Cation Distribution in Monodispersed MFe₂O₄ (M = Mn, Fe, Co, Ni, and Zn) Nanoparticles Investigated by X-ray Absorption Fine Structure Spectroscopy: Implications for Magnetic Data Storage, Catalysts, Sensors, and Ferrofluids

Spinel ferrite MFe2O4 (M = Mn, Fe, Co, Ni, and Zn) nanoparticles, coordinated with oleylamine, oleic acid, and trioctylphosphine oxide, were effectively synthesized by the microwave-assisted rapid synthesis in 1-dodecanol. Spinel ferrite nanoparticles were structurally analyzed using HRTEM, SAED, HAADF-STEM, XRD, XRF, XPS, and EXAFS measurements, which suggested that MFe2O4 nanoparticles have 3.2–4.3 nm in diameter and a spinel cubic structure with space group Fd3̅m. The metal composition retains an initially charged molar ratio (Fe: M = 2: 1), and both corresponding elements exist within a particle. EXAFS analysis performed quantitative determination of cation distribution in the tetrahedral and octahedral sites of the spinel structure to gain a deep insight into the influence of the cation distribution (i.e., the degree of inversion) to their magnetic properties. Compared with the magnetic properties of the bulk spinel structure, small spinel ferrite particles at nanoscale showed some promising applications for the magnetic data storage, catalysts, sensors, and ferrofluids, due to the significant effect of their particle sizes, surface coordinating environments, and the cation distributions.