Diamagnetism, Paramagnetism, and Ferromagnetism: Earth has a large number of elements present on its surface. Each day scientists are working on new alloys and compounds, increasing the list each day. We must classify these materials based on their magnetic properties.

Magnetism is a fundamental phenomenon, and its origin can be understood with the help of the motion of electrons in an atom. Like a circular current-carrying coil, the electrons in an atom move in circular orbits around the nucleus. The orbiting motion of an electron around the nucleus gives rise to the orbital magnetic moment. But electrons also possess a spin magnetic moment associated with the electron’s spinning around its own axis. It is the vector sum of the spin and orbital magnetic moment of all electrons in an atom that results in the net magnetic moment of an atom. All matter is composed of atoms, and thus, all materials are affected by the presence of a magnetic field in one way or the other.

An atom’s magnetic moment helps us measure its magnetic susceptibility, which tells us how a magnetic material would behave when kept in an external magnetic field. The magnetic substances can be classified as diamagnetic, paramagnetic, and ferromagnetic based on magnetic susceptibility. Let us learn in detail about the magnetic properties of these materials.

Molecular Theory of Magnetism

The molecular theory of magnetism was developed by Weber, and Ewing later modified it. According to the molecular theory:
1. Each molecule of a magnetic material behaves like one complete magnet in itself. Thus, each molecule, irrespective of its state of magnetism (i.e., it may be magnetised or unmagnetised), behaves like an independent magnet.
2. Thus, every molecule within a magnetic substance consists of a north pole and a south pole of equal magnetic strength.
3. When the substance is unmagnetised, the molecular magnets are oriented randomly, i.e., in no particular direction, and hence, they neutralise each other’s magnetic forces.
4. In a magnetised substance, the magnetic molecules are arranged in response to the external magnetic field. The molecular magnets are realigned in a way that the north poles of all the molecules point in one direction while their south poles point in a direction opposite to the direction in which north poles are aligned.
5. When the molecules of a magnet are fully aligned along a particular direction, the substance achieves magnetic saturation.
6. When a magnetised specimen is heated, its molecules acquire kinetic energy, resulting in them vibrating faster at their mean position. Thus, by heating the specimen, its magnetism would reduce.

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Magnetic Susceptibility

It is the property of a material that gives a measure of the degree of magnetisation of a material in the presence of an external magnetic field. It is a dimensionless constant, and it is denoted by \(\chi\)- Mathematically,
\(\chi = \frac{M}{H}\)
Where,
\(M:\)Magnetisation
\(H:\)Magnetic field intensity

Magnetic Permeability

It is the property that gives a measure of the resistance offered by a substance when placed in an external magnetic field. It gives an idea about the degree of penetration of a magnetic field into a material. It is a scalar quantity, and it is denoted by \(\mu .\) Mathematically,
\(\mu = \frac{B}{H}\)
Where,
\(B:\)Magnetic Induction
\(H:\)Magnetic field intensity
The SI unit of magnetic permeability is henry per meter or newton per ampere square.

Relative Magnetic Permeability

It is the ratio of permeability of a medium to the permeability of space. It is a dimensionless constant. Mathematically,
\({\mu _r} = \frac{\mu }{{{\mu _0}}}\)
where \(\mu \) is the permeability of the medium and \({\mu _0}\) is the permeability of space.
Also, \({\mu _r} = \chi + 1\)
where \(\chi\) is the magnetic susceptibility of the given substance.

Diamagnetism

The substances that are weakly magnetised when placed in an external magnetic field are known as diamagnetic substances. These substances are magnetised in a direction that is opposite to the direction of the external magnetic field. These substances tend to move away from the stronger part towards the weaker part of the external magnetic field, i.e. magnet attracts iron, but it would repel a diamagnetic substance. The type of magnetism associated with these diamagnetic materials is known as diamagnetism. Copper, gold, silver, bismuth, silicon, lead, and mercury, etc are few diamagnetic materials. The behaviour of magnetic field lines near a diamagnetic material can be shown from the diagram given below:

Like a small current-carrying loop, the electrons orbiting around the nucleus in an atom possess orbital angular momentum. Since electrons spin about their own axis, they possess spin angular momentum too.  In diamagnetic substances, the orbital and spin angular momentum is oriented so that the resultant magnetic moment in an atom is zero.

