was examined via scanning electron microscopy (SEM) at different magnifications. Elemental composition and atomic percentage ratios were determined using energy-dispersive X-ray spectroscopy (EDS), confirming that all compounds were consistent with the intended stoichiometry. The temperature-dependent magnetic properties and magnetic phase transition temperatures of the compounds were investigated via M–T measurements. These measurements were conducted under an applied field of 10 mT, following zero-field-cooled (ZFC) and field-cooled (FC) steps, within the temperature range of 5–380 K. Analysis of the Arrott plots confirmed that all samples exhibit a second-order magnetic phase transition. From the isothermal magnetization data near the transition region, the maximum magnetic entropy change (ΔSM ) under an applied field of 5 T was calculated as 8.42 J/kgK for the LCNM sample, whereas the highest relative cooling power (RCP) was obtained for the 0.50LCSM/0.50LCNM composite sample, with a value of 317 J/kg.