Properties of Magnetic Materials
It may be defined as the lagging of magnetization or Induction flux density (B) behind the magnetizing force (H). It may also be defined as a quality of a magnetic substance due to which energy is dissipated in it on the reversal of its magnetism
Let us take a un magnetized bar of iron AB and magnetize in by placing it within the magnetizing field of a solenoid (H). The Field can be increased or decreased by increasing or decreasing current through it. Let `H' be increased in step from zero up to a certain maximum value and the corresponding of induction flux density (B) is noted. If we plot the relation between H and B, a curve like OA, as shown in Figure, is obtained. The material becomes magnetically saturated at H = OM and has, at that time, a maximum flux density, established through it. If H is now decreased gradually (by decreasing solenoid current) flux density B will not decrease along AO (as might be expected) but will decrease less rapidly along AC. When it is Zero B is not zero, but has a definite value = OC. It means that on removing the magnetizing force H, the iron bar is not completely demagnetized. This value of B (=OC) is called the residual flux density.
To demagnetize the iron bar we have to apply the magnetizing force H in the reverse direction. When H is reversed by reversing current through the solenoid, then B is reduced to Zero at point D where H - OD. This value of H required to wipe off residual magnetism is known as coercive force and is a measure of the coercivity of materials i.e. its `tenacity' with which it holds on to its magnetism. After the magnetization has been reduced to zero value of H is further increased in the negative i.e. reverse direction, the iron bar again reaches a state of magnetic saturation represented by point E. By taking H back from its value corresponding to negative saturation (=OL) to its value for positive saturation (=OM), a similar curve EFGA is obtained. If we again start from G, the same curve GACDEFG is obtained once again. It is seen that B always lags behind H the two never attain zero value simultaneously. This lagging of B, behind H is given the name Hysteresis' which literally means `to lag behind.' The closed Loop ACDEFGA, which is obtained when iron bar is taken through one complete cycle of reversal of magnetization, is known as Hysteresis loop.