Physics of Magnetism

124 CHAPTER 12. PERMANENT MAGNETS

segments for electric motors, anisotropic rings for loudspeakers, and large anisotropic

blocks for ore separators. Applications in which the use temperature can become substan­

tially higher than room temperature may profit from the fact that the ferrite magnets have

a high chemical stability and that the coercivity increases rather than decreases with tem­

perature. A distinct disadvantage of magnets made of hard ferrites is their low

value (see Table 12.5.1). It requires generally magnets of comparatively large size and this

restricts their application to magnetic devices in which weight and space are not a concern.

12.8. ALNICO MAGNETS

Nowadays, the Alnico alloys have become a less important group of permanent-magnet
materials. They contain Fe, Co, Ni, and A1 with small amounts of Cu and Ti as additives. The
Alnicos, like the sintered and magnets discussed in Section 12.5,
are fine-particle magnets, consisting of ferromagnetic particles in a non-magnetic matrix.
However, there is an important difference where the rare-earth-based magnets are concerned.
In the Alnico alloys, the fine-particle structure is not the result of powder metallurgy but
the result of a metallurgical precipitation reaction that takes place in the solidified ingots of
the alloy.
The important role played by the microstructure of Alnico alloys is most conveniently
discussed by means of alloys of the composition although Alnico alloys have in
general a much more complicated composition, including Co. The pseudobinary section
FeNiAl in the phase diagram is shown in Fig. 12.8.1. Permanent-magnet alloys close in
composition to are commonly prepared by a homogenization treatment at 1250°C.
It can be seen in Fig. 12.8.1 that the alloy consists of one single phase at this temperature.
However, at lower temperatures there is a miscibility gap. The presence of this miscibility
gap causes the phase to decompose into two different phases and when an alloy of
a composition falling into the gap region is kept at temperatures confined within the gap
for some time. This second heat treatment is of prime importance for the formation of a
microstructure in the alloy ingot that gives it the desired hard-magnetic properties.
Both the Fe-rich particles phase) and the non-ferromagnetic or weakly ferromag­
netic NiAl-rich matrix phase) have the bcc structure. This circumstance is one of the
reasons that the phase separation of into and upon annealing at a temperature within
the gap proceeds by so-called spinodal decomposition rather than by the normal nucle­
ation and growth process. This has important consequences for the microstructure and the
magnetic properties of the alloys, as will be discussed below.
Although the decomposition proceeds spontaneously, the rate of the spinodal decompo­
sition of the phase into and is diffusion limited. This means that the decomposition
process will reach completion within a reasonable time only if the atoms are able to diffuse
to a sufficient extent. Atomic motion during diffusion requires an activation energy that
can be supplied only if the temperature is sufficiently high. Therefore, the decomposition
rate is sufficiently high only at relatively high temperatures (850°C). The nature of the
spinodal-decomposition process is such that the concentrations of the Fe atoms in the two
phases show a periodic variation (sinusoidal) and the amplitude of the composition fluctu­
ations increases with time until the phase separation into and is complete. The whole