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Hysteresis Curve

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 magnetizing field which results into a residual magnetism.
The lack of retracibility as shown in Fig. 24 is called hysteresis and the curve is known as hysteresis loop.
Fig. 24


When H is reduced, I reduces, but is not zero when H = 0. The remainder value OC of magnetization 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 magnetizing field is called retentivity or residual magnetism.

Coercivity or coercive force

When magnetic field H is reversed, the magnetization 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 coercivity.
Magnetic hard substance (steel)  High coercivity
Magnetic soft substance (soft iron)  Low coercivity
  • When field H is further increased in reverse direction, the intensity of magnetisation attains saturation value in reverse direction (i.e. point E)
  • 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 by BCDEFGB. This curve is known as hysteresis curve.

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