Simple Fabrication of ϵ‐Fe₂O₃ Nanoparticles Containing Silica Monoliths with Enhanced Coercivity

Abstract Most of large‐size permanent room‐temperature hard magnets contain rare‐earth elements. Here, we present the preparation of a large‐size rare‐earth free hard magnet based on the loading of the ordered mesoporosity of a preformed ordered mesoporous silica monolith obtained by a sol‐gel route with the Fe(NO₃)₃ salt, followed by a simple calcination in air. The investigation of the influence of the thermal treatment temperature as well as the Fe/Si ratio on the microstructure of the ϵ‐Fe₂O₃/SiO₂ nanocomposites reveals well‐controlled ϵ‐Fe₂O₃ particles size and size distribution. Their magnetic study allows us i) to specify the size of the particles with an enhanced coercivity and ii) to show a linear correlation between the magnetic coercivity of the materials and the mass percent of these particles in the nanocomposites. A ϵ‐Fe₂O₃ based bulk hard magnet with the highest coercive field of 18 kOe at room temperature was obtained by this synthesis process.