Weiss Molecular Theory of Ferromagnetism Curie Temperature

WEISS MOLECULAR THEORY OF FERROMAGNETISM CURIE TEMPERATURE

The metals like Fe, Co, Ni etc exhibit magnetisation even in the absence of external field. Therefore Weiss gave a molecular field theory (i.e.,) he postulated the existence of an internal molecular field (H_i).

This internal field is responsible for spontaneous magnetisation of a ferromagnetic material, so that only the material possess magnetisation even in the absence of an external field.

The net (or) effective magnetic field is represented by

H_e = H+H_i ……(1)

where H is external field

H_i is the internal molecular field and it is proportional to the intensity of magnetisation.

i.e., H_i ∞ I

H_i =λI …….(2)

Where λ is weiss constant.

Substituting equation (2) in (1), we have

H_e = H+ λI

From Langevin theory, the intensity of magnetisation of the ferromagnetic material is given by

where N is the number of atoms

 (or) 

 …….(3)

Where I/H=X_m and

Curie constant 

Equation (3) can be rewritten as

 (or) 

(or) 

This is known as Curie-Weiss law and is known as curie temperature. From the above relation, we have three cases

Case (i) When T=Ɵ, X_m→ ∞

i.e., the material is ready to attain external magnetisation.

Case (ii) When T> Ɵ, X_m= 1/+ ve i.e., X_m is positive.

In this case, the thermal agitation opposes the tendency of weiss molecular field to align the molecular magnets. So it becomes paramagnetic above curie temperature.

Case (iii) When T<Ɵ, i.e., the temperature is below the curie temperature, the material behaves as ferromagnetic material.

It is due to the reason that, at low temperature (T<Ɵ) weiss field energy is great and sufficient to overcome the thermal agitation. Hence, the alignment of molecular magnets results in the magnetisation of the material even in the absence of the applied field.

Curie Temperature

Ferromagnetic material have a critical temperature below which they behave as ferromagnetic and above which they behave as paramagnetic. This critical temperature is known as ferromagnetic Curie temperature.