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

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

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

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.