Green synthesized ferrites have attracted significant attention for suitable electronic and memory applications; however, a direct correlation between morphology-driven defect states, dielectric response, and resistive switching behavior remains insufficiently explored. In this work, octahedral (NiFe2O4, NFO) nanoparticles were synthesized via a neem Azadirachta indica leaf extract-assisted sol–gel route and subsequently integrated into \(\hbox {Al}_{2}\hbox {O}_{3}/\hbox {Al/NiFe}_{2}\hbox {O}_{4}\) /Ag thin-film devices using RF magnetron sputtering. X-ray diffraction (XRD) and Rietveld refinement confirmed the formation of a single-phase cubic spinel (Fd \(\bar{3}\) m) structure with an average crystalline size of 51 nm. FTIR revealed characteristic tetrahedral and octahedral metal-oxygen stretching vibrations, while Raman spectroscopy confirmed the inverse spinel structure through well-defined A1g, F2g, and Eg phonon modes, indicating high phase purity and lattice ordering. FE-SEM analysis revealed clear octahedral morphology, contributing to the enhancement of grain-boundary-induced defect states. UV–vis diffuse reflectance spectroscopy indicated a direct optical bandgap of 2.0 eV, and the evaluated Urbach energy (0.32 eV) confirmed the presence of localized defect states associated with structural disorder. Magnetic studies demonstrated soft ferrimagnetic behavior with a saturation magnetization of 51 emu/g and coercivity of 27.29 Oe. Dielectric measurements exhibited frequency-dependent dispersion dominated by Maxwell–Wagner interfacial polarization. The fabricated Al2O3/Al/NiFe2O4/Ag device shows stable bipolar resistive switching with an ON/OFF ratio of \(\sim\) 1.5 and endurance over 50 cycles. Conduction analysis revealed ohmic, Schottk2y emission, Poole–Frenkel emission, and space-charge-limited current conduction mechanisms governed by oxygen vacancy-mediated charge transport. These results establish a structure and vibrational property correlation to device, highlighting eco-synthesized octahedral NiFeO4 as a promising candidate for next-generation resistive memory and neuromorphic computing devices.
