Impact of oxygen deficiencies on magnetic properties of La0.725□0.275MnO3−δ compounds

Abstract

In this study, oxygen-deficient La_0.725□_0.275MnO_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 Rc 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.