Improvement of magnetic properties and intergranular microstructure of Nd-Fe-B magnets by intergranular addition of MgO oxide

In this work we have studied in detail the effect of MgO on magnetic properties and intergranular microstructure. We have found that both coercivity and thermal stability can be remarkably enhanced by the intergranular addition of MgO. For Nd22Fe71B7 magnets with 2 wt % MgO addition, the coercivity at room temperature and 180 degrees C are enhanced from 17.0 and 3.2 kOe to 22.1 and 5.2 kOe, respectively, and the reversible and irreversible flux loss from room temperature to 180 degrees C is reduced from 25.4% and 5.2% to 20.5% and 0.5%, respectively, Microstructural studies reveal that a new intergranular Nd-O-Fe-Mg phase with a composition close to Nd70O23Fe3Mg2 appears in the magnets with MgO addition. The improvement of magnetic properties by the MgO addition is believed to be due to the appearance of a Nd-O-Fe-Mg intergranular phase, which probably hinders the propagation of the domain walls between Nd2Fe14B grains. It is further found that the addition of Mg or O alone into the intergranular regions of the magnets does not lead to the formation of this Nd-O-Fe-Mg intergranular phase, and thus, cannot substantially improve the coercivity and the thermal stability of the magnets. (C) 1997 American Institute of Physics.