Unveiling A-site substituent's influence on magnetocaloric response in La2-xNaxNiMnO6 (0.0 ≤ x ≤ 1.0) double perovskite manganite materials

In this study, the compounds of La2-xNaxNiMnO6 (x = 0.0, 0.1, 0.2, 0.3, 0.5, and 1.0) double perovskite manganites have been produced by employing the sol-gel method to investigate their magnetocaloric effect. From X-Ray Diffraction analysis, all compounds crystallized in rhombohedral structure form with the R3<overline>c\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\rm{R}}\bar{3}{\rm{c}}$$\end{document} space group confirmed using the Rietveld refinement technique. The compounds show a magnetic phase transition from ferromagnetic to paramagnetic according to the temperature-dependent magnetization measurement. The phase transition temperatures sharply increased with Na doping (x = 0.1) to 277.8 K which can be considered as under room temperature level and then slowly decreased with further Na concentration to 262.3 K. Using an external magnetic field-dependent magnetization measurements, the isothermal magnetization curves were obtained, and these curves used to find the magnetic entropy change values, which give info about compounds' characteristics of magnetic cooling performance. Under 5 T magnetic field change, the maximum magnetic entropy change value increased 10 times with a small amount of Na doping (x = 0.1), and compounds' values changed from 0.21 to 1.29 Jkg-1K-1 for x = 0.0 to 1.0 compounds, respectively. Arrott plots were graphed by using isothermal magnetization curves to demonstrate all the compounds have a 2nd order magnetic phase transition which supports that these compounds can be candidates as magnetic refrigerants.