Effects of diffusing DyZn film on magnetic properties and thermal stability of sintered NdFeB magnets

The DyZn film on sintered NdFeB magnet surface was prepared through DC magnetron sputtering. The magnetic properties, thermal stability and microstructure of annealed and DyZn diffused magnets were compared. The coercivity enhances significantly from 1174.10 kA/m to 1711.40 kA/m by a striking increment of 45.7% with only a slight reduction in remanence from 1.20 T to 1.19 T after DyZn thermal diffusion treatment. The thermal stability of NdFeB magnets can be improved by grain boundary diffusion. The temperature coefficient of coercivity (β) and remanence (α) are improved from −0.506%/°C, −0.120%/°C to −0.481%/°C, −0.105%/°C in the range of 20 °C-180 °C, respectively. The irreversible magnetic flux losses of DyZn diffused magnet are reduced to 37.5% at 180 °C for 2 h and 29.8% in HAST for 288 h, respectively. Dy elements diffusing along the grain boundary phase mainly aggregate between the grain boundary phase and the main grain epitaxial layer via SEM and EPMA observations. The optimization of microstructure and composition of grain boundary phases and the formed (Nd,Dy)2Fe14B transition layer are the main reasons for the significant enhancement of intrinsic coercivity.