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Ferromagnets
A third type of magnetic material is a ferromagnet. In this
material, there are domains in which the magnetic fields of the
individual atoms align, but the orientation of the magnetic fields
of the domains is random, giving rise to no net magnetic field.
This is illustrated below.
Figure 9.13:
A ferromagnet
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A useful property of ferromagnets is that when an external magnetic field
is applied to them, the magnetic fields of the individual domains tend to
line up in the direction of this external field, due to the nature of the
magnetic forces, which causes the external magnetic field to be enhanced.
This is illustrated below.
Figure 9.14:
A ferromagnet in an external magnetic field
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This enhancement of the external magnetic field is the reason one often
finds the loops of wire as used in an electromagnet, such as in
Fig. 9.10, are wrapped around a ferromagnetic core.
Another area where ferromagnetic materials are employed is in magnetic
recording devices, such as for cassette tapes, floppy discs for computers,
and the magnetic stripe on the back of credit cards. These devices
essentially take information in the form of electrical signals and
permanently encode it into a magnetic material. The way this is done
is illustrated below.
Figure 9.15:
Magnetic recording / reading
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As an (AC) electrical signal passes through the wire loop, a magnetic field
is produced which passes through the ferromagnetic core,
which in turn produces a magnetic
field in the vicinity of a moving magnetic tape. This magnetic field
aligns the magnets of atoms on the tape that happen to be passing by it
at that instant. A short time later the direction of the current reverses,
which reverses the direction of the magnetic field, which subsequently
reverses the orientation of the next atom on the tape which passes by.
In this way information
stored in the electrical signal is encoded in a particular orientation
of the magnetic fields of individual atoms.
Next: Currents from magnetism
Up: Magnetic materials
Previous: Permanent magnets
modtech@theory.uwinnipeg.ca
1999-09-29