The grain boundary region of NdFeB sintered magnets were modified by reduction-grain boundary diffusion (r-GBD) process using Heavy Rare-Earths (HRE = Dy or Tb) metal which generated via reduction of HRE compounds such as oxide or fluoride with Ca metal vapor. After modification, magnetic properties were measured accurately by superconducting magnet-based vibrating sample magnetometer (SCM-VSM). The coercivity of DyF 3 and TbF 3 treated magnets with the assistance of reduction by Ca metal vapor (H cJ = 1451 kA/m and 1778 kA/m, respectively) were effectively enhanced compared to untreated magnets (H cJ = 1003 kA/m) without severe decreasing of remanence value. The resultant coercivity was higher than that of the magnets modified with DyF 3 (H cJ = 1319 kA/m) and TbF 3 (H cJ = 1580 kA/m) without using any reducing agent such as Ca metal vapor.

Effective grain boundary diffusion (GBD) process with DyAl co-sorption is applied to enhance the coercivity of NdFeB sintered magnets. The coercivity of the magnet (H cJ = 1789 kA•m ¹1 ) subjected to the present GBD treatment was observed to be superior to that of the untreated magnet (H cJ = 1003 kA•m ¹1 ) and the conventional GBD magnet (H cJ = 1661 kA•m ¹1 ) treated with DyAl alloy. In the present GBD magnet, the DyAl co-sorption process facilitated Dy diffusion into the center region of the magnet (thickness: 3.5 mm), resulting in high coercivity. Further, magnetic domain observations were made using magnetic force microscopy (MFM) to observe the thermal demagnetization behavior of the present GBD magnet. The present GBD magnet suppressed the continuous domain reversal of adjacent grains; thus, the partially persistent single-domain structure remained, even at 453 K.