Hysteresis Loop

Hysteresis loop
Definition:
The magnetization of ferromagnetic substances due to a varying magnetic field
lags behind the field. This effect is called hysteresis.
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.
Introduction:
Hysteresis loops are generated from the observation of ferromagnetic
materials. Ferromagnetic materials are the most common of the five classes of
magnetic materials: diamagnetic, paramagnetic, ferrimagnetic,ferromagnetic, and antiferrom
agnetic. Without a magnetic field, ferromagnetic materials exhibit paramagnetic behavior
wherein their magnetic dipole moments are random and disordered as seen in Figure
1a. Once a ferromagnetic material is introduced to a magnetic field, however, their dipole
moments align parallel and in the same direction resulting in a much stronger magnetic
field. These dipole moments are so highly ordered that when removed from the magnetic
field, there is still some remnant magnetization. In order to reduce the magnetic flux back to
zero, a coercive force must be applied wherein the dipole moments cancel each other
out. This hysteresis loop therefore summarizes the pathway that a ferromagnetic material
takes from the addition and removal of a magnetizing force.
Magnetization Curve:
If an alternating magnetic field is applied to a soft magnetic
material, the magnetic induction (B) changes with the magnetic field (H). The hysteresis
loop, describing the relation between H and B, is called the magnetization curve.
Magnetic Hysteresis Loop:
Then the B-H curve follows the path of a-b-c-d-e-f-a as the
magnetizing current flowing through the coil alternates between a positive and negative
value such as the cycle of an AC voltage. This path is called a Magnetic Hysteresis Loop.
Magnetic flux density (B) and the magnetizing force (H):
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.
Soft and hard Magnetic Martials:
Magnetic Hysteresis results in the dissipation of
wasted energy in the form of heat with the energy wasted being in proportion to the area of
the magnetic hysteresis loop. Hysteresis losses will always be a problem in AC transformers

where the current is constantly changing direction and thus the magnetic poles in the core
will cause losses because they constantly reverse direction.
Rotating coils in DC machines will also incur hysteresis losses as they are alternately
passing north the south magnetic poles. As said previously, the shape of the hysteresis loop
depends upon the nature of the iron or steel used and in the case of iron which is subjected
to massive reversals of magnetism, for example transformer cores, it is important that the B-
H hysteresis loop is as small as possible.
In the next tutorial about Electromagnetism, we will look at Faraday’s Law
of Electromagnetic Induction and see that by moving a wire conductor within a stationary
magnetic field it is possible to induce an electric current in the conductor producing a simple
generator.
Properties:
1. 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.)
2. 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 level.
3. 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.)
4. Permeability, A property of a material that describes the ease with which a
magnetic flux is established in the component.

5. 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.
Applications:
There are a great variety of applications of the hysteresis in ferro magnets.
Many of these make use of their ability to retain a memory, for example magnetic tape, hard
disks, and credit cards. In these applications, hard magnets (high coercivity) like iron are
desirable so the memory is not easily erased.
Soft magnets (low coercivity) are used as cores in electromagnets. The nonlinear response
of the magnetic moment to a magnetic field boosts the response of the coil wrapped around
it. The low coercivity reduces that energy loss associated with hysteresis.
Importance of Hysteresis loops:
Hysteresis loops are important in the construction
of several electrical devices that are subject to rapid magnetism reversals or require memory
storage. Soft magnetic materials (i.e. those with smaller and narrower hysteresis areas) and
their rapid magnetism reversals are useful in electrical machinery that require minimal
energy dissipation. Transformers and cores found in electric motors benefit from these
types of materials as there is less energy wasted in the form of heat. Hard magnetic
materials (i.e. loops with larger areas) have much higher retentivity and coercivity. This
results in higher remnant magnetization useful in permanent magnets where
demagnetization is difficult to achieve. Hard magnetic materials are also useful in
memory devices such as audio recording, computer disk drives, and credit cards. The high
coercivity found in these materials ensure that memory is not easily erased.
Adeel Rasheed

Hysteresis Loop

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