Epitaxial Growth Mechanism and Magnetic Properties of Heterogeneous Core–Shell Grains in (Nd, Y)‐Fe‐B Alloys

ABSTRACT The construction of core–shell grains can enhance the coercivity of Nd 2 Fe 14 B magnets. However, it is a significant challenge to elucidate the formation of these core–shell grains particularly regarding the spontaneous core–shell structure in Y‐substituted Nd 2 Fe 14 B alloys. We demonstrate the evolution of Nd‐rich shells through two‐step heat treatment of rare‐earth‐rich (RE‐rich) NdY‐Fe‐B alloys providing insight into the self‐assembly of shells in neodymium magnets. Significantly, the RE‐rich phase after first‐step heat treatment contains Nd, Y, Fe, and B, which drives the epitaxial growth of the 2:14:1 shell during subsequent heat treatment. Additionally, adjusting the first‐step heat treatment temperature modulates the chemical composition of the RE‐rich phase, enabling precise control over the thickness and composition of the epitaxial shell formed after second‐step heat treatment. The strain concentration induced at the core–shell interface by this epitaxial shell impedes the displacement of domain walls contributing to the enhancement of coercivity. This work provides a route for designing and understanding core–shell structures in Nd‐Fe‐B magnets.