Electron Theory of Magnetism: Each electron in an atom is revolving in an orbit around the nucleus. The revolving electron is equivalent to a tiny loop of current. Therefore, it possesses some orbital magnetic dipole moment
\(|\vec M_i|\) = current x area of the loop.
In addition to the orbital motion, every electron is assumed to have a spin motion around its axis. Therefore, another dipole magnetic moment called spin magnetic moment \(\vec M_s\) is also associated with electron.
The vector sum of \(\vec M_i\) and \(\vec M_s\) provides the net dipole moment \(\vec M\) to the electron.
(a) Diamagnetism: In a diamagnetic substance \(\vec M_i\) and \(\vec M_s\) cancel each other for every atom so that atom has no net magnetic dipole moment.
Therefore, motion of all the electrons in an atom of a diamagnetic material is assumed to be reduced to motion of two electrons revolving with the same angular velocity in a circular orbit of the same radius, but in opposite sense.
The magnetic moment of the two being equal and opposite, cancel each other (in the absence of any external magnetic field) so that net magnetic moment of each atom is zero. Fig
Let a uniform external magnetic field \(\vec B\) be applied perpendicular to the plane of rotation of electron and directed away from the reader. Each electron experiences Lorentz force F = Bev. According to Fleming’s left hand rule,the Lorentz force F acts radially outwards on the electron revolving anticlockwise, tending to decrease the vel. of electron \(\vec v - \vec {\bigtriangleup v}\). As a result of magnetic moment of electron decreases to \(\vec M - \vec {\bigtriangleup M}\). Fig. On the electron revolving clockwise, the Lorentz force acts radially inwards tending to increase the velocity of electron to \(\vec v - \vec {\bigtriangleup v}\) . Therefore, the magnetic moment of electron increases to \(\vec M - \vec {\bigtriangleup M}\). Fig. The vector addition magnetic moments gives rise to net dipole moment 2∆\(\vec M\) directed towards the reader i.e opposite to external field \(\vec B\).
The net magnetic moment 2∆\(\vec M\) so developed is called induced magnetic moment. The magnetic moment induced in different atoms and vectorially to give a net magnetisation of the material in a direction opposite to \(\vec B\). This accounts for a diamagnetic behaviour of materials.
Further, as the appearance of induced magnetic moment in atoms is not affected by thermal motion of the atoms, therefore, magnetic susceptibility of such materials does not depend on temperature of the materials.
(b) Paramagnetism: In such materials, every atom has some permanent dipole moment. In the absence of an external magnetic field, the atomic dipoles are randomly oriented so that average magnetic moment per unit volume of the material is zero. Therefore a paramagnetism material does not behave as a magnet in the absence of an external magnetic field, Fig.
When external magnetic field \(\vec B\) is applied, it tries to aling the atomic dipole magnetic moments in the direction of the field. This is why the specimen gets magnetised in the direction of magnetising field. This is paramagnetism. Due to thermal motion the alignment is not complete Fig. but if the temperature of the specimen is lowered and strength of magnetic field is increased, complete alignment is also possible Fig.
When we raise the temperature of the material, the atomic dipoles acquire kinetic energy. This tends to disorient the dipoles. That is why magnetic susceptibility of the paramagnetic material decreases with rise in temperature.
(c) Ferromagnetism: This is explained by Weiss on the basis of domain theory in addition to electronic theory. Every atom of ferromagnetic substance is a tiny magnetic dipole having a permanent dipole moment. Atoms form very large number of small effective regions called domains. Each domain contains about 1010 atoms. A special interaction takes place in each domain and is called exchange coupling. This renders dipole moment of all atoms in a particular direction.
Each domain is a strong magnet without external field. Domains are randomly oriented so that their resultant magnetic moment in any direction is zero as given in figure.
When external field is applied, it gets strongly magnetised which results in (i) Displacement of boundaries of domains Fig. (ii) Rotation of domains Fig.
When a ferromagnetic material is heated above a certain temperature (called curie temperature), the exchange coupling suddenly disappears. The substance, therefore, starts behaving as paramagnetic. Curie temperature for iron is about 750°C.
