These materials are feebly repelled by a magnet. When placed in a magnetizing field, they are feebly magnetized in a direction opposite to that of the field. Thus, the susceptibility of diamagnetic material is small and negative.
Further, the flux density in a diamagnetic material placed in a magnetizing field is slightly less than that in the free space. Thus, the relative permeability is slightly less than 1.
In a non-uniform magnetic field, a diamagnetic material tends to move from the stronger to the weaker part of the field.
The susceptibility of a diamagnetic material is independent of temperature.
Paramagnetic Materials
These materials are feebly attracted by a magnet. When placed in a magnetizing field, they are feebly magnetized in the direction of the field. Thus, they have a small positive susceptibility.
Further, the magnetic flux density in a paramagnetic material placed in a magnetizing field is slightly greater than that in the free space. Thus, the relative permeability for paramagnetic is slightly greater than 1.
The susceptibility of a paramagnetic material varies inversely as the kelvin temperature of the material.
Ferromagnetic Materials
These materials which are strongly attracted by a magnet, show all the properties of a paramagnetic material to a much higher degree.
For example, they are strongly magnetized in relatively weak magnetizing fields in the same direction as the field.
They have relative permeabilities of the order of hundreds and thousands. Similarly, the susceptibilities of ferromagnetics have large positive values.
Curie Temperature
Ferromagnetism decreases with the rise in temperature.
If we heat a ferromagnetic material, then at a definite temperature the ferromagnetic property of the material “suddenly” disappears and the material becomes paramagnetic.
The temperature above which a ferromagnetic material becomes paramagnetic is called the ‘Curie temperature‘ of the material. The Curie temperature of iron is 770°C and that of nickel is 358°C.
Explanation of Dia-, Para- and Ferromagnetism
The diamagnetic, paramagnetic and ferromagnetic behavior of materials can be explained on the basis of the atomic model.
We know that matter is made up of atoms. Each atom of any material has a positively-charged nucleus at its center around which electrons revolve in various discrete orbits. Each revolving electron is equivalent to a tiny current-loop (or magnetic dipole) and gives a dipole moment to the atom.
Besides this, each electron “spins” about its axis and this spin also produces a magnetic dipole moment. However, most of the magnetic moment of the atom is produced by electron spin, the contribution of the orbital spin is very small.
Explanation of Diamagnetism
The property of diamagnetism is generally found in those materials whose atoms (or ions or molecules) have an “even” number of electrons that form pairs. In the electron-pairs, the direction of the spin of one electron is opposite to that of the other.
So, the magnetic moment of one electron is neutralized by that of the other. As such, the net magnetic dipole of an atom (or ion or molecule) of a diamagnetic material is zero.
When diamagnetic material is placed in an external magnetic field B, the field modifies the motion of the electrons in the atoms (or ions or molecules).
That electron in each pair which is spinning in a direction to produce a magnetic field in the same direction as B is slowed down, while the other electron of the pair is accelerated. So, now the electrons of the pair do not neutralize the magnetic moments of each other.
Thus, a small magnetic moment is induced in each atom (or ion or molecule) which is proportional to B but pointing in the opposite direction.
Hence, the material is magnetized opposite to the external field B, and the field lines become less dense inside the diamagnetic material compared to those outsides.
If the temperature of the diamagnetic material is changed, there is no effect on its diamagnetic property. Thus diamagnetism is temperature-independent.
Explanation of Paramagnetism
The property of paramagnetism is found in those materials whose atoms (or ions or molecules) have an excess of electrons spinning in the same direction.
Hence, atoms (ions or molecules) of paramagnetic materials have a net non-zero magnetic moment of their own and behave like tiny bar magnets. Even then the paramagnetic materials do not exhibit any magnetic effect in the absence of an external magnetic field.
The reason is that the individual atomic magnets are randomly oriented and so the magnetic moment of the bulk of the material remains zero.
When a paramagnetic material is placed in an external magnetic field B then each atomic magnet experiences a torque which tends to turn the magnet in the direction of B. Hence, the atoms of the material are aligned in the direction of B.
As the material acquires a net magnetic dipole moment, that is, it is magnetized in the direction of and the field lines become denser inside the paramagnetic material compared to those outsides.
The atoms of material undergo thermal agitation. If the material is a gas, its atoms are in the state of random motion; and if it is solid, the atoms vibrate. This agitation disturbs the magnetic alignment of the atoms. Hence, normally, the magnetism in paramagnetic materials is very weak.
The magnetism increases on increasing the external magnetic field, or on reducing the temperature. Thus, the paramagnetism is temperature-dependent.
Curie’s Law of Paramagnetism
In 1895, Curie discovered experimentally that the intensity of magnetization M (magnetic moment per unit volume) of a paramagnetic material is directly proportional to the magnetic intensity H of the magnetizing field and inversely proportional to the kelvin temperature T. That is:
M = C (H/T)
This equation is known as Curie’s law of paramagnetism and C is called the Curie constant. The law, however, holds so long the ratio H/T does not become too large.
M cannot increase without limit. It approaches a maximum value corresponding to the complete alignment of all the atomic magnets contained in the material.
Explanation of Ferromagnetism
The difference in paramagnetism and ferromagnetism is only that of intensity. The ferromagnetic materials are such paramagnetic materials that acquire strong magnetism in an external magnetic field.
Like paramagnetic materials, atoms of ferromagnetic materials have a net non-zero magnetic moment of their own and behave like tiny magnets.
But in ferromagnetic materials, the atoms, due to certain mutual interaction, form innumerable small effective regions called ‘domains‘.
Each domain has 1017 to 1021 atoms whose magnetic axes are aligned in the same direction (but different from the atoms of the neighboring domain).
Thus, each domain, without any external magnetic field, is in the state of magnetic saturation, that is, it is a strong magnet. But, in the normal state of the material, the different domains are randomly distributed so that their resultant magnetic moment in any direction is zero.
When the material is placed in an external magnetic field, the magnetic moment or the magnetism of the material can increase in two different ways :
By the displacement of the boundaries of the domains: that is, the domains which are oriented favorably with respect to the external field increase in size, whereas those oriented opposites to the external field are reduced.
By the rotation of the domains: that is, the domains rotate until their magnetic moments are aligned more or less in the direction of the external magnetic field.
When the external field is weak, then the material is magnetized mostly by the displacement of the domains, but in strong fields, the magnetization takes place mostly by the rotation of the domains.
The field lines inside the ferromagnetic material are much denser than outside. When the external field is removed, the ferromagnetic material is not completely demagnetized, but some residual magnetism remains in it.
If we hammer a magnet or heat it above its Curie temperature, then the thermal agitation of the atoms becomes so vigorous that the domains are disturbed and the resultant magnetic moment of the magnet becomes almost zero, that is, the magnetism vanishes.
