Curie – Weiss theory which explains the qualitative explanation of the ferromagnetism. We also call it as molecular field theory. In this theory we treat the Ferromagnetic materials as a special case of paramagnetic material. In which there is an internal field besides in addition to applied external field.
The total field experienced by the ferromagnetic material is given as
M – magnetization
Sub equation (2) in equation (1)
As we know, magnetic susceptibility (χ) is given by
Assuming temperature dependence of magnetic susceptibility (χ) is given by curie law
Where C – is constant
T – is Temperature
This equation is known as curie law for paramagnetism.
By comparing equation (4) and (5)
θ – characteristic temperature (or) curie temperature we call this above equation (6) as curie – Weiss law.
Case I: T > θ
For Ferromagnetic material θ = is positive and this curie – Weiss law holds good for ferromagnetic material. The behaviour of ferromagnetism doesnot exist to above the certain characteristics temperature (θ) – is identified as critical temperature (or) curie temperature of the material. Material behaves as paramagnetic material.
When T > θ
Internal field of the magnetic material vanishes. Now the atomic magnetic moment will not aligned with respect to one another.
Case (II): T < θ
Temperature below the characteristic temperature (or) curie temperature the magnetic material behaves as Ferromagnetic material. Which means there is spontaneous magnetization.
There is a strong internal field which aligns the atomic magnetic moments even in the absence of external applied field.