Hysteresis loop

Suppose a ferromagnetic material which is in a complete demagnetized state, i.e. one in which B = H = 0 be subjected to increasing magnetic field strength H and the corresponding flux density B measured. The diagram below shows resulting relationship between B and H on the the curve Oab. At a particular value of H, shown as Oy, it becomes very inconvineint to increase the flux density any more. The material is said to be saturated. Thus b_y is the point of saturation flux density.

If the value of H is now reduced it is found that the flux density follows curve bc. When H is reduced to 0, flux remains in the iron. This remanent flux density or remanence is shown as Oc in the diagram above.WhenH is increased in the opposite direction, the flux density decreases until, at a value shown as Od, the flux density has been reduced to 0. The magnetic field strength Od needed to eliminate the residual magnetism ,i.e. reduce B to 0, is called the coercive force.Further increase of H in the reverse direction causes the flux density to increase in the reverse direction until saturation is reached, as shown by curve de. If H is varied backwards from Ox to Oy, the flux density follows the curve efgb, similar to curve bcde.It is seen from Figure  that the flux density changes lag behind the changes in the magnetic field strength. This effect is called hysteresis. The closed figure bcdefgb is referred to as the hysteresis loop (i.e. the B/H loop).