First-principles calculations of crystal field parameters of Nd ions near surfaces and interfaces in Nd-Fe-B magnets

We study the local magnetic anisotropy due to Nd ions located around surfaces of Nd₂Fe₁₄B within the crystal field theory based on first-principles calculations. The Nd ions on the (001) surface exhibit A₂⁰ < 0, that is, the in-plane anisotropy, as reported previously, but those on the (100) surface exhibit A₂⁰ > 0, which is consistent with the bulk c-axis uniaxial anisotropy. The physical origin of these conflicting local anisotropies can be understood from the 6p and 5d valence charge distributions realized in the Nd₂Fe₁₄B structure.