In the presence of a magnetic field, the electrons with the orbital magnetic moment in the same direction slow down. In contrast, electrons with the orbital magnetic moment in the opposite direction speed up. Due to this, the given substance gets a net magnetic moment in the direction opposite to that of the applied field and hence is repelled by the external field. Diamagnetism is almost in all substances, but its effect can be so weak that it gets drifted towards para or ferromagnetism.

Superconductors can be defined as metals that are cooled to very low temperatures and exhibit perfect conductivity and diamagnetism. These are considered to be the most exotic diamagnetic materials. The phenomenon of perfect diamagnetism as exhibited by superconductors is known as the Meissner effect. The field lines are completely expelled from superconductors, and thus, these are repelled by a magnet.

Thus, for a superconductor: \(\chi = \, – 1\) and \({\mu _r} = 0\)

The superconductors are used for designing magnets that are used for running magnetically levitated trains.

Characteristics of Diamagnetic Materials

1. The magnetic moment of every atom in a diamagnetic material is zero.
2. An external magnetic field weakly repels the diamagnetic substances.
3. When diamagnetic material is placed in a non-uniform magnetic field, it moves from the stronger part of the external field towards the weaker side of the field.
4. These materials get weakly magnetised in a direction that is opposite to the direction of the field.
5. Magnetic susceptibility in diamagnetic substances turns out to be negative, i.e. \(\chi < 0.\)

Paramagnetism

The substances that get weakly magnetised when placed in an external magnetic field are called paramagnetic substances. These substances get magnetised in the direction of the external magnetic field. The properties of these materials differ from Ferro and diamagnetic materials. The paramagnetic materials possess the tendency to move from the weaker to the stronger part of the applied external magnetic field. The type of magnetism associated with these paramagnetic materials is known as paramagnetism: calcium, lithium, tungsten, aluminium, platinum, etc., are a few examples of such substances.

Each individual atom or ion in a paramagnetic material has a permanent magnetic dipole moment due to its spin. Still, due to the continuous random motion of the atoms, the direction of orientation of the magnetic moments is random. Thus, the net magnetic moment of such material is zero.

Although in the presence of a strong external magnetic field, at low temperatures, the atomic dipole moments of the individual atoms can be made to align along the direction of the external magnetic field. The behaviour of magnetic field lines around a paramagnetic material can be shown from the diagram below:

The field lines get concentrated within the material, enhancing the field inside it. It has been found experimentally that the magnetisation of a paramagnetic material varies inversely with the absolute temperature \(T.\) Mathematically,
\(M = C\frac{{{B_0}}}{T}\)
Also,
\(\chi = \frac{{{C_{{\mu _0}}}}}{T}\)…..(1)
Here \(C\) is Curie’s constant, and equation (1) represents Curie’s law. For a paramagnetic material, the value of both \(\chi\) and \(\mu \) not only varies with temperature and depends on the temperature of the material. The magnetisation value increases with the field strength or decreases in temperature until it reaches the saturation level \(\left( {{M_s}} \right).\)

Characteristics of Paramagnetic Material

1. Atoms or molecules in a paramagnetic material are considered magnetic dipoles because they have a resultant magnetic moment.
2. These materials are weakly attracted towards the external magnetic field.
3. The paramagnetic material tends to move from the weaker to the stronger part of the field when placed in a non-uniform field.
4. On removing external magnetic, these substances lose their magnetism.
5. The value of magnetic susceptibility for these materials is small and positive, i.e., \(\chi > 1.\)

Ferromagnetism

The substances that are strongly magnetised when placed in an external magnetic field are called ferromagnetic substances. The direction of the magnetisation produced in these substances is along the direction of the external magnetic field. Ferromagnetic materials tend to retain their magnetic moment even when the applied field is removed. These materials move from the weaker part to the stronger part of the externally applied field- iron, cobalt, and nickel are a few examples of ferromagnetic materials. The magnetism associated with ferromagnetic materials is known as ferromagnetism.

