Effect of preparation conditions on coercive force of Mischmetal-Cobalt magnets

Mischmetal-Cobalt sintered magnets(MMCo
x
) without sintering additive were prepared under various conditions, and the optimum condition to obtain high coercive forces was examined. In powder compacts which were prepared after milling for 16 hr(16 hr-milled samples), maximum intrinsic coercive force of 3.5 kOe was obtained for samples with the nominal compositional ratio of x=4.6. In sintered magnets, higher coercive forces were obtained for MM-richer compositions than those of powder compacts. The maximum value was 9 kOe for 16 hr-milled samples with x=3.6 which were sintered at 1000°C. Optimum particle size to obtain high coercive forces was about 3μm for 16 hr-milled samples with
x= 3.2\sim5.0
It was found from X-ray diffraction patterns that high coercive forces are due to the co-existence of a small amount of the MM
5
Co
19
phase with the main phase of MMCo
5
.