Types of Magnetic Materials

Science > Physics > Magnetism > Types of Magnetic Materials

In this article, we shall study types of magnetism, types of magnetic material, and Curie temperature. On the basis of magnetic behaviour magnetic materials are classified into three types: diamagnetic, paramagnetic, and ferromagnetic substances.

Origin of Magnetism:

The origin of magnetism in substances can be explained by considering the circular motion of electrons. The electrons in atoms move in circular orbits around the nucleus which is equivalent to a circular coil carrying current. The orbital motion of electrons gives rise to an orbital magnetic moment.

In addition, the electrons spin about its own axis constituting a spin magnetic moment.  The resultant magnetic moment of an atom is the vector sum of the orbital and spin magnetic moment.

On the basis of magnetic properties, substances are classified into three groups namely diamagnetic, paramagnetic and ferromagnetic.

Diamagnetic substances:

Those substances which are weekly magnetized when placed in an external magnetic field, in a direction opposite to the applied field are called diamagnetic substances. The magnetism exhibited by these substances is called diamagnetism.

Examples: Copper, gold, antimony, bismuth, silver, lead, silicon, mercury, water, air, hydrogen, nitrogen, etc.

Explanation of Diamagnetism:

The orbital motion of electrons gives rise to an orbital magnetic moment. In addition, the electrons spin about its own axis constituting a spin magnetic moment.  The resultant magnetic moment of an atom is the vector sum of the orbital and spin magnetic moment. In an atom, electrons can have clockwise or anticlockwise spin. Similarly, the electrons can revolve around the nucleus in a clockwise or anticlockwise direction.

In diamagnetic substances, the orbital magnetic moments and magnetic moments of atoms are oriented in such a way that the vector sum of the magnetic moment of an atom is zero.

When a diamagnetic substance is placed in an external magnetic field, the induced e.m.f. in each atom increases.  As a result, the speed of electrons revolving in one direction increases and those revolving in opposite direction decreases.  Thus the substance as a whole acquires a net magnetic moment in a direction opposite to the applied field.

Characteristics of Diamagnetic Substances:

• The magnetic moment of every atom is zero.
• They are weakly repelled by an external magnetic field.
• When placed in a non-uniform magnetic field, they tend to move from the stronger to the weaker part of the field.
• In an external magnetic field, they get weakly magnetized in the direction opposite to that of the field
• When a rod of diamagnetic substance is suspended in a uniform magnetic field, it comes to rest with its length perpendicular to the directions of the field.

• For diamagnetic substances magnetic susceptibility is negative.
• In the absence of an external magnetic field, the net magnetic moment of diamagnetic substance is zero.
• On removal of the external magnetic field, diamagnetic substances lose their magnetism.
• If a watch glass containing a small quantity of diamagnetic liquid is placed on two dissimilar magnetic poles, the liquid shows a depression in the middle.

If a magnetic field is applied to a diamagnetic liquid in one arm of U-tube, the liquid level in that arm is lowered.

If diamagnetic gas is introduced between pole pieces of magnet, it spreads at a right angle to the magnetic field.

Paramagnetic Substances:

Those substances which are weekly magnetized when placed in an external magnetic field in the same direction as the applied field are called Paramagnetic substances.  They tend to move from weaker to the stronger part of the field. The magnetism exhibited by these substances is called paramagnetism.

Examples: Aluminium, platinum, manganese, chromium, sodium, calcium, lithium, tungsten, niobium, copper chloride, crown glass, oxygen etc.

Explanation of Paramagnetism:

In paramagnetic substances, the orbital and spin magnetic moments of atoms are oriented in such a way that, each atom has a permanent magnetic dipole moment. However, due to thermal motion (vibration), the direction of the magnetic moments of the atoms have random orientations.  As a result of this, the net magnetic moment of a paramagnetic substance is zero.

When a paramagnetic substance is placed in an external magnetic field, each atomic magnets tend to align in the direction of the field.  Thus a paramagnetic substance acquires a net magnetic moment (magnetization).

However, the degree of alignment depends directly on the strength of the external field and inversely on the temperature of the specimen.

