Effect of Ca2+ Doping on the Structural, Magnetic and Magneto–Transport Properties of La1−xCaxMnO3 Manganites: Insights from XPS and 55Mn IFNMR Studies

In this study, La1-xCaxMnO3 (LCMO) samples (x = 0.3, 0.4 and 0.5) were synthesized using a sol–gel method and their structural, electronic, magnetic and magneto–transport properties were investigated. Structural analysis by XRD confirmed orthorhombic perovskite structure. FTIR studies indicated the changes in bond length and bond angle of Mn–O/Mn–O–Mn bonds; FESEM/EDAX confirmed a polycrystalline nature. Magneto–transport studies revealed a decrease in ferromagnetic metal–paramagnetic semiconductor transition temperature with the increase in Ca2+ content. Magneto–transport studies also revealed that the resistivity in the ferromagnetic metallic region was predominantly governed by the extrinsic spin–polarized tunneling (SPT) mechanism along with double-exchange interaction, whereas charge transport in the paramagnetic semiconducting region at higher temperatures was due to variable-range hopping conduction. Magnetoresistance was at its maximum near the transition temperature, but decreased at lower temperature (77 K). XPS and 55Mn IFNMR confirmed coexistence of Mn3+/Mn4+, while IFNMR results at 77 K confirmed high-frequency electron exchange among Mn3+ and Mn4+ ions due to double exchange interaction. VSM studies at 300 K showed that the paramagnetic nature of the samples and susceptibility decreased with the increase in Ca2+ content. The close agreement between the magnetoresistance behavior and the 55Mn IFNMR results confirms the role of the intrinsic DE interaction in governing the magnetotransport properties of LCMO.