Magnetic and magnetocaloric properties in La_0.7Ca_0.3−xNa_xMnO₃ exhibiting first-order and second-order magnetic phase transitions

Polycrystalline orthorhombic samples La_0.7Ca_0.3−xNa_xMnO₃ (x = 0–0.09) were prepared by solid-state reaction. The study of magnetic properties revealed that the ferromagnetic-paramagnetic (FM-PM) transition temperature (T_C) increases from 255 to about 271 K with increasing Na-doping content (x) from 0 to 0.09, respectively. Around the T_C, we have found the samples showing a large magnetocaloric (MC) effect with maximum values of magnetic entropy change (|ΔS_max|) of 7–8 J kg⁻¹ K⁻¹ and relative cooling power RCP = 232–236 J/kg for the samples x = 0.03–0.09 in a magnetic-field interval ΔH = 40 kOe. Detailed analyses of isothermal magnetization data M(T, H) based on Banerjee's criteria indicated a first-to-second-order magnetic-phase transformation taking place at a threshold Na-doping concentration x_c ≈ 0.06. This could also be observed clearly from the feature of entropy universal curves. An assessment of the magnetic-ordering exponent N = dLn|ΔS_m|/dLnH demonstrates an existence of short-range magnetic order in the samples. We believe that the changes of the magnetic properties and MC effect in La_0.7Ca_0.3−xNa_xMnO₃ caused by Na doping are related to the changes in the structural parameters and Mn⁴⁺/Mn³⁺ ratio, which are confirmed by the geometrical and electronic analyses based on X-ray diffraction and X-ray absorption fine structure. - Highlights: • Geometrical and electronic structures of La_0.7Ca_0.3−xNa_xMnO₃. • Threshold of first-to-second-order phase transformation in La_0.7Ca_0.3−xNa_xMnO₃. • Large magneto-caloric effect with |ΔS_max| ≈ 7–8 J kg⁻¹ K⁻¹, and RCP = 232–236 J/kg. • Universal curve of magnetic-entropy change.