Tuning the Crystallite Size, Shape, and Magnetic Properties of Fe₃O₄ Nanoparticles Using Annealing

This study examines the effect of annealing on 135 nm Fe₃O₄ nanospheres and establishes a direct correlation among particle shape, crystallite size, and magnetic properties. Polycrystalline nanospheres and highly crystalline nanocubes with an equivalent diameter/body diagonal of 135 nm were synthesized via solvothermal and thermal decomposition methods, respectively. Scanning electron microscopy (SEM) revealed that the nanospheres developed smoother surfaces and gradually transformed toward a cubic morphology upon annealing, with increasing temperature and duration. Vibrating sample magnetometry (VSM) measurements showed that both saturation magnetization and coercivity increased with annealing as the particles evolved toward cube-like morphology and larger crystallite size, indicating that the magnetic properties of Fe₃O₄ nanoparticles are strongly dependent on crystallite size and shape. Nanospheres annealed between 500 and 850 °C exhibited increases in both crystallite size and saturation magnetization; however, coercivity decreased at 850 °C, where the crystallite size was maximal. Annealing at 700 °C for 12 h resulted in enhanced crystallite size and improved magnetic properties. Prolonged annealing at 700 °C (24 h) yielded the largest crystallite size but led to a significant reduction in saturation magnetization. This study demonstrates a clear correlation between magnetic properties, crystallinity, and morphology in nanoparticles beyond the superparamagnetic size regime (e.g., 135 nm). It further provides a strategy for tuning structural parameters that govern magnetic behavior and establishes an alternative, more facile route to obtain Fe₃O₄ nanospheres with crystallite sizes and magnetic properties comparable to nanocubes obtained via direct synthesis.