Tuning the microstructure and magnetocaloric properties of Na⁺-substituted La_0.67Ca_0.33-xNaxMnO₃ porous nanospheres

This study investigated the effects of Na+ substitution on the microstructure and magnetocaloric properties of La0.67Ca0.33-xNaxMnO3 (x = 0, 0.05, 0.10, 0.15, 0.20) porous nanospheres synthesized by the solvothermal method. The structural analysis showed that Na+ substitution increased the unit cell volume and distorted the lattice structure of La0.67Ca0.33-xNaxMnO3 porous nanospheres. Na+ doping also reduced the diameter uniformity of the porous nanospheres and caused them to relax and disintegrate. The magnetocaloric measurements revealed that Na+ doping increased the Curie temperature values. The La0.67Ca0.23Na0.1MnO3 composition exhibited a maximum magnetic entropy change of 3.57 J/kg.K, while La0.67Ca0.18Na0.15MnO3 achieved the largest relative cooling power of 308.18 J/kg under an applied field H = 5 T. The critical parameters of the La0.67Ca0.33-xNaxMnO3 porous nanospheres were examined using the modified Arrott plot, Kouvel-Fisher method and critical isotherm, indicating significant changes in critical parameters due to Na+ doping. The results suggested that Na+-substituted La0.67Ca0.33_xNaxMnO3 porous nanospheres could be promising candidates for magnetic refrigeration near room temperature.