DIA, PARA and FERRO MAGNETISM

( 1 ) Paramagnetic materials: A material is called paramagnetic, if its molecules / atoms possess permanent magnetic dipole moment. Normally, the molecules are so arranged that their magnetic dipole moments are randomly distributed with zero resultant. Symbolically,
molecular / atomic magnetic dipoles can be represented by tiny current loops as shown in the figure.
When kept in uniform magnetic field, these tiny dipoles align themselves in the direction of the magnetic field as shown in the following figure to the left side.
Due to thermal oscillations, all the dipoles do not attain 100 % parallel positions to the external magnetic field. In the figure, just one dipole is shown parallel to the external magnetic field, B . Here micro magnets arrange themselves parallel to the external magnetic field in such a way that the magnetic South Pole of one micro-magnet is adjacent to the magnetic North Pole of the immediately next micro magnet. As the magnetic dipole moments of all are pointing in the same direction, they add up vectorially to produce a resultant magnetization of the material and the resultant poles are as shown in the figure.

 The magnetic field lines produced inside the material are in the same direction as that of the
external magnetic field increasing the density of the magnetic field lines inside the material as
shown in the above figure to the right hand side.
In the adjoining figure, a paramagnetic material is
kept in a non-uniform magnetic field which
increases towards the right. The resultant north
pole of the magnetized material is in a strong
magnetic field as compared to its south pole.
Thus, there is a resultant force on it towards right ( i.e., the stronger magnetic field region ). This shows that whenever a paramagnetic material is placed in a non-uniform magnetic field, it is attracted towards the region of strong magnetic field. As this force is very weak, light weight paramagnetic materials are used in the experiments to observe this effect. Aluminium, Sodium, Calcium, Oxygen at STP and copper chloride are few examples of paramagnetic materials. The magnetic susceptibility, of these materials is positive. On increasing the temperature, the magnetic dipoles get more randomly arranged due to increased thermal oscillations. This results in the decrease in the value of magnetization, M
( 2 ) Diamagnetic materials: - The atoms / molecules of Gold, Silver, Copper, Water and Bismuth etc. do not possess permanent magnetic dipole moments. The orbital motion of the electrons and their spin is
such that their total magnetic dipole moment is zero. Such materials are called diamagnetic materials.
On keeping such materials in a magnetic field, the electron orbital motion changes in such a way that magnetic dipole moments are induced on the atoms / molecules in the direction opposite to the external magnetic field as shown in the left hand side figure below.

The external magnetic field and the magnetic field generated inside the material oppose one
another, reduced into reduced magnetic field inside the material as compared to the external
magnetic field. The density of field lines also reduces inside the material as shown in the right-hand side figure above.
If the diamagnetic substance is kept in a nonuniform magnetic field, then the magnetic south pole of the substance is in the strong magnetic field and the north pole is in the weak magnetic field as shown in the figure. As the force on Spole is more than on N-pole, it experiences a
resultant force towards the region of the weaker
magnetic field.
Magnetic susceptibility of a diamagnetic substance is negative. Certain substances, when they behave as superconductors, have susceptibility = - 1 and Km = 0. If such a substance is kept in an external magnetic field, and then brought into its superconducting condition, then all the magnetic field lines are expelled from the substance ( Meissner effect ).
( 3 ) Ferromagnetic substances: -
The atoms of iron, cobalt and nickel possess
permanent magnetic dipole moments due to
the spin of electrons in outermost orbits, but
still they do not behave as paramagnetic
materials. This is because the atoms of these
three elements have a strong bonding with
neighbouring atoms. The atoms are arranged
in such a way that the magnetic dipole
moment ( due to spin ) of one atom and that
of its neighbouring atoms are in the same
direction as shown in the figure.
Despite this, they do not behave as
permanent magnets. This is because the
strong bonding between the atoms is
restricted to a limited region called domains
which can be explained with the help of
quantum mechanics. Every domain in such
materials has a resultant magnetic dipole
moment and all such dipole moments are randomly directed resulting into zero net magnetic dipole moment. Hence the substance does not behave as a permanent magnet. Using etching techniques, coupled with powerful microscopes, one can observe such domains as shown in the figure.
Domains are approximately of the order of 1 mm in size and may consist of nearly 10exp(11)
atoms. The materials with such a constitution are called ferromagnetic materials