Paramagnetic, and Ferromagnetic Materials
When a material is placed within a magnetic field,
the magnetic forces of the material's electrons will be affected.
This effect is known as Faraday's Law of Magnetic Induction. However,
materials can react quite differently to the presence of an external
magnetic field. This reaction is dependent on a number of factors,
such as the atomic and molecular structure of the material, and
the net magnetic field associated with the atoms. The magnetic
moments associated with atoms have three origins. These are the
electron motion, the change in motion caused by
an external magnetic field, and the spin of the electrons.
In most atoms, electrons occur in pairs. Electrons
in a pair spin in opposite directions. So, when electrons
are paired together, their opposite spins cause their magnetic
fields to cancel each other. Therefore, no net magnetic field
exists. Alternately, materials with some unpaired electrons
will have a net magnetic field and will react more to an external
field. Most materials can be classified as diamagnetic, paramagnetic or ferromagnetic.
Diamagnetic materials have a weak, negative
susceptibility to magnetic fields. Diamagnetic materials are slightly
repelled by a magnetic field and the material does not retain
the magnetic properties when the external field is removed. In diamagnetic
materials all the electron are paired so there is no permanent net magnetic moment per atom. Diamagnetic properties
arise from the realignment of the electron paths under the influence
of an external magnetic field. Most elements in the periodic table,
including copper, silver, and gold, are diamagnetic.
Paramagnetic materials have a small, positive
susceptibility to magnetic fields. These materials are slightly
attracted by a magnetic field and the material does not retain
the magnetic properties when the external field is removed. Paramagnetic
properties are due to the presence of some unpaired electrons,
and from the realignment of the electron paths caused by the
external magnetic field. Paramagnetic materials include magnesium,
molybdenum, lithium, and tantalum.
Ferromagnetic materials have a large,
positive susceptibility to an external magnetic field. They exhibit
a strong attraction to magnetic fields and are able to retain
their magnetic properties after the external field has been removed.
Ferromagnetic materials have some unpaired electrons so their
atoms have a net magnetic moment. They get their strong magnetic
properties due to the presence of magnetic domains. In these domains,
large numbers of atom's moments (1012
to 1015) are aligned parallel
so that the magnetic force within the domain is strong. When a
ferromagnetic material is in the unmagnitized state, the domains
are nearly randomly organized and the net magnetic field for the
part as a whole is zero. When a magnetizing force is applied,
the domains become aligned to produce a strong magnetic field
within the part. Iron, nickel, and cobalt are examples of ferromagnetic
materials. Components with these materials are commonly inspected
using the magnetic particle method.