Hysteresis - M Versus H Behaviour

HYSTERESIS - M VERSUS H BEHAVIOUR

When a ferromagnetic material is made to undergo through a cycle of magnetisation, the variation of M (Magnetisation) with respect to H (Applied field) can be represented by a closed hysteresis loop (or) curve. i.e., it refers to the lagging of magnetisation behind the magnetising field.

If a magnetising field H is applied to a ferromagnetic material and if H is increased to 'H_max’ the material acquires the magnetism. So the magnetisation (M) also increases, represented by 'oa' in the Fig. 3.23.

Now if the magnetic field is decreased from Hmax to zero, the magnetisation will not fall rapidly to zero, but falls to 'b' rather than zero. This shows that even when the applied field is zero (or) removed, the material still acquires some magnetisation (ob) which is so socalled residual magnetism (or) Retentivity.

Now, to remove this residual magnetism, the magnetic field strength is reversed and increased to - 'H_max' represented as 'oc' so called coercivity and hence we get the curve 'bcd'. Then the reverse field (-H) is reduced to zero and the corresponding curve 'de' is obtained and by further increasing H to Hmax the curve 'efa' is obtained.