We have investigated the magnetic properties and structure of Fe 3 O 4 films on Si substrates before and after annealing at various temperatures, T a . The saturation magnetization and the coercivity in the Fe 3 O 4 films are observed at all T a . They markedly increase with increasing T a up to 873 K, and their values become maximum at T a ¼ 873 K. Furthermore, they slightly decrease above T a ¼ 873 K. The Fe 3 O 4 phase mainly exists in the films at all T a . In addition, the -Fe 2 O 3 phase (hematite) is also formed in the films above T a ¼ 873 K. On the basis of these results, it is found that the growth of Fe 3 O 4 phase by annealing is mainly dominant in the change of magnetic properties in the Fe 3 O 4 film up to T a ¼ 873 K and that the formation of -Fe 2 O 3 phase influences the change of magnetic properties in the Fe 3 O 4 film above T a ¼ 873 K.

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