Influence of Nd Oxide Phase on the Coercivity of Nd-Fe-B Thin Films

Abstract

To understand the coercivity mechanism of Nd-Fe-B sintered magnets, the microstructure of grain boundary composed of Nd₂Fe₁₄B and Nd-rich phases has been studied. However, the influence of Nd-rich phase, which contains some amount of oxygen, on microstructure and coercivity has not been clear. In this study, the influence and the interfacial microstructure between the Nd₂Fe₁₄B phase and the Nd-rich phase were investigated using Nd-Fe-B thin films. Furthermore, the microstructural change of Nd-oxide (Nd-O) phase was investigated using oxidized Nd thin films. The coercivity (H_cJ) of the Nd-Fe-B thin films decreased by about 80% from the level of as-deposited film (H_cJ(as-depo)) after oxidation and annealing at low temperature (∼350°C). From TEM observation of the Nd-Fe-B film, some steps along the surface of the Nd₂Fe₁₄B phase contacting with the hcp Nd₂O₃ phase were observed. Investigation of the microstructural change of Nd oxide phase was carried out using Nd thin films. The as-deposited Nd film was composed of the dhcp Nd (α-Nd) phase, and the fcc NdO_x phase formed at the surface of α-Nd phase after oxidation. After annealing at 350°C, the hcp Nd₂O₃ phase crystallized from the fcc NdO_x phase, and it resulted in large roughness at the boundary with the α-Nd phase. From the results described above, the crystallization of hcp Nd₂O₃ phase causes damage at the surface of Nd₂Fe₁₄B phase during the annealing at low temperature, which results in the decrease of coercivity.