When the paramagnetic’ substance is removed from the magnetic field, the alignment is once again disturbed by thermal vibrations and it gets demagnetized.  For this, reason, paramagnetic substances cannot be used as permanent magnets.

Characteristics of Paramagnetic Substances:

• Every atom is a magnetic dipole having a resultant magnetic moment.
• They are weakly attracted by an external magnetic field.
• When placed in a non-uniform magnetic field, they tend to move from the weaker to the stronger part of the field.
• In an external magnetic field, they get weakly magnetized in the same direction to that of the field
• When a rod of a paramagnetic substance is suspended in a uniform magnetic field, it comes to rest with its length parallel to the directions of the field.
• In absence of an external magnetic field, the magnetic moments of atomic magnets are randomly arranged, hence the net magnetic moment of the paramagnetic substance is zero.
• On removal of the external magnetic field, paramagnetic substances lose their magnetism.
• If a watch glass containing a small quantity of paramagnetic liquid is placed on two dissimilar magnetic poles, the liquid shows an elevation in the middle.

• If a magnetic field is applied to the paramagnetic liquid in one arm of U-tube, the liquid level in that arm rises.

• If paramagnetic gas is introduced between pole pieces of magnet, it spreads in the direction of the magnetic field.
• For paramagnetic substances, magnetic susceptibility is positive and small.
• The susceptibility decreases with an increase in temperature.

Ferromagnetic substances:

Those substances which are strongly magnetized in an external magnetic field in the same direction as the external applied field and retain its magnetic moment even after the removal of the external field are Called Ferromagnetic substances.  They have a very strong tendency to move from weaker to the stronger parts of the external field. The magnetism exhibited by these substances is called ferromagnetism.

Examples: Iron, cobalt, nickel.

Explanation of Ferromagnetism on the Basis of Domain Theory:

Ferromagnetism is a special case of Paramagnetism.  In ferromagnetic substances, to the magnetic dipole moment of atoms, the contribution of the spin magnetic moment is very large.

According to the domain theory, a ferromagnetic substance consists of a large number of small units (regions) known as Domains.  A domain ‘is an extremely small region containing a large number of atomic magnets having magnetic axes aligned in the same direction due to a strong exchange coupling. When a ferromagnetic substance is kept in the magnetic field, the permanent alignment of the domain due to a strong interaction (force) takes place this force is known as exchange coupling. In one domain the magnetic dipole moments of all the atoms are aligned in the same direction. Hence each domain has a resultant magnetic dipole moment.  This permanent alignment is due to a strong interaction (force) known as exchange coupling.

However, in the absence of an external magnetic field, various domains have random orientations and hence their resultant magnetic moment is zero.

When a ferromagnetic substance is subjected to an external magnetic field, each domain experience a torque.  As a result of this, some domains rapidly rotate and remain aligned parallel to the direction of the field. This is called domain rotation or flipping.

At the same time, those domains whose magnetic axes are nearly in line with the external magnetic field grow in size at the cost of the neighbouring domains.  This is called domain growth.

As the strength of the external magnetic field is increased, more and more domains flip and align in the direction of the external magnetic field.  Finally, at a certain stage, practically all domains get aligned in the direction of the field.  This is known as magnetic saturation.  At this stage, a ferromagnetic substance behaves as a permanent magnet and retains its magnetic property (residual magnetism) even if the external magnetic field is removed.

Characteristics of Ferromagnetic Substances:

They are strongly magnetized when placed in an external magnetic field.

• The substances are made up of a large number of small domains. The atomic magnets in one domain are aligned in the same direction due to strong interaction is known as exchange coupling.
• They do not lose magnetism when the external magnetic field is removed.
• When heated above Curie temperature they become paramagnetic.
• They are strongly attracted by external magnetic field.
• When placed in a non-uniform magnetic field, they tend to move from the weaker to the stronger part of the field.
• In an external magnetic field, they get strongly magnetized in the same direction to that of the field
• When a rod of a ferromagnetic substance is suspended in a uniform magnetic field, it comes to rest with its length parallel to the directions of the field.
• In the absence of an external magnetic field, the magnetic moments of domains are randomly arranged, hence the net magnetic moment of a ferromagnetic substance is zero.
• On removal of the external magnetic field, ferromagnetic substances do not lose their magnetism. i.e. they are permanent magnets.
• For ferromagnetic substances, magnetic susceptibility is positive and large.

