Understanding Permanent Magnets
Theory and Applications
Modern permanent magnets play a vital role in a wide
range of industrial, consumer and defense products.
Efficient use of permanent magnets in these devices
requires a basic understanding of magnetic theory. To
achieve this end it is helpful to understand that all
magnetic fields are the result of electrons in motion.
Figure 1 - Magnetic field resulting from current flow in
a coil.
In the electrical circuit, Figure 1, a DC voltage is developed
in the battery which causes a current, I, to flow through the
wires to the load. This current flow, which is the movement
of electrons between atoms in the conductor, causes a
magnetic field to be established around the wire. The
magnitude of the field is measured in ampere-turns per
meter in the International System (SI) or in oersteds in the
gram-centimeter-second (cgs) system and is designated by
the symbol H.
In permanent magnets the electrons-in-motion
phenomenon still explains the magnetic field produced
within the magnet.
Figure 2 - Electron shells in an atom of iron.
As shown in figure 2, within the third electron shell of the
iron atom, there exists an imbalance in the spin direction of
the electrons. This imbalance creates a magnetic moment in
the iron atom. However, this atomic magnetic moment
alone is insufficient to cause ferromagnetism. Additionally,
there must be cooperative interatomic exchange forces that
maintain neighboring atoms parallel. The present theory
states that these parallel groups of atoms form domains or
areas within the ferro-magnetic body that are magnetized
to saturation, but that the magnetization direction between
the domains need not be parallel. When the magnet is
demagnetized it is only demagnetized from the viewpoint
of the external observer. The domains are not demagnetized
but are fully magnetized with neighboring domains
being magnetized in opposite and mutually canceling
directions. The magnet becomes “magnetized” when an
external magnetizing field is applied to the magnet of
sufficient magnitude to cause all the domains to rotate and
align in the direction of the applied field.