The effect of Tb4O7 powders addition on the magnetic properties for Nd–Fe–B sintered magnets with low rare earth content

Abstract The coercivity increased by 2.62 kOe with the formation Tb-rich shell by Tb diffusion after the dissolution of initial 0.5 wt.% Tb4O7. With the increase of Tb4O7 additive amount, the increment of coercivity decreased first and then increased for Nd-Fe-B magnets. The dissolution of more Tb4O7 powder increased the Tb concentration gradient and promoted the diffusion of Tb into the inner area of grains. It resulted in the increase of the thickness of Tb-rich shell and the decrease of Tb efficiency. Increasing Tb4O7 addition beyond the O solubility in limited Nd-rich phase slowed down its dissolution. Thanks to the slow sustained release of Tb, Tb efficiency increased as the decrease of diffusion flux into the inner area of grains. The remaining (Nd,Tb,O)-rich phases can also enhance the coercivity.