Each atom in a ferromagnetic material posses a dipole moment similar to that of a paramagnetic material. Still, they interact in a way to align themselves spontaneously along a common direction over a microscopic volume known as a domain. There is a net magnetisation associated with each domain, and the size of a domain is about \(1\,{\rm{mm}}\) and contains almost \({10^{11}}\) atoms.

Initially, there is no bulk magnetisation across these domains. But in the presence of an external magnetic field, these domains align themselves in the direction of the externally applied field, and their size grows. When placed in an external magnetic field, these domains experience torque, and it causes the domains to rotate and remain parallel to the direction of the field.

The magnetic field lines around a ferromagnetic material can be visualised from the diagram given below:

The field lines are highly concentrated with this material.

Hard ferromagnetic materials:  The ferromagnetic materials in which magnetisation still remains even after the external magnetic field is removed are called hard ferromagnetic materials. Examples of such materials are – Alnico (an alloy of aluminium, nickel, copper, and cobalt) and lodestone. These are used for making permanent magnets.

Soft ferromagnetic materials: The ferromagnetic materials in which the magnetisation disappears on removing the external magnetic field are called soft ferromagnetic materials. Examples of such materials are soft iron, cobalt, nickel, and gadolinium.

The ferromagnetic property varies with the temperature of the material, and at a sufficiently high temperature, a ferromagnet becomes a paramagnet. The temperature at which a ferromagnetic material converts into a paramagnetic material is known as Curie’s temperature and its represented by \({T_c}.\) The susceptibility of a ferromagnetic material at a temperature \(T,\) where \(T > {T_c}\) can be given as:
\(\chi = \frac{C}{{T – {T_c}}}\)

Characteristics of Ferromagnetic Materials

1. Ferromagnetic substances consist of a large number of small domains.
2. In the absence of an external magnetic field, these substances do not lose their magnetism.
3. These substances,when heated above the curie point, become paramagnetic.
4. These materials are strongly attracted to the external magnetic field.
5. The ferromagnetic materials move from the weaker to the stronger part of the field when the magnetic field is non-uniform.
6. The magnetic susceptibility value of a ferromagnetic material is a very large positive value, i.e. \(\chi \gg 1.\)

Summary

The substances that are weakly magnetised when placed in an external magnetic field are known as diamagnetic substances. These substances are magnetised in a direction that is opposite to the direction of the external magnetic field. These substances tend to move away from the stronger part towards the weaker part of the external magnetic field. The type of magnetism associated with these diamagnetic materials is known as diamagnetism.

The substances that get weakly magnetised when placed in an external magnetic field are called paramagnetic substances. These substances get magnetised in the direction of the external magnetic field. The paramagnetic materials tend to move from the weaker to the stronger part of the applied external magnetic field. The type of magnetism associated with these paramagnetic materials is known as paramagnetism.

The substances that are strongly magnetised when placed in an external magnetic field are called ferromagnetic substances. The direction of the magnetisation produced in these substances is along the direction of the external magnetic field. Ferromagnetic materials tend to retain their magnetic moment even when the applied field is removed. These materials move from the weaker part to the stronger part of the externally applied field. The magnetism associated with ferromagnetic materials is known as ferromagnetism.

Frequently Asked Questions (FAQs) on Diamagnetism, Paramagnetism, and Ferromagnetism

Q.1. What is the value of magnetic susceptibility for paramagnetic materials?
Ans: The value of magnetic susceptibility for paramagnetic materials is small and negative.

Q.2. What is the SI unit of magnetic susceptibility?
Ans: Magnetic susceptibility is dimensionless.

Q.3. What is a domain?
Ans: The individual atom in a ferromagnetic material posses a dipole moment, and they tend to align themselves along a single direction over a macroscopic volume called domain.

Q.4. How can you convert a ferromagnetic material into paramagnetic material?
Ans: A ferromagnetic material can be converted into a paramagnet by heating it above the curie temperature.

Q.5. Who gave the molecular theory of magnetism?
Ans: The molecular theory of magnetism was given by Weber.

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