Synthesis of nanocrystalline barium-hexaferrite from nanocrystalline goethite using the hydrothermal method: particle size evolution and magnetic properties

To characterize particle size/magnetic property relationships, 9 to 50 nm in diameter barium hexaferrite, BaFe₁₂O₁₉ (BHF), particles were prepared using a new synthesis route. By replacing the conventional 50 to 100 nm particles of goethite with nanocrystalline goethite produced via the microwave anneal method of Knight and Sylva [1], nanocrystalline BHF was synthesizer using the hydrothermal method. Evolution of particle size and morphology with respect to concentration and heat treatment time is reported. Hysteresis properties, including coercivity (0.2 - 1.0 kOe), magnetization saturation (0.1-33.4 emu/g), and magnetization remanence (0.004 - 22.5 emu/g) are discussed as a function of particle size. The magnetization saturation and remanence of the 7 nm particles is nearly zero, suggesting the superparamagnetic threshold size for BHF is around this size. In addition, the equilibrium morphology of BHF crystals was calculated to be truncated hexagonal prisms which was verified by experiment, and the isoelectric point, pH of 4.1, was measured for 18 nm BHF particles.