Enhanced magnetic properties and improving thermal stability for sintered Nd-Fe-B magnets prepared by two-step grain boundary diffusion processes

Two-step grain boundary diffusion processes (GBDP) were carried out to obtain the diffused magnets with excellent magnetic properties and high thermal stability, using Dy and Al coated films as diffused sources in turn, prepared by magnetron sputtering. It is found that through the two-step diffusion with Dy/Al films, the highest coercivity, up to 1382 kA/m is achieved, increasing by 42.3% compared to as-prepared magnet, moreover, the maximum energy product is improved significantly as well. Besides, whether for as-prepared magnets or one-step diffused magnets with Al film and Dy film, respectively, the thermal stability and magnetic properties of two-step diffused magnet with Dy/Al film, are the highest among magnets. Microstructural investigation reveals that Dy is preferred to diffuse along the RE-rich phase and forming the "shell" of (Nd, Pr, Dy)(2)Fe14B phase around the outer region of main phase, restraining the nucleation of anti-magnetization domain, while the Al tends to locate in the RE-rich region, playing the role of modifying the interface between RE-rich phase and 2:14:1 main phase. Eventually, the temperature coefficient of coercivity (vertical bar beta vertical bar) and remanence (vertical bar alpha vertical bar) with 0.65%/K and 0.12%/K, can be obtained, respectively.