SECTION 12.7. HARD FERRITES 123
Solid bodies of permanent magnets based on M ferrites can be made either by sintering
or by plastic bonding. Of both types of magnets, anisotropic as well as isotropic forms are
applied. The former types are characterized by higher remanences owing to the magnetically
induced alignment of the powder particles during processing. In the latter magnets, the
orientation of the powder particles has a random distribution. The anisotropic sintered form
is the most important.
The starting materials for the preparation of are and pos
sible additives and/or Appropriate amounts of powders of these materials are
dry-mixed and the resulting mixture is subsequently granulated in a disc pelletizer to gran
ules of roughly 5 mm diameter. During the pre-firing process, performed at about 1250°C
in air, the raw materials react to form the compound Comminution of the hard
pre-fired granules is achieved by wet milling with steel balls. This latter process leads to
a thick suspension (slurry) in which the fine powder particles (preferably single crystals)
have sufficient mobility to align themselves along the preferred magnetization direction
when an external field is applied during wet pressing. The resultant compacts are first dried
and then sintered in air at about 1250°C. Anisotropic shrinkage occurs during the sintering
process. For this reason, the pole faces of the sintered bodies have to be ground afterwards
when accurate dimensional control of the magnets is required. For more details regarding
processing, the reader is referred to the reviews by Kools (1986) and McCaig and Clegg
(1987).
Permanent magnets made of hard ferrites can be characterized as low-cost low-
performance magnets. Their application is widespread, main applications being anisotropic