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 HC 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 HC 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 HC 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.