part is magnetized, these domains are caused to align in the direction
of the coercing field. Some of the domains are more difficult to align
than others, and so some small degree of magnetization may be reached
at a low field, and more at a higher one, until all the domains have
turned (or reformed) in the direction of the external field. In magnetically
"soft" materials, such as low carbon steel, as soon as the
external field is removed, some or all of the domains go back to a
field-canceling jumble of directions. In magnetically "hard"
permanent materials, however, most or all of the domains remain aligned,
resulting in a net external magnetic field.
Many modern magnet materials are said to be ANISOTROPIC, meaning
that they have a preferred axis of magnetic field, built-in during
manufacture. The part may be magnetized in either direction along
this axis, north-south or south-north. The part is extremely resistant
to magnetizing in any other direction, however. Many of these materials
are easier to magnetize the first time, from the virgin state, than
they are to remagnetize again in the opposite direction. For this
reason, if a part made of material with these characteristics must
be remagnetized, it must be aligned in the fixture in the same way
that it was originally magnetized, not north pole to south pole,
and so on.