First-Principles Study on the Role of B in Nd₂Fe₁₄B

Rare-earth-Fe based magnetic compounds R₂Fe₁₇ (R = rare earth) are not capable of becoming strong magnets due to their low Curie temperatures. Sagawa tried to make the ferromagnetism of the compounds stronger through magneto-volume effect. He added boron (B) to Nd₂Fe₁₇ in order to stretch the Fe-Fe distances, which led the discovery of Nd₂Fe₁₄B, the strongest magnet known[1]. However, according to the first-principles studies on (Y, Gd)₂Fe₁₄B with the same structure as Nd₂Fe₁₄B[2], the magnetic moment m_s of Fe is reduced after adding B, implying that B does not enhance the magnetic properties of Nd₂Fe₁₄B.
We perform first-principles calculations for Nd₂Fe₁₄B in order to understand the role of B in terms of m_s, magnetization, and stability. From our analysis, we find that B does not work effectively on improving the m_s and magnetization of Nd₂Fe₁₄B, because the total magnetic moment of Fe is reduced by adding B. We also calculated the formation energies of Nd₂Fe₁₄X, where X represents B, C, N, O, and F. The obtained formation energies are consistent with experimental results.
[1] M. Sagawa et al., J. Appl. Phys. 55, 2083 (1984).
[2] M. Yamaguchi and S. Asano, Physica B: Cond. Matt. 254, 73 (1998).