Biomedical Engineering Reference
In-Depth Information
magnets as
“lodestones” (also spelled loadstone; lode means to lead or to
attract), which contain “magnetite,” a natural magnetic material Fe
3
O
4
.
Finally, Oersted showed that magnetic ef ects could be produced by mov-
ing electrical charges, whereas Faraday and Henry showed that electric
currents could be produced by moving magnets.
13.5.2 Classii cation of Magnetic Materials
In materials, a central positive nucleus is surrounded by electrons in orbits.
An electron placed in an orbit is quite analogous to a small current loop.
h is electron experiences a torque in an external magnetic i eld so as to
orient its dipole in the i eld's direction. h is orbital magnetic dipole's direc-
tion is opposite to the direction of the applied magnetic i eld. It results in
“negative susceptibility.” Such materials are called “diamagnetic materials.”
A second “moment” is caused by “electron spin.” h is phenomenon is quite
dii cult to comprehend and cannot be explained easily by a simple model.
In an atom, spins of electrons in shells (which are not completely i lled)
contribute towards the “magnetic moment” of an atom. A third “moment”
in atom is caused by “nuclear spin.” However, it has a negligible ef ect
on the overall magnetic properties of material. On the basis of magnetic
behavior, materials are classii ed as shown in Figure 13.22.
•
Diamagnetic:
Diamagnetic materials have a weak, negative
susceptibility to magnetic i elds. Diamagnetic materials are
slightly repelled by a magnetic i eld and the material does
not retain the magnetic properties when the external i eld
is removed. In diamagnetic materials all the electrons are
paired so there is no permanent net magnetic moment per
atom. Diamagnetic properties arise from the realignment of
the electron paths under the inl uence of an external mag-
netic i eld. Most elements in the periodic table, including
copper, silver and gold, are diamagnetic.
•
Paramagnetic:
Paramagnetic materials have a small, posi-
tive susceptibility to magnetic i elds. h ese materials are
Magnetic materials
Diamagnetic
Paramagnetic
Ferromagnetic
Antiferromagnetic
Ferrimagnetic
Figure 13.22
Classii cation of magnetic materials.
