Classification of magnetic materials:

1. Diamagnetic substances: Diamagnetic substances are those which develop feeble magnetization in the opposite direction of the magnetizing field. Such substances are feebly repelled by magnets and tend to move from stronger to weaker part of the field.

Ex: copper zinc, lead, water, sodium chloride etc.

Origin of diamagnetism: In atoms of some materials like Cu, Pb, etc the magnetic moments due to different electrons cancel out. In such atoms, electrons occur in pairs with one of them revolving clock wise and other anticlock wise around the nucleus. Net magnetic moment of an atom is zero as shown in fig.

When such an atom is placed in a magnetic field B, the speed of revolution of one electron increases while other decreases. The magnetic of one electron is m +Δm whereas of the other is m –mΔ. Then net magnetic moment is 2Δm opposite to B. this sample moves from stronger to the weaker field. The behavior of diamagnetic materials is independent of temperature.

Properties of diamagnetic substances:

1. When placed in an external field, a diamagnetic substance develops feeble magnetization in the opposite direction of the applied field.
2. When a rod of diamagnetic substance is suspended freely in a uniform magnetic field, it aligns itself perpendicular to the magnetizing field.
3. When placed in a non-uniform magnetic field, diamagnetic substance moves from stronger to the weaker parts of the field.
4. When a rod of diamagnetic material is placed in a magnetic field, place are induced on it in a direction opposite to that of inducing field. So the lines of force prefer to pass through the surrounding air than to pass through the material itself. i.e. the lines of force get expelled or repelled.
5. The relative permeability is positive but less than 1.
6. The magnetization of diamagnetic substance lasts so long as the magnetizing field is applied.

1. Paramagnetic substances: Paramagnetic substance are those which develop feeble magnetization in the direction of the magnetizing field. Such substances are feebly attracted by magnets and tend to move from weaker to stronger part of the field. Ex: manganese, aluminium, oxygen, sodium, platinum etc.

Origin of paramagnetism: According to Langevin, the atoms or molecules of a paramagnetic material possess a permanent magnetic moment either due to presence of some unpaired electron or due to the non-cancellation of the spins of two electrons. In the absence of external magnetic field, there is no net magnetization due to their ceaseless random motion as shown in fig.

When a strong enough field B₀ is applied and the temperature is law enough, the field B₀ tends to align the atomic dipole s in its own direction, producing a weak magnetic moment in the direction of B₀. The material tends move from a weak field region to a strong field region.

Currie’s law: It states that the intensity of magnetization M of a paramagnetic material is

1. Directly proportional to the magnetizing field intensity H.
2. Inversly proportional to the absolute temperature T.

Therefore,

M ∝ H/T or M = C. H/T or M/H = C/T

X_m = C/t, where C is curie constant and X_m is the susceptibility. 

Ferromagnetic Substances: Which develop strong magnetization in the direction of the magnetizing field. They are strongly attracted by magnets and tend to move from weaker to stronger parts of a magnetic field.

Ex: Iron, cobalt, nickel etc.

Origin of Ferromagnetism: Weiss explained ferromagnetism on the basis of domain theory, the individual atom of these substance have large magnetic moments and interact with neighbouring atom and align themselves spontaneously in a common direction over macroscopic region called domain. In the absence of external magnetic field, these domains are randomly distributed so that the net magnetic moment is zero.

When a ferromagnetic material is placed in a magnetic field, all the domains align themselves along the direction of the field leading to strong magnetization of the material along he direction of the field. That is why the ferromagnetic substances are strongly attracted by magnets.

Types of ferromagnetic materials:

1. Soft ferromagnetic material: In which the magnetization disappear on the removal of the external magnetizing field. Such material have narrow hysteresis loop.

They have low retentivity, low corcivity and low hysteresis loss but have high relative magnetic permeability. They are used as cores of solenoids and transformers.

Ex: soft iron, stalloy etc.

2. Hard Ferromagnetic material: Which retain magnetization even after the removal of the external magnetizing field. Such materials have wide hysteresis loop. They have high retentivity, high corcivity and large hysteresis loss. They are used for making permanent mangnet.

Ex: Steel, alnico, Iodestone etc.