Interfacial microstructure of Nd ₂Fe ₁₄B/Nd-Rich and coercivity in Nd-Fe-B films

This study provides the relationship between phase change of Nd-rich and microstructural changes of Nd ₂Fe ₁₄B/Nd-rich interface in Nd-Fe-B films with different coercivity values. In as-deposited Nd-Fe-B film, flat interface of Nd ₂Fe ₁₄B/dhcp Nd is mainly observed. After oxidation, oxidized region was observed at the surface of Nd ₂Fe ₁₄B phase, and Nd ₂Fe ₁₄B/fcc NdO _x interface were observed. In the Nd-Fe-B film subsequently annealed at 350°C after oxidation, the coercivity of which decreases to around 20% of the value of as-deposited film, and the hep Nd ₂O ₃ phase exists dominantly at the surface of Nd ₂Fe ₁₄B phase. In addition, there are some damaged regions at the surface of Nd ₂Fe ₁₄B phase. On the contrary, both the metastable C-Nd ₂O ₃ and amorphous phases existed dominantly at the Nd ₂Fe ₁₄B/Nd-rich interface of the film annealed at 650°C. The coercivity of this film recovered to the same value of as-deposited film. These results indicate that the crystallization of hep Nd ₂O ₃ phase include some damages to neighboring Nd ₂Fe ₁₄B phase decrease coercivity. During annealing at 550 to 650°C, Nd-rich liquid phase appears and modifies the surface of Nd ₂Fe ₁₄B, and the coercivity recovers. After cooling, this liquid phase seems to crystallize into a C-Nd ₂O ₃ or an amorphous phases at the surface of Nd ₂Fe ₁₄B phase. In other words, the C-Nd ₂O ₃ or amorphous phases can exist without decreasing coercivity.