Effects of trace elements on the crystal field parameters of Nd ions at the surface of Nd$_2$Fe$_{14}$B grains

TL;DR

This study uses first-principles calculations to explore how trace elements like O and Cu influence the crystal field parameters of Nd ions on Nd$_2$Fe$_{14}$B grain surfaces, affecting magnetic anisotropy.

Contribution

It provides new insights into how specific trace elements alter the surface magnetic properties of Nd$_2$Fe$_{14}$B grains through positional dependence.

Findings

O and Cu can induce negative $A_2^0$ at the (001) surface when close to Nd ions.

At the (110) surface, O can cause negative $A_2^0$, but Cu tends to keep $A_2^0$ positive.

Cu atoms are more likely to maintain positive local anisotropy than O atoms near grain surfaces.

Abstract

Using first-principles calculations, we investigate the positional dependence of trace elements such as O and Cu on the crystal field parameter $A_2^0$, proportional to the anisotropy constant $K_u$ of Nd ions placed at the surface of Nd$_2$Fe$_{14}$B grains. The results suggest the possibility that the $A_2^0$ parameter of Nd ions at the (001) surface of Nd$_2$Fe$_{14}$B grains exhibits a negative value when the O or Cu atom is located near the surface, closer than its equilibrium position. At the (110) surface, however, O atoms located at the equilibrium position provide a negative $A_2^0$, while for Cu additions $A_2^0$ remains positive regardless of Cu's position. Thus, Cu atoms are expected to maintain a positive local $K_u$ of surface Nd ions more frequently than O atoms when they approach the grain surfaces in the Nd-Fe-B grains.