PGTRB: Physics :The Electron Theory of Magnetism.

 PGTRB: Physics :The Electron Theory of Magnetism.

1. The paramagnetic diamagnetic and ferromagnetic behavior of substances can be explained in an elementary way in terms of the electron theory of matter.
2. Each electron is supposed to be revolving in an orbit around the nucleus.
3. Each moving electron behave like a tiny current loop and possesses orbital magnetic dipole moment. 
4. Each electron is spinning about an axis through itself.
5. This spin also gives rise to a magnetic dipole moment.
6. Magnetic dipole moment of an atom is the vector sum of the orbital and spin magnetic dipole moments of its electrons.

Explanation of Diamagnetism:

• Diamagnetism occurs in those substances whose atoms consists of an even number of electron.
• The electron of such atoms are paired.
• The electron in each pair have orbital motions as well as spin motions in opposite sense.
• The resultant magnetic dipole moment of the atom is thus zero.
• When such a substance is placed in a magnetic field, the field does not tend to align the atoms of the substance.
• The field, modifies the motion of the electron in orbits which are equivalent to tiny current-loops.
• The electron moving in a direction so as to produce a magnetic field in the same direction as the external field is slowed down, while the other is accelerated(Lenz’s law).
• The electron pair, and hence the atom, thus acquire an effective magnetic dipole moment which is opposite to the applied field.
• Diamagnetic material M is opposite to H.
• The Susceptibility of a diamagnetic substance is negative and is very small.

Explanation of Paramagnetism:

• In paramagnetic materials, the magnetic fields associated with the orbiting and spinning electrons do not cancel out.
• There is a net intrinsic moment in it.
• The molecules in it behave like little magnets.
• When such a a substances is placed in an external magnetic field, it will turn and line up with its axis parallel to the external field.
• Thus it tends to move further into the field, there is force of attraction.
• The diamagnetic force of repulsion is also present, but it is not so strong as the attracting force arising from the magnetic properties of the material.
• Since, M and B are in the same direction in paramagnetics, the susceptibility is positive.
• When a paramagnetic substance is heated, the thermal agitation of its atoms increase. 
• The alignment of the dipoles becomes more difficult. 
• The magnetisation of paramagnetic substances decreases as the temperature of the substances increases.

Explanation of Ferromagnetism:

• Ferromagnetic substance are very strongly magnetic.
• Example: ferromagnets are the transition metals,
• Fe, Co, and Ni.
• A ferromagnet has a spontaneous magnetic moment- a magnetic moment even is zero applied field.
• The atoms or molecules of ferromagnetic materials have a net intrinsic magnetic dipole moment which is primarily due to the spin of the electrons.
• The interaction between the neighbouring atomic magnetic dipoles is very strong.
• It is called spin exchange interaction and is present even in the absence of an external magnetic field.
• It turns out that the energy of two neighbouring atomic magnets due to this interaction is the least when their magnetic moments are parallel.
• The neighbouring magnetic moments are, therefore, strongly constrained to take parallel orientation.
• This effect of the exchange interaction to align the neighbouring magnetic dipole moments parallel to one another spreads over a small finite volume of the bulk.
• This small(1-0.1 mm across) volume of the bulk is called a domain.
• All magnetic moments within a domain will point in the same direction, resulting in a large magnetic moment. 
• Thus the bulk material consists of many domains.
• The domains are oriented in different directions.
• The total magnetic moment of a sample of the substance is the vector sum of the magnetic moments of the component domains.
• In an unmagnetised piece of ferromagnetic material, the magnetic moments of the domains themselves are not aligned.
• When an external field is applied, those domains that are aligned with the field increase in size at the expense of the others.
• In a very strong field, all the domains are lined up in the direction of the field and provide the high observed magnetisation.
• If a ferromagnetic material is heated to a very high temperature, the thermal vibrations may become strong enough to offset the alignment within a domain.
• At such temperature, the material loses its ferromagnetic property and behaves like a paramagnetic material.
• The critical temperature above which a ferromagnetic material becomes a paramagnetic is called the Curie temperature.