Abstract
In this study, oxygen-deficient La(0.725)□(0.275)MnO(3-δ) (δ = 0.00, 0.15, 0.25 and 0.35) compounds were synthesized using the sol-gel method. The elemental composition was confirmed by energy-dispersive X-ray spectroscopy (EDX) measurements, while X-ray photoelectron spectroscopy (XPS) measurements were employed to quantify and validate the oxygen deficiency levels. X-ray diffraction (XRD) analysis revealed that all samples crystallize with a rhombohedral structure in the R3̄c space group and exhibit nanometric crystallite sizes. Magnetic measurements demonstrated that the Curie temperature (T (C)) and magnetization (M) are strongly dependent on the oxygen deficiency (δ). The field-cooled/zero-field-cooled (FC/ZFC) magnetization curves reveal a pronounced magnetic irreversibility in all compounds, which becomes more marked as the δ value increases. This behavior is closely related to the enhancement of the magnetic anisotropy (MA) with increasing δ. Furthermore, analysis of the inverse magnetic susceptibility (χ (-1)(T)) shows clear deviations from the Curie-Weiss law at temperatures above the magnetic transition. These deviations are clearly observed for the compounds with δ = 0.00 and 0.15, become significantly weaker for δ = 0.25, and completely disappear for δ = 0.35. This evolution indicates that the magnetic inhomogeneities responsible for the non-Curie-Weiss behavior are progressively suppressed as δ increases, concomitant with strengthening of the magnetic anisotropy. Meanwhile, hysteresis loop measurements revealed a difference between the theoretical and experimental magnetization saturation values for the samples with δ = 0.00, 0.15 and 0.35. This disparity was assigned to a significant antiferromagnetic (AFM) contribution and to magnetic disorder on the nanoparticle surface. In contrast, the good agreement between the theoretical and experimental magnetic saturation for the compound with δ = 0.25 was attributed to the predominance of double exchange (DE) interactions.