First-principles phonon calculations of neodymium-magnet compounds

We report first-principles investigations of the light-element effect on phonon states in neodymium-magnet compounds. It is found that B in Nd2Fe14B contributes to acoustic phonon hardening. As for the substitution of other light elements for B, frequencies of phonons that originate from light elements decrease with increasing the atomic number. However, imaginary phonons in Nd2Fe14F are attributed to the electronic states and the atomic configurations, not to the heavier mass of F. In addition, as the effect of the magnetic disordering on the phonon state, we show that the phonons of the paramagnetic Nd2Fe14B are softer than those of the ferromagnetic Nd2Fe14B. This suggests that the phonon effect decreases the Curie temperature through the stabilization of the paramagnetic state.