Distinguishing Between Diamagnetic and Paramagnetic Substances:

Diamagnetic Substance	Paramagnetic Substance
The magnetic moment of every atom of diamagnetic substance is zero.	Every atom is a magnetic dipole having a resultant magnetic moment.
They are weakly repelled by the external magnetic field.	They are weakly attracted by the external magnetic field.
When placed in a non-uniform magnetic field, they tend to move from the stronger to the weaker part of the field.	When placed in a non-uniform magnetic field, they tend to move from the weaker to the stronger part of the field.
In an external magnetic field, they get weakly magnetized in the direction opposite to that of the field	In an external magnetic field, they get weakly magnetized in the same direction to that of the field
When a rod of diamagnetic substance is suspended in a uniform magnetic field, it comes to rest with its length perpendicular to the directions Of the field.	When a rod of a paramagnetic substance is suspended in a uniform magnetic field, it comes to rest with its length parallel to the directions of the field.
For diamagnetic substances magnetic susceptibility is negative.	For paramagnetic substances, magnetic susceptibility is positive and small.
In the absence of an external magnetic field, the net magnetic moment of diamagnetic substance is zero.	In the absence of an external magnetic field, the magnetic moments of atomic magnets are randomly arranged, hence the net magnetic moment of the paramagnetic substance is zero.
If a watch glass containing a small quantity of diamagnetic liquid is placed on two dissimilar magnetic poles, the liquid shows a depression in the middle.	If a watch glass containing a small quantity of paramagnetic liquid is placed on two dissimilar magnetic poles, the liquid shows an elevation in the middle.
If a magnetic field is applied to a diamagnetic liquid in one arm of U-tube, the liquid level in that arm is lowered.	If a magnetic field is applied to the paramagnetic liquid in one arm of U-tube, the liquid level in that arm rises.
If diamagnetic gas is introduced between pole pieces of magnet, it spreads in the direction at the right angle to the magnetic field.	If paramagnetic gas is introduced between pole pieces of magnet, it spreads in the direction of the magnetic field.

Distinguishing Between Ferromagnetic Substances and Diamagnetic Substances

Ferromagnetic Substance	Diamagnetic Substance
The substances are made up of a large number of small domains. The atomic magnets in one domain are aligned in the same direction due to strong interaction is known as exchange coupling.	The magnetic moment of every atom of diamagnetic substance is zero.
They are strongly magnetized when placed in an external magnetic field.	They are weakly repelled by an external magnetic field.
When placed in a non-uniform magnetic field, they tend to move from the weaker to the stronger part of the field.	When placed in a non-uniform magnetic field, they tend to move from the stronger to the weaker part of the field.
In an external magnetic field, they get strongly magnetized in the same direction to that of the field	In an external magnetic field, they get weakly magnetized in the direction opposite to that of the field
On removal of the external magnetic field, ferromagnetic substances do not lose their magnetism. i.e. they are permanent magnets.	On removal of the external magnetic field, diamagnetic substances lose their magnetism.
When a rod of a ferromagnetic substance is suspended in a uniform magnetic field, it comes to rest with its length parallel to the directions of the field.	When a rod of diamagnetic substance is suspended in a uniform magnetic field, it comes to rest with its length perpendicular to the directions Of the field.
For ferromagnetic substances, magnetic susceptibility is positive and large.	For diamagnetic substances magnetic susceptibility is negative.
In the absence of an external magnetic field, the magnetic moments of domains are randomly arranged, hence the net magnetic moment of a ferromagnetic substance is zero.	In the absence of an external magnetic field, the net magnetic moment of diamagnetic substance is zero.

