Correlation between grain size and Curie temperature in oxide ferromagnetic compounds

Among oxide ferromagnets such as ferrite, there are some materials with the same composition and crystal structure that have different Curie temperatures depending on the firing conditions. For example, it has been reported that the Curie temperature T$_C$ of BaFe$_{12}$O$_{19}$ was enhanced when the particle size became small. However, this report dealt with fine particles of about 5 to 20 nm. Therefore, we have prepared BaFe$_{12}O$_{19} crystals with a large grain size and have systematically investigated the relationship with T$_C$. BaCO$_3$ and α-Fe$_2$O$_3$ were used as raw materials and weighed to achieve the desired stoichiometric composition. These wet-mixed raw material powders were pre-fired at 900°C. After calcination, they were ground in a planetary ball mill and compacted under pressure. They were then sintered for 5 hours at various conditions between 1000 and 1300°C. All the obtained samples had the M-type structure, and there was no difference in the lattice parameter. The grain size was about 0.3~1.0 µm, and the grain size tended to increase with increasing firing temperature. The T $_C$ obtained by magnetization measurement and suggestive thermal analysis was 430~450°C. The T$_C$ decreased with increasing sintering temperature.