Influence of Grain Boundary on Coercivity in Rare-earth Permanent Magnet with Grain Surface Defects

Coercivity (H_cj) microstructure-sensitive characteristic of Nd-Fe-B or Sm-Co magnet results in their complex and rigorous fabrication process. To improve H_cj by optimizing microstructure in magnets has been researched as a hot topic. Micromagnetic simulations (MS) have successfully clarified the factors affecting H_cj, such as grain size, grain defect, grain boundary (BG) and disorientation. But few studies have focused on a collective effect of above factors while the real magnets involve many unfavorable factors to H_cj.
We investigate the effect of BG between two different non-ideal Nd₂Fe₁₄B grains on H_cj by finite element MS. One grain contains surface defects with a reverse magnetization field (H_defect) and connects with another non-defect grain through BG (thickness-T). Variation of H_cj with T can be divided into three parts: (1) When T is very thin, the magnet belongs to the rigid model and H_cj = H_defect; (2) When T becomes thicker, the magnet transforms into the exchange-spring model. BG acts as pinning points and H_cj increases with T; (3) When T is large, the magnet is the decoupled model. BG is as a new nucleation site with a reverse magnetization field (H_BG) and H_cj = min (H_defect, H_BG).