Biomedical Engineering Reference
In-Depth Information
13.1.2.2.2.1 Domain Structure and Hysteresis Loop
Magnetic domain theory was developed by French physicist Pierre-Ernest
Weiss who suggested the existence of magnetic domains in ferromagnetic
materials [9]. Later on, in 1966, Carey and Isaac developed some tech-
niques for observation of the magnetic domains [10]. In ferromagnetic
materials, a magnetic domain describes a region within a magnetic mate-
rial which has uniform magnetization. h is means that the individual mag-
netic moments of the atoms are aligned with one another and they point in
the same direction. When heated above a certain temperature, called the
Curie temperature, a piece of ferromagnetic material undergoes a phase
transition and the uniform magnetization within a domain spontaneously
disappears. h e regions separating magnetic domains are called domain
walls, where the magnetization rotates coherently from the direction in
one domain to that in the next domain.
A great deal of information can be obtained about the magnetic proper-
ties of ferromagnetic materials by studying their hysteresis loop. A hyster-
esis loop shown in Figure 13.3 shows the relationship between the induced
magnetic l ux density (B) and the magnetizing force (H). It is ot en referred
to as the B~H loop.
13.1.2.2.2.2 Curie Temperature and Phase Transition
h e ferromagnetic materials have a transition temperature called the
Curie point (T
c
). At a temperature T > T
c
the crystal does not exhibit
B
Flux density
Saturation
Retentivity
Coercivity
-H
H
Magnetizing force
in opposite direction
Magnetizing force
Flux density
in opposite direction
Saturation
in opposite direction
-B
Figure 13.3
B-H Hysteresis loop of ferromagnetics.
