On magnetic ordering in heavily sodium substituted hole doped lanthanum manganites

• Higher substitution of more than 50 percent of monovalent ion, sodium for La sites in lanthanum manganites scarce in literature.
• Structural studies using XRD and further structure refinement by Rietveld refinement confirmed orthorhombic pbnm spacegroup.
• Ferromagnetic behavior at room temperature with saturation magnetization decreasing with increase in sodium concentration.
• M vs T measurements using FC ZFC proved coexisting FM/AFM behavior arising from exchange interactions between different valence states of Mn ions.
• Disparity in ratio of Mn valence ions indicated presence of vacancies providing the role of vacancies and oxygen stoichiometry in deciding magnetic inhomogeneity.

Mixed valence manganite system with monovalent sodium substituted lanthanum manganites form the basis of the present work. Lanthanum manganites belonging to the series La1−xNaxMnO3 with x=0.5–0.9 were synthesized using modified citrate gel method. Variation of lattice parameters and unit cell volume with Na concentration were analyzed and the magnetization measurements indicated ferromagnetic ordering in all samples at room temperature. Low temperature magnetization behavior indicated that all samples exhibit antiferromagnetism along with ferromagnetism and it has also been observed that antiferromagnetic ordering dominates ferromagnetic ordering as concentration is increased. Evidence for such a magnetic inhomogeneity in these samples has been confirmed from the variation in Mn3+/Mn4+ ion ratio from X-ray Photoelectron Spectroscopy and from the absorption peak studies using Ferromagnetic Resonance Spectroscopy.