Influences of Oxidation State of Nd-Rich Phase on the Coercivity of Nd-Fe-B/Nd Thin Films

Abstract

The Nd-rich phase of Nd-Fe-B sintered magnets, which contains some amount of oxygen, plays an important role for generating coercivity. However, the influence of interfacial microstructure between Nd 2 Fe 14 B and Nd-rich phases has not been clear. In this study, Nd 2 Fe 14 B/Nd(-O) interface was prepared by thin film technique and the influence of oxidation state of Nd-rich phase on the coercivity was investigated. A Nd-Fe-B layer was deposited by ultra high vacuum (UHV) magnetron sputtering, and the film was oxidized under low vacuum condition (low oxidation state) or Ar atmosphere (high oxidation state). A Nd layer was deposited on the oxidized Nd-Fe-B layer, and the film was annealed at 250-650°C for 60min. The coercivity of films oxidized in low oxidation state was recovered by annealing at 250-650°C, and an amorphous phase was observed at the interface between Nd 2 Fe 14 B and hcp Nd 2 0 3 (+ fcc NdO x ) phases in the film annealed at 350°C. On the other hand, the coercivity of films oxidized in high oxidation state was lower than that of films oxidized in low oxidation state, and it decreased more about 20% by annealing at 250-350°C. However, an amorphous phase was not observed at the interface of these films. After annealing above 550°C, the coercivity of films oxidized in both low and high oxidation state recovered drastically to the almost same value of as-deposited film. From the SAD patterns of TEM observation, a metastable C-Nd 2 O 3 phase was present in the Nd-rich phase of these films. In addition, it is known that the wettability of Nd-rich phase improves at temperatures around 550°C. Therefore, it is considered that the increase of coercivity is related to the improvement of fluidity of Nd-rich phase or the existence of C-Nd 2 O 3 phase.