SECTION 12.8. ALNICO MAGNETS 125
process takes only a very short time when performed at 850°C, being completed in a matter
of seconds or minutes. At lower temperatures, longer times are required.
The formation and growth of the particles to their final shape and size occurs almost
entirely during the spinodal-decomposition reaction at 850°C. The driving force for this
reaction is the reduction of the interfacial energy between the particles and the matrix
Although the interfacial energy is small it is sufficiently large to
promote particle growth.
In general, a third heat treatment is required for good quality magnets. The main effect
of this third heat treatment, performed at 600°C, is to increase the difference between
the saturation polarization of the Fe-rich particles and the surrounding matrix (NiAl-rich).
During annealing, a continuous change in the composition takes place due to the diffusion
of Fe atoms to the ferromagnetic particles.
The magnetic anisotropy of Alnico magnets is due to the ferromagnetic particles.
However, this is not the type of magnetocrystalline anisotropy as found, for instance, in
rare-earth-based magnets (see Section 12.4). In Alnico alloys, the magnetic anisotropy is
due to the rod-like shape of the ferromagnetic particles. It originates from the difference
in demagnetizing factor (see Chapter 8) in the two extremal directions of the particles,
which requires that the easy magnetization direction is along the long direction of the rod-
shaped particles. The spinodal decomposition alone does not produce a sufficiently large