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 (HcJ) of the Nd-Fe-B thin films decreased by about 80% from the level of as-deposited film (HcJ(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 hep 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 NdOx, phase formed at the surface of α-Nd phase after oxidation. After annealing at 350°C, the hcp Nd 2O3 phase crystallized from the fee NdOx phase, and it resulted in large roughness at the boundary with the α-Nd phase. From the results described above, the crystallization of hcp Nd 2O3 phase causes damage at the surface of Nd 2Fe14B phase during the annealing at low temperature, which results in the decrease of coercivity. © 2010 The Japan Institute of Metals.