Explain paramagnetism on the basis of electron theory.

Explanation

Paramagnetism depends depends on the presence of permanent atomic ( or molecular) magnetic dipole moments. The inherent net atomic magnetic moment results from a particular combination of the spin and orbital magnetic moments of its electrons.
The spin magnetic moments of the electrons in matter are affected by the internal magnetic field created by the magnetic moments of surrounding electrons. This internal field `(~ 10^(-2) T " to " 10^(-1)T)` causes the spin magnetic moments to precess about the field direction . At normal temperature, the thermal motion of the electrons produces constant fluctuations in the internal field so that the spin magnetic moments have random directions, Fig. (a) . In the absence of an external magnetizing field, therefore, a paramagnetic material is not magnetized.

Magnetic dipole moments in a paramagnetic sample
(a) in the absence of a magnetizing field (b) in an external field
(c ) at very low temperature and in strong field.
When the applied field strength is greater than that of the internal field, the spin magnetic moments tend to align paralled or antiparallel to the external field direction. But the randomizing effect of thermal agitation prevents complete alignment , Fig . (b) . Therefore, at room temperature, when a paramagnetic material is placed in a magnetic field , it is weakly magnetized in the direction of the magnetizing field.
If the external field is very large or the temperature is very low, the magnetic dipole moments are effectively aligned parallel to the field and the magnetization reaches saturation, Fig (c ) .