WEISS THEORY OF FERRO MAGNETISM

Ferro magnetic substances have strong magnetic properties such as permeability and susceptibility.
The reason for these properties is every atom in the Ferro substance is like a small magnet. Hence every atom is a dipole.
According to Weiss theory a group of small atomic magnets form a region called domain.
Within a domain the dipole direction of group of atomic magnets is same. But the direction of dipoles for all domains is random.
In the normal condition i.e. in the absence of external magnetic field all the domains form a closed chain.
When the substance is placed in an external field and the field is increased then the substance is magnetized. The dipole arrangement of domains gradually is aligned along the direction of external field.
Then the domain chain is broken and slowly the substance gets magnetized. The Ferro substance attains crystal structure.
In this way the Ferro substance possess an internal magnetic field and hence magnetic moment.
The intensity of magnetization I = magnetic moment / volume of the substance.
‘I ‘value increases gradually and gets saturated at a certain point. At this saturated state the Ferro substance has maximum magnetic moment as all tiny dipoles are aligned along the direction of external field.

If β is smolecular strength, the internal magnetic field developed with in the ferro substance is 𝑩i= βi
The external magnetic field is denoted by B.
The resultant effective field on Ferro substance is 𝑩𝒆 = B + Βi

At any instant the ratio of intensity of magnetization I and saturated intensity of magnetization Is is constant,

𝐈 / Is = constant.
𝐈 / 𝐈s = coth(a )-(𝟏/a)
Where a = m B / KT
m =magnetic moment of one gram mole of the substance, K = Boltzmann constant ant T= absolute temperature of the substance.
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