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For ferromagnetic materials, by removing external magnetic field i.e. H = 0. The magnetic moment of some domains remain aligned in the applied direction of previous magnetising field which results into a residual magnetism.
The lack of retracibility as shown in figure is called hysteresis and the curve is known as hysteresis loop.
(i) When magnetising field (H) is increased from O, the intensity of magnetisation I increases and becomes maximum. This maximum value is called the saturation value.
(ii) When H is reduced, I reduces but is not zero when H = 0. The remainder value OC of magnetisation when H = 0 is called the residual magnetism or retentivity.
The property by virtue of which the magnetism (I) remains in a material even on the removal of magnetising field is called Retentivity or Residual magnetism.
(iii) When magnetic field H is reversed, the magnetisation decreases and for a particular value of H, denoted by Hc, it becomes zero i.e., Hc = OD when I = 0. This value of H is called the corecivity.
(iv) So, the process of demagnetising a material completely by applying magnetising field in a negative direction is defined Corecivity. Corecivity assesses the softness or hardness of a magnetic material. Corecivity signifies magnetic hardness or softness of substance :
Magnetic hard substance (steel) ——> High corecvity
Magnetic soft substance (soft iron) ——> Low corecivity
(v) When field H is further increased in reverse direction, the intensity of magnetisation attains saturation value in reverse direction (i.e. point E)
(vi) When H is decreased to zero and changed direction in steps, we get the part EFGB.
Thus complete cycle of magnetisation and demagnetisation is represented