An investigation on the behavior of fine-grained magnetite particles as a function of size and surface modification

By using a KNO₃-aging ferrous hydroxide gel method, Fe₃O₄ particles with sizes ranging from 35 to 1500 nm were synthesized. The particles were covered with a silica coating to form Fe₃O₄-SiO₂ core-shell structures by using the improved conventional Stöber polycondensation method. The thickness of the SiO₂ covering on magnetite particles surface varies from 10 to 20 nm. The morphology, size and composition of the particles were determined by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The particles with and without coating with SiO₂ were pressed into slices with an oil press at 10 MPa. Subsequently, the coercive forces H_C of the particles were measured by VSM at room temperature, and the critical size for a single domain was estimated. The shape of the particles is basically spherical when the size is smaller than 800 nm, while it is hexagonal for larger particles. The H_C of Fe₃O₄-SiO₂ core-shell structure was larger than that of the uncoated Fe₃O₄ particles by 20%, which was explained to be due to the reduction of inter-particle magnetostatic interaction, supported by an agreement with the packing factor. The dependence of H_C on magnetic particle size could be explained and fitted by the Heewell-Knozam stacking density equation and object-oriented micromagnetic computing framework (OOMMF) micromagnetic software. the results agree well with the experimental data.