Magnetic Properties of Nd–Fe–B Sintered Magnets Produced by Reduction-Grain Boundary Diffusion Process with Heavy Rare-Earths Compounds and Ca Metal Vapor

Abstract

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 DyF₃ and TbF₃ 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 DyF₃ (H_cJ = 1319 kA/m) and TbF₃ (H_cJ = 1580 kA/m) without using any reducing agent such as Ca metal vapor.