Ferromagnetic materials get their magnetic properties
not only because their atoms carry a magnetic moment but also
because the material is made up of small regions known as magnetic
domains. In each domain, all of the atomic dipoles are coupled
together in a preferential direction. This alignment develops
as the material develops its crystalline structure during solidification
from the molten state. Magnetic domains can be detected using
Magnetic Force Microscopy (MFM) and images of the domains like
the one shown below can be constructed.
Magnetic Force Microscopy (MFM) image showing the magnetic
domains in a piece of heat treated carbon steel.
During solidification, a trillion or more atom moments are aligned
parallel so that the magnetic force within the domain is strong
in one direction. Ferromagnetic materials are said to be characterized
by "spontaneous magnetization" since they obtain saturation
magnetization in each of the domains without an external magnetic
field being applied. Even though the domains are magnetically
saturated, the bulk material may not show any signs of magnetism
because the domains develop themselves and are randomly oriented
relative to each other.
Ferromagnetic materials become magnetized when the
magnetic domains within the material are aligned. This can be
done by placing the material in a strong external magnetic field
or by passing electrical current through the material. Some or
all of the domains can become aligned. The more domains that are
aligned, the stronger the magnetic field in the material. When
all of the domains are aligned, the material is said to be magnetically
saturated. When a material is magnetically saturated, no additional
amount of external magnetization force will cause an increase
in its internal level of magnetization.