C hapter 4 Material Classes, structure, and properties
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it almost completely. The result is that most magnetic materials,
unless manipulated in some way, adopt a domain structure with
minimal external field; that’s the same as saying that, though magnetic, they are not magnets.
How can they be magnetized? Placed in a magnetic field, created,
say, with a solenoid, the domains already aligned with the field
have lower energy than those aligned against it. The domain wall
separating them feels a force, the Lorentz force, pushing it in a direction to make the favorably oriented domains grow at the expense
of the others. As they grow, the magnetization of the material
increases, moving up the M−H curve, finally saturating at M s , when
the whole sample is one domain oriented parallel to the field.
4.7 optiCal Behavior
Electromagnetic (e-m) radiation permeates the entire universe.
Observe the sky with your eye and you see the visible spectrum,
Flux lines of
external field, B
Magnetized
material
Flux lines
(a)
(b)
Domain walls
(d)
(c)
Figure 4.63
Domains allow a compromise in the cancellation of
the external field while retaining magnetization of
the material itself. The arrows show the direction
of magnetization.