Diamagnetic substance: Diamagnetic substances are those substances which are repelled by a magnet. From microscopic point of view, these are the substances whose atomic orbitals are completely filled. The cause of magnetization for these substances is the orbital motion of electron in which velocity of the electron is affected by the external magnetic field. Some examples of diamagnetic substances are antimony, bismuth, copper, lead, gold, silver, zinc, quartz, mercury, alcohol, sodium chloride, water, hydrogen, air, argon etc.
Properties of diamagnetic substances:
(1) When placed in a non-uniform magnetic field, it tends to move from stronger to weaker regions of the magnetic field. For example, when a diamagnetic liquid in a watch glass is placed on two pole pieces lying close to each other, we observe a depression in the middle and when the pole pieces are placed sufficiently apart, then we observe depressions at the sides .
(2) A diamagnetic rod when placed in a uniform magnetic field, the rod aligns itself in a direction perpendicular to the direction of the magnetic field.
(3) The permeability of a diamagnetic substance is less than one.
(4) When it is placed in a magnetic field, it develops weak magnetization in a direction opposite to the direction of the magnetizing field.
(5) As soon as the magnetizing field is removed, it loses its magnetization.
(6) The magnetic susceptibility does not depend upon temperature. It has a small negative value.
Paramagnetic substances are the substances which are feebly attracted by a magnet. The atomic orbitals of these substances are partially filled and hence there exists unpaired electrons the cause of magnetisation for these materials is spin and orbital motion of electrons. Some examples include aluminium, chromium, manganese, oxygen, platinum, alkali and alkaline earth metals.
Properties of paramagnetic substances:
(1) Removing the magnetizing field, the paramagnetics lose their magnetisation.
(2) It develops weak magnetisation along the direction of magnetic field.
(3) The magnetic lines of force prefer to pass through the material.
(4) When suspended freely in a uniform magnetic field, it aligns itself in the direction of magnetic field.
(5) It moves from weaker to stronger parts of the magnetic field.
(6) The magnetic permeability is slightly greater than one.
These are the substances which are strongly attracted by a magnet and can be magnetised. Ferromagnetic substances are paramagnetic in nature lent their behaviour is much more intense. Ferromagnetism can be explained on the basis of domain theory. The cause of magnetisation for these substances is the formation of domains.
(i) Domain is a region in which magnetic moment of all atoms is in the same direction.
(ii) At normal temperatures these domains are distributed randomly throughout the entire material. Hence net magnetic moment of the substance is zero.
Some examples of ferromagnetic substances are iron, cobalt, nickel, gadolinium, dysprosium etc.
Properties of ferromagnetic substances
(1) When placed in a non-uniform magnetic field, it moves from weaker to stronger parts of the magnetic field. For example, when a ferromagnetic liquid in a watch glass is placed on two closely placed pole pieces, it is observed that the liquid moves the side to the middle and when the pole pieces are moved apart, the liquid gets depressed in the middle.
(2) The magnetic lines of force tend to pass through the material.
(3) When suspended freely in a uniform magnetic field, it aligns itself parallel to the direction of the magnetic field.
(4) When it is placed in a magnetic field, it develops strong induced magnetism.
(5) With the removal of the magnetizing field, it does not lose its magnetisation.
(6) Its permeability is extremely large compared to that of free space. Hence B >> H.
(7) Magnetic susceptibility has a very large positive value.
(8) When placed in a magnetic field, it is strongly magnetized in the direction of the magnetic field.
(9) The magnetic susceptibility decreases with increase of temperature. That’s why the ferromagnetism decreases with rise of temperature. Maximum at absolute zero of temperature and drops to zero at Curie temperature.
Graphical representation of magnetic susceptibilities of dia-, para- and ferromagnetic materials