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Questions  >>  CBSE XII  >>  Physics  >>  Magnetism and matter

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Describe ferromagnetism on the basis of domain theory.How does Curie’s law get modified for ferromagnetic substances?

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In materials like Fe,Ni,Co,the individual atoms are associated with large magnetic moments of neighbouring atoms interact with each other and align themselves spontaneously in a common direction over macroscopic regions called domains.Each domain has a typical size of about 1mm and contains about $10^{11}$ atoms.So each domain possesses a strong magnetic moment .In the absence of any external magnetic field,these domains are randomly distributed so that the net magnetic moment is zero  When a ferromagnetic material is placed in a magnetic field,all the domains align themselves along the direction of the field leading to the strong magnetisation of the material along .That is why the ferromagnetic substances are strongly attracted by magnets.The alignment of domains may occur in either of the following two ways: 1.By displacement of the boundaries of domains.When the external field B₀ is weak the domain aligned in the direction of B₀ grow in size while those oppositely directed decrease in size. 2.By rotation of domains:When the external field is B₀ strong ,the domains rotate till their magnetic moments get aligned in the direction of B₀ Modified curie law for ferromagnetic substances.When a ferromagnetic sample is heated,its magnetisation decreases due to increase in the randomisation of its domains.At a sufficiently high temperature,the domain structure disintegrates and the ferromagnetic substance becomes Paramagnetic The temperature at which a ferromagnetic substance becomes paramagnetic is called Curie temperature or Curie Point Tc.It varies with temperature asxm=c’/T-Tc Where C’ is constant .This is modified Curie law for a ferromagnetic material above the curie temperature.It is also known as Curie-Weiss law.This law states that the susceptibility of a ferromagnetic substance above the curie temperature is inversely proportional to the excess of temperature above curie temperature.
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