Shape-directed modification of truncated octahedral to coffin-like cobalt-doped ferrite particles by changing the hydrothermal reaction conditions.

Truncated octahedral cobalt ferrite-based nanoparticles were synthesised using a precursor-derived coprecipitation reaction followed by a hydrothermal step. The nanoparticles were characterised regarding their shape anisotropy and size distribution as a function of reaction parameters such as time, temperature, metal salt concentration, base molarity, pressure, and reactor filling volume. Notably, temperature and molarity serve as critical synthesis factors that reduce the polydispersity of the particles to values below 0.1, which is an exceptionally favourable result compared to aqueous coprecipitation reactions. Furthermore, an increase in filling volume resulted in higher proportions of coffin-like nanoparticles due to alterations in flow velocity. The crystallographic assignment of the nanoparticle facets was analysed using high-resolution transmission electron microscopy, selected area diffraction, and angle-resolved scanning electron microscopy measurements. This analysis revealed that the {222} facet exhibited preferential growth in the coffin-like particles. By elucidating the significant reaction parameters for enhanced shape anisotropy, it is possible to increase the magnetic anisotropy constants by assuming a prolate spheroid structure with uniaxial shape combined with cubic magnetocrystalline anisotropy. Contributions of the strain and surface anisotropy are discussed briefly to ensure a comprehensive overview.