First-principles Study on Crystalline Nd-Fe alloys as Candidate Structures for Grain-Boundaries in Nd-Fe-B Sintered Magnets*

Due to technological progress in industry, the coercivity enhancement of Nd-Fe-B sintered magnets at high temperatures is highly demanded. It has been identified that the grain-boundaries (GBs) are known as Nd-Fe alloys with fcc diffraction patterns. However, the detailed atomic arrangement on the atomic scale is still unclear. Therefore, in this study, we investigated Nd-rich alloys with Fe as a candidate structures for GBs.
First, we performed first-principles calculations for cuprite, fluorite, half-Heusler, and sphalerite Nd-Fe alloys in a wide range of the Fe composition ratio in order to determine the most stable fcc Nd-Fe alloy. We found that only the fluorite Nd-Fe alloy keeps the high crystallinity in a range of around 65 to 100 at.%. In contrast, cuprite, half-Heusler, and sphalerite alloys become amorphous-like structures after optimizing structures.
Second, we added metallic elements such as Al, Co, Cu, and Ga into the fluorite Nd-Fe alloys and optimized the structures in order to check the stability. Only the formation energies of the fluorite Nd-Fe-Ga alloys show the negative values. These results indicate that Ga promotes the stabilization of Nd-Fe alloys more than any other metallic elements.