Distinguishing Between Ferromagnetic Substances and Paramagnetic Substances

Ferromagnetic Substance	Paramagnetic Substance
The substances are made up of a large number of small domains. The atomic magnets in one domain are aligned in the same direction due to strong interaction is known as exchange coupling.	Every atom is a magnetic dipole having a resultant magnetic moment.
They are strongly magnetized when placed in an external magnetic field.	They are weakly attracted by an external magnetic field.
When placed in a non-uniform magnetic field, they tend to move from the weaker to the stronger part of the field.	When placed in a non-uniform magnetic field, they tend to move from the weaker to the stronger part of the field.
In an external magnetic field, they get strongly magnetized in the same direction to that of the field	In an external magnetic field, they get weakly magnetized in the same direction to that of the field
For ferromagnetic substances, magnetic susceptibility is positive and large.	For paramagnetic substances, magnetic susceptibility is positive and small.
In the absence of an external magnetic field, the magnetic moments of domains are randomly arranged, hence the net magnetic moment of a ferromagnetic substance is zero.	In the absence of an external magnetic field, the magnetic moments of atomic magnets are randomly arranged, hence the net magnetic moment of the paramagnetic substance is zero.

Effect of Heat on Ferromagnetic Substance or the Concept of Curie Temperature:

It is the temperature required to destroy the alignment of domains and to make a ferromagnetic substance demagnetized. Above Curie temperature. a ferromagnetic substance behaves as paramagnetic.  When a ferromagnetic substance is heated, the exchange coupling between neighbouring atoms becomes loose and ultimately the domain structure gets vanished.

If the heating is continued then at Curie temperature, the exchange coupling disappears and the domain structure is destroyed and hence the substance becomes paramagnetic.

Curie temperature is the characteristic property of the substance. It is different for different materials. e.g.  Fe (1043 K), Ni (631 K), Co (1394 K), Gadolinium (317 K), Fe2O3 (893 K)

Scientific Reasons:

Diamagnetic substances are weakly repelled by a magnet

The magnetic moment of every atom of diamagnetic substance is zero. In an external magnetic field, they get weakly magnetized in the direction opposite to that of the field.

Hence when placed in a non-uniform magnetic field, they tend to move from the stronger to the weaker part of the field. Hence diamagnetic substances are weakly repelled by an external magnetic field.

If a watch glass containing a small quantity of diamagnetic liquid is placed on two dissimilar magnetic poles, the liquid shows a depression in the middle.

When the dissimilar poles are separated by a small distance then the magnetic field is stronger at midway than at the poles. In an external magnetic field, diamagnetic substances get weakly magnetized in the direction opposite to that of the field. Hence when placed in a non-uniform magnetic field, they tend to move from the stronger to the weaker part of the field. Thus the liquid at the centre moves from stronger to weaker section of the field creating a depression at the centre.

If a watch glass containing a small quantity of paramagnetic liquid is placed on two dissimilar magnetic poles, the liquid shows an elevation in the middle.

When the dissimilar poles are separated by a small distance then the magnetic field is stronger at midway than at the poles. In an external magnetic field, paramagnetic substances get weakly magnetized in the same direction to that of the field. Hence when placed in a non-uniform magnetic field, they tend to move from the weaker to the stronger part of the field. Thus the liquid at the edge moves from weaker to a stronger section of the field creating an elevation at the centre.

Paramagnetic substances cannot be used for making permanent magnets.

In the absence of an external magnetic field, the magnetic moments of atomic magnets are randomly arranged, hence the net magnetic moment of the paramagnetic substance is zero. In an external magnetic field, they get weakly magnetized in the same direction to that of the field.

On removal of the external magnetic field, the magnetic moments of atomic magnets again become randomly arranged.  Hence the paramagnetic substances lose their magnetism. Thus paramagnetic substances are temporary magnets. Hence Paramagnetic substances cannot be used for making permanent magnets.

Ferromagnetic substances are used for making permanent magnets.

In the absence of an external magnetic field, the magnetic moments of domains of ferromagnetic substance are randomly arranged, hence the net magnetic moment of a ferromagnetic substance is zero.  In an external magnetic field, they get strongly magnetized in the same direction as that of the field. The domain size increases.

On removal of the external magnetic field, ferromagnetic substances the increased domain structure is maintained and thus ferromagnetic substances do not lose their magnetism. Hence they are used in making permanent magnets.

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