Biomedical Engineering Reference
In-Depth Information
slightly attracted by a magnetic i eld and the material does
not retain the magnetic properties when the external i eld
is removed. Paramagnetic properties are due to the pres-
ence of some unpaired electrons and from the realignment
of the electron paths caused by the external magnetic i eld.
Paramagnetic materials include magnesium, molybdenum,
lithium and tantalum. In the presence of a i eld, there is now
a partial alignment of the atomic magnetic moments in the
direction of the i eld, resulting in a net positive magnetiza-
tion and positive susceptibility.
•
Ferromagnetic:
Ferromagnetic materials have a large posi-
tive susceptibility to an external magnetic i eld. h ey exhibit a
strong attraction to magnetic i elds and are able to retain their
magnetic properties at er the external i eld has been removed.
Ferromagnetic materials have some unpaired electrons so
their atoms have a net magnetic moment. h ey get their strong
magnetic properties due to the presence of magnetic domains.
In these domains, large numbers of atomic moments (10
12
to
10
15
) 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 orga-
nized and the net magnetic i eld for the part as a whole is zero.
When a magnetizing force is applied, the domains become
aligned to produce a strong magnetic i eld within the part.
Iron, nickel and cobalt are examples of ferromagnetic materials.
Components with these materials are commonly inspected using the mag-
netic particle method.
•
Ferrimagnetic:
Like ferromagnetic, ferrimagnets retain
their magnetization in the absence of a i eld. However, like
antiferromagnets, neighboring pairs of electron spins like to
point in opposite directions. h ese two properties are not
contradictory because in the optimal geometrical arrange-
ment, there is more magnetic moment from the sublattice of
electrons that point in one direction, than from the sublat-
tice that points in the opposite direction.
•
Antiferromagnetic:
In an antiferromagnet, unlike a fer-
romagnet, there is a tendency for the intrinsic magnetic
moments of neighboring valence electrons to point in oppo-
site directions. When all atoms are arranged in a substance
