The Hysteresis Loop
A great deal of information can be learned about the magnetic
properties of a material by studying its hysteresis loop. A
hysteresis loop shows the relationship between the induced magnetic
flux density (B) and the magnetizing force (H).
It is often referred to as the B-H loop. An example hysteresis
loop is shown below.
The loop is generated by measuring the magnetic flux of a
ferromagnetic material while the magnetizing force is changed.
A ferromagnetic material that has never been previously magnetized
or has been thoroughly demagnetized will follow the dashed line
as H is increased. As the line demonstrates, the greater
the amount of current applied (H+), the stronger the
magnetic field in the component (B+). At point "a"
almost all of the magnetic domains are aligned and an additional
increase in the magnetizing force will produce very little increase
in magnetic flux. The material has reached the point of magnetic
saturation. When H is reduced to zero, the curve will
move from point "a" to point "b." At this
point, it can be seen that some magnetic flux remains in the
material even though the magnetizing force is zero. This is
referred to as the point of retentivity on the graph and indicates
the remanence or level of residual magnetism in the material.
(Some of the magnetic domains remain aligned but some have lost
their alignment.) As the magnetizing force is reversed, the
curve moves to point "c", where the flux has been
reduced to zero. This is called the point of coercivity on the
curve. (The reversed magnetizing force has flipped enough of
the domains so that the net flux within the material is zero.)
The force required to remove the residual magnetism from the
material is called the coercive force or coercivity of the material.
As the magnetizing force is increased in the negative direction,
the material will again become magnetically saturated but in
the opposite direction (point "d"). Reducing H
to zero brings the curve to point "e." It will have
a level of residual magnetism equal to that achieved in the
other direction. Increasing H back in the positive direction
will return B to zero. Notice that the curve did not
return to the origin of the graph because some force is required
to remove the residual magnetism. The curve will take a different
path from point "f" back to the saturation point where
it with complete the loop.
From the hysteresis loop, a number of primary magnetic properties
of a material can be determined.
Retentivity - A measure of the residual flux density
corresponding to the saturation induction of a magnetic
material. In other words, it is a material's ability to
retain a certain amount of residual magnetic field when
the magnetizing force is removed after achieving saturation.
(The value of B at point b on the hysteresis curve.)
Residual Magnetism or Residual Flux - the
magnetic flux density that remains in a material when the
magnetizing force is zero. Note that residual magnetism
and retentivity are the same when the material has been
magnetized to the saturation point. However, the level of
residual magnetism may be lower than the retentivity value
when the magnetizing force did not reach the saturation
Coercive Force - The amount of reverse magnetic
field which must be applied to a magnetic material to make
the magnetic flux return to zero. (The value of H
at point c on the hysteresis curve.
Permeability, m -
of a material that describes the ease with which a magnetic
flux is established in the component. Reluctance
- Is the opposition that a ferromagnetic material shows
to the establishment of a magnetic field. Reluctance is
analogous to the resistance in an electrical circuit.