MgO(001)単結晶基板上におけるγʹ-Fe₄N薄膜のエピタキシャル成長

抄録

  Fe-N thin films are prepared on MgO(001) single-crystal substrates at temperatures ranging from room temperature to 600 °C by varying the ratio of N₂ partial to total pressure in sputtering from 0 to 5%. The effects of substrate temperature and N₂ partial pressure ratio on the formation of γʹ phase (sc-based L’1 phase) are systematically investigated. Epitaxial α phase (bcc-based A2 phase) is formed in the films prepared by sputtering in pure Ar gas for all the investigated temperatures. On the other hand, the structure is delicately influenced by the substrate temperature when the films are prepared in Ar-N₂ mixture gases. Epitaxial films are obtained at temperatures higher than 300 °C, whereas the films prepared at temperatures lower than 200 °C involve poly-crystals. When the N₂ partial pressure ratio is 2.5%, the films prepared at temperatures ranging between 200 and 400 °C consist of a mixture of α and γʹ phases and the volume fraction of γʹ phase decreases with increasing the substrate temperature. When the N₂ partial pressure ratio is 5%, γʹ single-crystal films are obtained at temperatures ranging from 300 to 400 °C. The N site ordering parameter slightly increases as the substrate temperature increases from 200 to 400 °C. The films prepared at temperatures higher than 500 °C in the N₂ partial pressure ratios of 2.5 and 5% primarily consist of α phase. The present study has shown that high substrate temperature prevents the formation of γʹ phase and employment of a moderate substrate temperature around 300–400 °C is suitable to obtain a γʹ single-crystal film.