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:

Distinguishing Between Ferromagnetic Substances and Diamagnetic Substances

Distinguishing Between Ferromagnetic Substances and Paramagnetic Substances

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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Science > Physics > Magnetism > Types of Magnetic Materials

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In this article, we shall study the concept of a magnetic field, magnetic lines of force, types of magnetic material.

When a magnetic needle is kept on a wooden table it comes to rest in the north-south direction. If another magnet is kept on a table the needle comes to rest in some other direction such that one of its poles is in the direction towards the nearer pole of the magnet. Thus the property of the space in which the magnetic needle is kept is changed.

The space around a magnet in which the needle of a compass rests in a direction other than geographical north-south direction is called a magnetic field. It is a vector quantity. It has a magnitude as well as direction. The direction of the field at a point is in the direction in which the needle of the compass rests when it is kept at that point.

Magnetic Lines of Force:

The magnetic field can be represented by drawing lines called as magnetic lines of force. A magnetic line of force is defined as a curve drawn in the magnetic field in such a way that the tangent to the curve at any points gives the direction of the field.

Plotting of Magnetic Lines of Force:

Draw a line in the middle of a paper fixed on a drawing board to represent the magnetic axis of the magnet. Place the magnet on the paper such that its magnetic axis coincides with the line drawn on the paper. Mark the outline of the magnet. Keep a small plotting magnetic needle (plotting compass) and place it on paper such that its south pole is directed towards point 0 at the edge of the bar magnet, and mark the point b1 in the direction in which the north pole of the compass points. Repeat the procedure by shifting the compass to a new point such that its south pole is directed towards point 1 to get point 2, 3, 4,…., Draw a smooth curve through all plotted points, which give the magnetic lines of force.

Experiment to Show Nature of Lines Formed Due to Bar Magnet:

Place a magnet below a sheet of stiff paper and spread iron filings on the top of the paper uniformly. Tap the paper gently. We observe that the iron filings arrange themselves along the curved lines as shown.

These lines are called magnetic lines of force and are formed due to induced magnetism in iron filings and aligning in the direction of the field created y the magnet. If the compass needle is placed at any point its needle rests along the magnetic line of force.

Pattern of Magnetic Lines of Force:

Bar Magnet:

Horse Shoe Magnet:

Two Bar Magnet Unlike Poles Facing Each Other:

Two Bar Magnet Like Poles Facing Each Other

Characteristics of Magnetic Lines of Force:

• Magnetic lines of force are imaginary (hypothetical).
• They are closed, continuous curves.
• Magnetic lines of force always emerge or start from the north pole and terminate on the south pole. Inside the magnet, the lines of force move from the south pole to the north pole.
• The lines of force emerged or terminate normally to the surface of the magnet.
• When two dissimilar poles of two magnets are brought near each other the lines of force assist each other hence there is an attraction between two poles.
• When two similar poles of two magnets are brought near each other the lines of force oppose each other and there is repulsion between the two poles.
• The lines of force never intersect each other if they do so it means that there are two directions for the magnetic field at the point of intersection, which is not possible
• Lines of force have a tendency to shirk.
• Lines of force exert lateral pressure on each other.
• The strength of the magnetic field depends on the number of lines per unit area. This quantity is called the intensity of the magnetic field. If the lines of force are crowded together then the magnetic field is strong. We can observe that the lines of force are crowded near the poles, where the magnetic strength of the field is more.
• If the lines of force are equally spaced then the field is uniform.

Types of Magnetic Fields:

• If a magnetic induction has the same magnitude and direction at all the points in the region, then the magnetic field is said to be uniform.
• If a magnetic induction varies in both the magnitude and the direction at all the points in the region, then the magnetic field is said to be nonuniform.
• Earth’s magnetic field may be regarded as a uniform over a small region.

Representation of Magnetic Field:

Uniform Magnetic Field:

Uniform Magnetic Field Perpendicular to Plane of Paper and Coming Out of It:

Uniform Magnetic Field Perpendicular to Plane of Paper and Going into It:

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.

Types of Magnetic Substances:

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. They tend to move from stronger to weaker parts of the field. Examples: Copper, Gold, Antimony, Bismuth, Water, Air, Hydrogen, etc.

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 a stronger part of the field. Examples:  Aluminium, Platinum, Manganese, Crown glass, oxygen, etc.

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. Examples: Iron, cobalt, nickel

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. e.g. Fe (770 °C), Ni (360 °C), Co (1150 °C).

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