Abstract
The Curie temperature (Tc) of magnetite (Fe3O4) ultra-fine particles prepared by the co-precipitation method was investigated. The sizes of particles, which are controlled by the ratio of FeSO4·7H2O and NaNO2, ranged from 5 nm to 50 nm. The Curie temperature (Tc) is determined by the relationship M ∼ (1-T/Tc)β. The temperature (Tc) decreased with the size of Fe3O4 particles. It is considered that the Curie temperature can be controlled by varying the size of ultra-fine particles. On the other hand, when the particles were heated at high temperature, the particle size became large on account of the sintering effect. This caused an apparent increase in the Curie temperature.
