Improved coercivity and squareness in bulk hot-deformed Nd–Fe–B magnets by two-step eutectic grain boundary diffusion process

Abstract We introduce a novel method to develop high coercivity with improved squareness in bulk hot-deformed Nd–Fe–B magnets with a thickness of 5.6 mm. A low-melting-point Tb20Dy10Nd40Cu30 eutectic diffusion source was infiltrated into the thick magnets through grain boundaries as the 1st step, followed by the 2nd step eutectic diffusion using Nd80Cu20. The 1st step led to the formation of rare earth (RE)-rich intergranular phase as well as heavy rare earth (HRE)-rich shell surrounding platelet-shaped Nd2Fe14B grains. A large concentration gradient toward the center of the thick magnet caused a large switching field distribution, which was the main limitation of this method after the first step and in the conventional infiltration process. The second step Nd-Cu diffusion makes the HRE-shell more uniform throughout the entire volume, which improved the squareness factor from 0.83 to 0.91 with a slight coercivity increase from 2.38 T to 2.43 T. Magneto-optical Kerr effect microscopy showed unlike the conventional infiltration processed magnet, strong pinning of the magnetic domain wall motion at the side grain boundaries occur even in the near center of the two step diffusion processed magnet.