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.
The loop is generated by measuring the magnetic flux B of a ferromagnetic material while the magnetizing force H 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. 
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 back down 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.) 
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. 
From the hysteresis loop, a number of primary magnetic properties of a material can be determined.
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. 
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. 
3 Coercive Force - The amount of reversed 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 
Reluctance is analogous to the resistance in an electrical circuit.
A However, the level of residual magnetism may be lower than the retentivity value when the magnetizing force did not reach the saturation level.
B The value of B at point b on the hysteresis curve.
C It is the opposition that a ferromagnetic material shows to the establishment of a magnetic field.
D The curve will take a different path from point "f" back to the saturation point where it will complete the loop.
E As the magnetizing force is reversed, the curve moves to point "c", where the flux has been reduced to zero.
F The atoms become too excited by the heat to remain pointing in one direction for long.
G As the line demonstrates, the greater the amount of current applied (H+), the stronger the magnetic field in the component (B+).
