Diamagnetic Substances

Substances that are freely magnetised by magnetic fields are called diamagnetic substances. In this case, magnetization occurs in the opposite direction to the magnetic field. Diamagnetic substances are repellent to the external magnetic field. Their rotation around the nucleus determines the orbital angular momentum of electrons in an atom. In the case of a diamagnetic material, the atom’s magnetic moment is nil. According to Lenz’s law, when induced dipoles are introduced in diamagnetic materials by an external magnetic field, the induced dipoles oppose the external field.

Discovery of Diamagnetism

In 1778, Anton Brugmans realised that magnetic fields repelled bismuth, which led to the discovery of diamagnetism. Michael Faraday accepted this as a property of matter and concluded that every material could be affected by a magnetic field. Faraday first named the phenomenon diamagnetic, which later evolved into diamagnetism.

Properties of Diamagnetic Substances

• The magnetic moment of the atoms in diamagnetic substances is nil because of the paired electrons in the atoms.
• The material reacts weakly to the external magnetic field, so it tends to move from a stronger to a weaker region in a non-uniform field.
• The magnetization strength (I) is a tiny, negative number that varies with the magnetic field.
• The magnetic susceptibility of these materials is very negligible.
• Diamagnetic materials do not vary with temperature. This means that Cure’s law does not apply to these materials.
• Diamagnetic rods rest with their length perpendicular to the direction of a homogeneous magnetic field because the strongest fields are at the poles.
• Due to the repulsion property, a diamagnetic liquid in a U-tube depresses in the arm placed between two poles of a magnet.
• The magnetic dipole moment is negligible and is opposite in direction to the magnetic field.
• A diamagnetic liquid placed between two close pole pieces of glass creates depressions in the centre, where the field is greatest, and liquid collects on the sides.
• When a liquid is poured over a watch glass between pole pieces that are sufficiently spaced apart, liquid collects in the centre, where the field is weakest.
• Electrons in atoms shift in their orbital motion by applying the applied field, resulting in a dipole moment.

Applications

Diamagnetism is the repulsion of all the magnetic fields inside a substance. Strong magnetic fields levitate diamagnetic substances and magnets if the diamagnetic field is strong and large enough. Superconductors exhibit no internal magnetic fields as a result of their diamagnetic properties. A powerful permanent magnet can easily levitate certain materials, creating the Meissner effect. The interior of a superconductor is free of magnetic fields due to its expulsion from a magnetic field applied to it. This is the Meissner effect, due to which superconductors behave as a perfect diamagnet. Superconductors that operate at high temperatures (-100 K) often require special processing and cryogenic fluids.

Diamagnetic Levitation

Diamagnetic levitation takes place when a diamagnetic material is brought close to a material that produces a magnetic field. Diamagnetic material will repel magnetic field-producing material. However, this repelling force is generally not strong enough to overcome the forces of gravity on the surface of the Earth. To achieve diamagnetic levitation, diamagnetic and magnetic materials must combine in a way that overcomes the Earth’s gravity.

Diamagnetic levitation can be achieved in the ways given below:

• Substances like pyrolytic carbon help to achieve diamagnetic levitation. A simple method of diamagnetic levitation can be developed by placing pyrolytic graphite over a strong rare-earth magnet. In the levitation process, pyrolytic graphite is lifted above the magnet.
• Magnetic fields cannot penetrate the interior of a superconductor. The flow of current creates a magnetic field inside the superconductor, thus balancing out the external fields that are not penetrating the material. Magnets will levitate if placed above a superconductor with a strong permanent magnet.

Example of Diamagnetic Levitation

In the presence of 15 Tesla magnetic fields, a frog is levitated. Water molecules within the frog’s body are magnetic, which produces the levitation force. This best demonstrates diamagnetism.

Superconductors

In a magnetic field, a superconductor behaves like a perfect diamagnetic material and excludes the magnetic field, keeping out all the flux lines. Diamagnetic superconductors are generally materials with a volume susceptibility equal to 1 (dimensionless). Since they tend to expel all magnetic fields, they can be considered true diamagnets.

Meissner Effect

Whenever a permanent magnet is brought near a superconductor, the magnet produces a magnetic field opposite to that of the superconductor. The interior of a superconductor is free of magnetic fields due to its expulsion from a magnetic field applied to it. This is the Meissner effect, due to which superconductors behave as a perfect diamagnet.

Conclusion

Diamagnetic materials repel magnetic fields to penetrate inside them. Furthermore, they induce an opposite magnetic field, which is repulsive. The material reacts weakly to the external magnetic field, so it tends to move from a strong to a weak region in a non-uniform field. Material with paired electrons, therefore, shows diamagnetic properties. The magnetic moment of the atoms in diamagnetic substances is nil because of the paired electrons in the atoms. Magnets repel diamagnetic substances.