SECTION 14.2. SURVEY OF MATERIALS 151For a polycrystalline alloy, the magnetization directions of the various domains have a
random orientation, meaning that the induced anisotropy directions in the various domains
will also have a random orientation. This results in a constricted hysteresis loop as shown
by means of curve B in Fig. 14.2.2. At this stage, it is good to recall that the main effect
of a magnetic field when applied during annealing is to destroy the domain pattern and to
align the local magnetization in the field direction across the whole sample.
The value of the thermomagnetic anisotropy constant is generally of the order of
a few hundred As can be seen in Fig. 14.2.4, it increases with Fe concentration as
a result of an increased number of aligned atom pairs. The value of is generally higher
the lower the annealing temperature. More details can be found in the review of Ferguson
(1958).
It is important to bear in mind that the thermomagnetic anisotropy is generated by
annealing treatments performed below the Curie temperature. Because the pair formation
requires diffusion of atoms and because diffusion is a thermally activated process, too low