Weiss Theory
Weiss introduced the idea of domains of molecules.
According to Weiss Theory a ferro-magnetic substance contain atom with permanent magnetic moment as in a peramagnetic substance.
But due to special form of an interaction between adjacent atoms their exist strong force between neighbour atomic dipole which causes groups of them such that all atomic dipole of a group point in the same direction such as groups are called Domain and their size varies from 10-6cm³ to 10-2cm³.
Thus, every domain is magnetically saturated and the direction of magnetisation in different Domain is different.
When a ferro-magnetic substance is in unmagnetised state the magnetisation vector in different Domain are oriented in different direction such that the resultant magnetisation is zero when the the temperature of ferromagnetic substance is raise above Curie point the coupling suddenly disappear and the material becomes paramagnetic.
In passing from one Domain adjacent one the atomic magnetized vector gradually rotate from its original to it new direction over thickness of about thousand atomic dimension while the domain themselves have dimension of order of 10th of a millimetre the region between two domains is called Domain Wall.
When a magnetic field is applied the substance is magnetized and hence it is is necessary that some of magnetisation vector in Domain must align parallel to the applied field.
When the magnetising field is weak the magnetic polarization of the simple changes as a result of of motion of domain walls. In this process domain are whose Axis are are parallel or at a small angle with the field grow at the expenses of those which are unfavorably oriented.
This group is reversible as long as the field is very small if we turn the field of the magnetisation will return to zero.
This part of magnetization is not continuous process but it takes place in series of infinite small discrete steps because of the irregularities in crystal structure which might be due to strain, impurities, dirt and imperfections.
This can be shown by inserting the sample in a coil connected to an amplifier and loudspeaker if the failed surrounding the speaker is slowly increased individual clicks are heard in a speaker which represent small discrete flux increment. This is know as "Bark's Hussain effect".
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