Biomedical Engineering Reference
In-Depth Information
ferromagnetic, antiferromagnetic and ferrimagnetic. Diamagnetic is due to the
non-cooperative behavior of orbiting electrons when exposed to an external mag-
netic field. Diamagnetic substances are composed of atoms which have no net
magnetic moments, while in paramagnetic materials, some of the atoms or ions in
the material, due to unpaired electrons in partially filled orbital, have a net mag-
netic moment even without external magnetic field. Besides, the magnetization is
related to the temperature and their susceptibility is inversely proportional to the
absolute temperature. Unlike paramagnetic materials, the atomic moments in fer-
romagnetic materials exhibit very strong interactions produced by electronic
exchange forces. Spontaneous magnetization is the distinct characteristics of fer-
romagnetic materials. Ferromagnetic materials have specific maximum tempera-
ture called Curie temperature (T c ). For safe and effective tumor hyperthermia
therapy, it is of great significance to maintain the temperature in the entire tumor
at the optimal value during the treating procedure. Such particles can be effec-
tively heated at low temperatures, but they stop absorbing energy once their tem-
perature reaches T c and the heating would stop. As the particles temperature
decrease below T c , magnetic ordering re-establishes and the heating resumes.
Different magnetic materials have different T c , therefore, by varying synthesis
parameters of particles and post-treatment, one can utilize particles with suitable
T c in magnetic hyperthermia (Kuznetsov et al. 2007 ). The macro-features of anti-
ferromagnetic materials are similar with paramagnet. Their susceptibilities are
very small at the high temperature and increase as the temperature decreases.
When reaching certain temperature, the susceptibilities reach the maximum.
Antiferromagnets are composed of two kinds of exactly equal but opposite sub-
lattices, which results in no net magnetic moment. Compared with antiferromag-
nets, the two kinds of opposite sub-lattices are not equal, so ferrimagnets have a
net magnetic moment and their behaviors are similar to ferromagnets. The differ-
ences between several kinds of magnetic materials are shown in Table 1 .
Table 1 Comparison between several different magnetic materials
Diamagnet
Paramagnet
Ferromagnet
Antiferromagnet
Ferrimagnet
Permanent magnetic
moment
No
Ye s
Ye s
No
Ye s
Order of magnitude
Weak
Weak
Strong
Changing as the
temperature
Strong
Susceptibility
Negative
Positive
Positive
Positive
Positive
Magnetic properties
after removing
external field
Disappear
Disappear
Retain
Disappear
Retain
Typical materials
Bi, Cu,
Ag, Au
Transition
elements,
lanthano,
aluminum,
platinum
Fe, Co,
Ni and
many
of their
alloys
Non-metallic
compounds
such as
MnO
Magnetite
 
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