126 CHAPTER 12. PERMANENT MAGNETS
shape anisotropy in the primary ferromagnetic particles. Since the difference in
saturation magnetization of the particles and the matrix is relatively small, the effective
shape-anisotropy field of the particles is also small in spite of the elongation. Therefore,
the heat treatment at 600°C is desirable in order to increase the difference in magnetization
and the concomitant shape anisotropy. This, in turn, makes it possible to obtain the high
est coercivities and the optimal permanent-magnet properties. The tempering treatment at
about 600°C usually takes several hours.
In Fig. 12.8.2, a few examples are shown of how the intrinsic coercivity for
Fe-NiAl alloys can be varied with composition and heat treatment. In case A, the coerciv
ities were determined after quenching and after tempering treatments to give the optimal
coercivity. The results displayed by curve B were obtained by means of the more attrac
tive manufacturing route of continuously controlled cooling of these materials. It should,
however, be borne in mind that the overall magnetization of the magnets decreases
with decreasing Fe content. Therefore, the compositions corresponding to the maximum
coercivity need not necessarily correspond to the optimal composition of the ultimate
magnet.
The interfacial energy responsible for the growth of the ferromagnetic particles depends
on the crystallographic orientation of the boundary between the and the phase. There
fore, the particle growth is anisotropic, which results in an elongation parallel to the
directions of the cubic alloy. Significant improvements of the magnetic properties are there
fore generally obtained by controlled cooling of the alloys from 1200°C to about 800°C in
a saturating magnetic field. This thermomagnetic treatment leads to anisotropic magnets in
which the easy magnetization direction of the grains formed during the spinodal decompo
sition is parallel to the direction of the magnetic field applied during cooling. The elongation