Page 286 - Electrical Properties of Materials

268 Magnetic materials

Is it possible to describe more accurately the movement of domains?
One can go indeed a little further by taking into account the effect of
magnetostriction, which, as you may guess, is the magnetic counterpart of
electrostriction. Strictly speaking, one should distinguish between magneto-
striction and piezomagnetism, the magnetic counterpart of piezoelectricity. But
biased magnetostriction (see discussion on biased electrostriction in Section
10.11) is phenomenologically equivalent to piezomagnetism, and piezomag-
netism has not been much investigated anyway; thus most authors just talk
about magnetostriction. Disregarding the problem of nomenclature, the rel-
evant fact is that when a magnetic field is applied, the dimensions of the
∗
∗ This is, incidentally the cause of the material change, and conversely, strain in the material leads to changes in
humming noise of transformers. magnetization and may also affect the directions of easy magnetization. Now
if the material exhibits a large anisotropy and stresses are present as well,
then there will be local easy directions resisting the movement of domain
Domain boundary walls everywhere. The stresses may be caused by the usual defects in crystals
and particularly by impurities. In addition, a cluster of non-magnetic impur-
ity atoms might be surrounded by domains (see Fig. 11.8). This is a stable
configuration which cannot be easily changed.
How can we classify magnetic materials? There is a simple division into
soft and hard magnetic materials. Why soft and hard? Well, the hard materials
are those which are hard to magnetize and demagnetize. So materials which
are easy to magnetize and demagnetize should be called easy materials. In
fact, they are called soft materials, and there is nothing we can do about that.
We have to remember, though, that these are only very tenuously related to
mechanical properties, which may also be hard and soft.

Fig. 11.8 11.5 Soft magnetic materials
Non-magnetic impurity surrounded
Their main role is to enhance the magnetic effect produced by a current-
by a domain.
carrying coil. So, obviously, they should have large saturation magnetization
and large permeability. If the material is subjected to alternating voltages, then
an important consideration is to reduce losses caused by the induced eddy cur-
rents, which can be done by increasing resistivity. What else is needed in order
to reduce losses? A narrow hysteresis loop is needed as shown below.
The energy dissipated in a coil for a period T may be expressed with the aid
of the current and voltage as
T
E d = V(t)i(t)dt. (11.29)
0
Now, using Faraday’s law (that the voltage is proportional to the derivative
of the flux density) and Ampère’s law (that the magnetic field is proportional
to current) eqn (11.29) may be rewritten as

E d = C H d B, (11.30)

where C is a constant. Thus, clearly, the energy loss per cycle is proportional
to the area of the hysteresis loop.
The most important parameter determining the desirable properties of
soft magnetic materials is the frequency at which they are used. For d.c.