Magnetic and magnetocaloric properties in La0.7Ca0.3−xNaxMnO3 exhibiting first-order and second-order magnetic phase transitions

Polycrystalline orthorhombic samples La0.7Ca0.3−xNaxMnO3 (x = 0-0.09) were prepared by solid-state reaction. The study of magnetic properties revealed that the ferromagnetic-paramagnetic (FM-PM) transition temperature (TC) increases from 255 to about 271 K with increasing Na-doping content (x) from 0 to 0.09, respectively. Around the TC, we have found the samples showing a large magnetocaloric (MC) effect with maximum values of magnetic entropy change (|ΔSmax|) 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 xc ≈ 0.06. This could also be observed clearly from the feature of entropy universal curves. An assessment of the magnetic-ordering exponent N = dLn|ΔSm|/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 La0.7Ca0.3−xNaxMnO3 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.