Coexisting ferromagnetic and antiferromagnetic interactions in Mott–Hubbard insulating-type FeCr2O4 nanoparticles driven by localized d-electron states

Abstract

Spinel-type oxides are of great interest owing to their multifunctional properties and potential applications in spintronic devices. Here, we report a comprehensive investigation of the structural, electronic, and magnetic properties of polycrystalline FeCr₂O_4−δ (FCO) nanoparticles using X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), resonance photoemission spectroscopy (RPES), and extended X-ray absorption fine structure (EXAFS) analysis. XPS and valence band spectra reveal mixed oxidation states of Fe (Fe²⁺/Fe³⁺) and Cr (Cr²⁺/Cr³⁺), consistent with cluster-model calculations. The absence of finite density of states at the Fermi level confirms the insulating nature of nanoparticles. RPES measurements identify localized/incoherent Fe 3d and Cr 3d states near the Fermi level (3dⁿ⁻¹ final states), associated with the lower Hubbard band. A strong hybridization between Fe 3d–Cr 3d–O 2p orbitals is observed in the 5–9 eV binding energy range, dominated by O 2p contributions (3dⁿL final state). The combined electronic structure derived from occupied and unoccupied states demonstrates that the FCO nanoparticles exhibit a Mott–Hubbard insulating character at room temperature, in accordance with cluster-model calculations. EXAFS analysis at the Fe and Cr K-edges confirms short-range structural distortions and oxygen vacancies. Magnetically, the coexistence of ferromagnetic and antiferromagnetic interactions arises from exchange mechanisms involving the localized Fe 3d and Cr 3d states. These results establish the intrinsic correlations between the local structure, electronic configuration, and magnetism in the FCO nanoparticles, offering insights into their potential for spintronic and colossal magnetoresistance applications.