Preparation of Mn-diffused Sm-Fe-N core-shell powder by reduction-diffusion process

Although the addition of other elements such as Mn to Sm2Fe17Nx compounds can change the magnetic properties, it also decreases saturation magnetization. In order to exploit the advantages of additional elements in Sm2Fe17Nx powder while maintaining a high saturation magnetization, a structure of Sm2Fe17Nx-core/Sm2(Fe,M)17Nx-shell is promising. This is the first report of such a core-shell powder obtained by a reduction-diffusion process. Mn3O4 was mixed with Sm-Fe and Sm2O3 powders followed by the reduction by Ca above 860 °C, and then samples were nitrided and washed after reduction-diffusion process. Core-shell Sm-Fe-N fine powder with a Mn-enriched Sm2(Fe,Mn)17Nx shell was thus successfully obtained. The thickness of the Mn enriched region was about 0.8 μm, and the crystal structure of the core-shell powder was Th2Zn17. The saturation magnetization and coercivity of the core-shell powder were 138.8 A·m2·kg−1 and 1001.7 kA·m−1, respectively. The saturation magnetization was slightly smaller than that of Mn-free Sm-Fe-N powder, whereas the coercivity of the core-shell powder was higher than that of Mn-free Sm-Fe-N powder. In addition, the saturation magnetization and coercivity were higher than those reported for Sm-Fe-Mn-N powder. The thermal stability of the Mn-diffused Sm-Fe-N core-shell powder was improved compared with Mn-free Sm-Fe-N powder.