Callister - An introduction - 8th edition

820 • Chapter 20 / Magnetic Properties
B
Soft
Hard
Figure 20.19 Schematic magnetization curves
for soft and hard magnetic materials. (From
K. M. Ralls, T. H. Courtney, and J. Wulff,
Introduction to Materials Science and
Engineering. Copyright © 1976 by John Wiley
& Sons, New York. Reprinted by permission
of John Wiley & Sons, Inc.)
H
soft magnetic
material
Both ferromagnetic and ferrimagnetic materials are classified as either soft or
hard on the basis of their hysteresis characteristics. Soft magnetic materials are used
in devices that are subjected to alternating magnetic fields and in which energy
losses must be low; one familiar example consists of transformer cores. For this reason
the relative area within the hysteresis loop must be small; it is characteristically
thin and narrow, as represented in Figure 20.19. Consequently, a soft magnetic material
must have a high initial permeability and a low coercivity. A material possessing
these properties may reach its saturation magnetization with a relatively low
applied field (i.e., is easily magnetized and demagnetized) and still has low hysteresis
energy losses.
The saturation field or magnetization is determined only by the composition of
the material. For example, in cubic ferrites, substitution of a divalent metal ion such
as Ni 2 for Fe 2 in FeO–Fe 2 O 3 will change the saturation magnetization. However,
susceptibility and coercivity (H c ), which also influence the shape of the hysteresis
curve, are sensitive to structural variables rather than to composition. For example,
a low value of coercivity corresponds to the easy movement of domain walls as the
magnetic field changes magnitude and/or direction. Structural defects such as particles
of a nonmagnetic phase or voids in the magnetic material tend to restrict the
motion of domain walls, and thus increase the coercivity. Consequently, a soft magnetic
material must be free of such structural defects.
Another property consideration for soft magnetic materials is electrical resistivity.
In addition to the hysteresis energy losses described previously, energy losses
may result from electrical currents that are induced in a magnetic material by a
magnetic field that varies in magnitude and direction with time; these are called
eddy currents. It is most desirable to minimize these energy losses in soft magnetic
materials by increasing the electrical resistivity. This is accomplished in ferromagnetic
materials by forming solid solution alloys; iron–silicon and iron–nickel alloys
are examples. The ceramic ferrites are commonly used for applications requiring
soft magnetic materials because they are intrinsically electrical insulators. Their applicability
is somewhat limited, however, inasmuch as they have relatively small