A high coercivity of up to $5.1\mathrm{kOe}$ was induced in a large-grained ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ powder after milling for a short time $\left(1.5\mathrm{h}\right)$. It was found that the initial grain (particle) size played an important role in the microstructural evolution and in the magnetic properties of the milled ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ materials. The milling-induced microstructural evolution was analyzed using x-ray diffraction and transmission electron microscopy. The results indicated that the milling-induced high coercivity was associated with the highly-strained and defective microstructure. The enhancement in magnetic anisotropy was observed in large-grained ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ after milling, which might be mainly attributed to the stress anisotropy. In order to understand the coercivity mechanisms, detailed magnetic studies were carried out by the investigation of the field-dependent magnetization (demagnetization) behaviors and the magnetization reversal processes based on both the micromagnetic model and the phenomenological model. The results revealed that a domain wall pinning-controlled mechanism was responsible for the milling-induced high coercivity in ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ materials.

• Received 24 February 2006

DOI:https://doi.org/10.1103/PhysRevB.74.184427