Page 283 - Electrical Properties of Materials

Domains and the hysteresis curve 265

(a) (b) (c)

Fig. 11.4
The formation of domains (from
C. Kittel, Introduction to solid state
(d) (e) physics, John Wiley, New York).

dependence on applied magnetic fields. We must approach the problem, as
Lucretius said, ‘by exceedingly long and roundabout ways’.
However much I dislike talking about crystal structure, there is no escape
now because magnetic properties do depend on crystallographic directions. I
am not suggesting that magnets are ever made of single-crystal materials, but
in order to interpret some of the properties of ordinary polycrystalline magnets,
we have to know the magnetic properties of the single crystals.
In Fig. 11.5 the magnetization curve (B against H) of iron is plotted for
three different directions in the crystal. It may be seen that magnetization is
relatively easy in the AB direction and harder in the AC and AG directions,
or in other words, it is easier to magnetize iron along a cube edge than along
a face or a body diagonal. This does not mean, of course, that all magnetic

AB
2.0
AC
AG
1.5
B (T) D C
K (100) (110)
1.0
A
B
(111)
G
0.5
E F
Fig. 11.5
0.0 Magnetization curve of
0.0 1.6 3.2 4.8 single-crystal iron in three different
−1
H (A / m ) crystallographical directions.