Microstructure of high energy density Nd-Fe-B magnets

Magnets of the composition Nd15-xFe78.5+xB6.1M0.4 [M=Al,Ga,Cu and x=0-1.2] were prepared by the powdermetallurgical sintering route. The energy density product of 360 kJ/m(3) and the coercive field of 900 kA/m were obtained by the transverse die pressing method under optimised sintering conditions. The misalignment of the hard magnetic grains with a diameter of 2 similar to 5 mu m was in the order 17 degrees. The high oxygen content of the magnets (4000 ppm) was the limiting factor to decrease the Nd-content and increasing the volume fraction of 91 % of the hard magnetic grains. For a composition with x greater than or equal to 0.8 the coercive field decreased drastically. Electron microscope investigations showed that the formation of soft magnetic alpha-Fe grains with diameters > 100 nm shifts the nucleation process of reversed domains into the first quadrant of the demagnetisation curve.