Spontaneous and Saturation Magnetisation

SPONTANEOUS AND SATURATION MAGNETISATION

The molecular magnets in the ferromagnetic material is aligned in such a way that, they exhibit a magnetisation even in the absence of an external magnetic field. This is called Spontaneous magnetisation.

i.e., We know

H_e =H+H_i

Here H = 0, H_e = H_i

H_e = λI [Since H_i = λI]

where I is spontaneous magnetisation.

Temperature dependence of Spontaneous magnetisation

When the external field is zero.

H_e = λI …...(1)

From the Langevin theory, we can write intensity of the magnetisation I as

I = I_s L (α) ……(2)

where I_s is saturation magnetisation.

For ferromagnetics   

(or) …….(3)

By comparing equation (1) and (3), we get

 ………(4)

We know I_s = N_μ ……(5)

Dividing equation (4) by (5), we have

 (or) we can write the above equation as

A graph is plotted between I/I_s as a function of T/Ɵ as shown in Fig. 3.11.

From this graph we infer that,

(i) When temperature is low, Weiss field overpowers the thermal energy and it gives rise to maximum magnetisation.

i.e., I/I_s = 1

 (ii) When the temperature increased thermal energy increases which randomises more and more of the parallel spins and at curie temperature (T = Ɵ), all the parallel alignment of spins vanishes giving rise to zero value of spontaneous magnetisation and the material is to highly susceptible to get the external field. Thus, the substance become paramagnetic.