Page 288 - Electrical Properties of Materials

270 Magnetic materials

melting and elaborate sequences of rolling and annealing. The presently avail-
able amorphous materials have not quite reached the quality of supermalloy,
but they are quite close. The cobalt-based commercially available 2714 A ma-
terial has a saturation magnetization of 0.5 T with a maximum permeability of
a million. Another one, known as 2605S-3 A made of iron and chromium has a
saturation magnetization of 1.4 T and a maximum permeability over a quarter
of a million.
The latest line of soft magnetic materials are the nanocrystalline alloys with
grain sizes of the order of 10 nm. They have been around for about 15 years.
Typical representatives are Fe–B–Si–Cu–Nb alloys, which may reach relative
permeabilities over 100 000. The excellent soft magnetic properties may be
explained by the reduction in effective crystal anisotropy expected when grain
sizes are reduced below the bulk-domain wall thickness.
The situation is somewhat different in power applications, such as trans-
formers. There the traditional materials are cheaper, but amorphous materials
may still represent the better choice on account of lower losses; their higher
cost may be offset in the long term by lower power consumption (or even
possible future legislation in some countries requiring higher efficiency in
electrical equipment).
At higher frequencies, as mentioned before, high resistivities are required
for which a family of ferrites with chemical formula MO · Fe 2 O 3 (where M is
a metal, typically Ni, Al, Zn, or Mg) is used. If the metal M is iron, the material
is iron ferrite, Fe 3 O 4 , the earliest-known magnetic material.
Ferrites are usually manufactured in four stages. In the first stage the mater-
ial is produced in the form of a powder with the required chemical composition.
In the second stage the powder is compressed, and the third stage is sintering
to bind the particles together. The fourth stage is machining (grinding, since
the material is brittle) to bring the material to its final shape.
For the properties of a number of soft magnetic materials see Table 11.1.

11.6 Hard magnetic materials (permanent magnets)

What kind of materials are good for permanent magnets? Well, if we want large
flux density produced, we need a large value of B r . What else? We need a large
H c . Why? A rough answer is that the high value of B r needs to be protected. If
for some reason we are not at the H = 0 point, we do not want to lose much
flux, therefore the B–H curve should be as wide as possible.
A more rigorous argument in favour of large H c can be produced by taking
account of the so-called demagnetization effect, but in order to explain that,
I shall have to make a little digression and go back to electromagnetic the-
ory. First of all, note that in a ring magnet [Fig. 11.10(a)] B = B r = constant
everywhere in the material to a very good approximation. Of course, such a
permanent magnet is of not much interest because we cannot make any use of
the magnetic flux. It may be made available, though, by cutting a narrow gap
in the ring, as shown in Fig. 11.10(b). What will be the values of B and H in
the gap? One may argue from geometry that the magnetic lines will not spread
out (this is why we chose a narrow gap, so as to make the calculations simpler)
and the flux density in the gap will be the same as in the magnetic material.
But, and this is the question of interest, will the flux density be the same in the