Microstructure of sintered Fe-Nd-B magnets

The importance of microstructure in determining the magnetic properties, especially the coercivity, of sintered Fe-Nd-B magnets is discussed. The importance of non-ferromagnetic phases which may increase internal demagnetizing fields or reduce intergranular coupling is reviewed. A metastable, magnetically soft phase with a Curie temperature of 245°C, present in the intergranular regions of sintered magnets, is dissolved by the 600°C heat treatment, improving the coercivity of annealed magnets. The grain-size distribution and its evolution during sintering are presented. Anisotropic grain shapes are discussed in relation to coercivity.