Diamagnetism is a fundamental property of materials that causes them to be repelled by magnetic fields. Unlike ferromagnetic or paramagnetic materials, diamagnetic materials exhibit weak and negative magnetic susceptibility, meaning they develop an induced magnetic field in the opposite direction of the applied field.
In this topic, we will explore the susceptibility of diamagnetic materials, their properties, behavior, and applications in various industries.
Understanding Magnetic Susceptibility
What Is Magnetic Susceptibility?
Magnetic susceptibility (χ) is a measure of how much a material becomes magnetized when exposed to an external magnetic field. It determines whether a material is:
- Diamagnetic (χ < 0) – Weakly repelled by a magnetic field.
- Paramagnetic (χ > 0, but small) – Weakly attracted to a magnetic field.
- Ferromagnetic (χ >> 0) – Strongly attracted and can retain magnetization.
The Susceptibility of a Diamagnetic Material
For diamagnetic materials, the susceptibility (χ) is always negative and small, typically in the range of -10⁻⁶ to -10⁻⁵. This means that when an external magnetic field is applied, the material generates a weak internal magnetic field in the opposite direction, leading to repulsion.
Formula for Magnetic Susceptibility
The relationship between magnetic induction (B) and magnetizing field (H) is given by:
where:
- B = Magnetic induction
- H = Magnetizing field
- μ₀ = Permeability of free space
- χ = Magnetic susceptibility
For diamagnetic materials, since χ is negative, the magnetic induction B is slightly reduced compared to the applied field.
Properties of Diamagnetic Materials
Diamagnetic materials exhibit several key properties:
1. Negative Susceptibility
Diamagnetic materials always have a negative magnetic susceptibility (χ < 0), meaning they oppose the applied field.
2. No Permanent Magnetism
Unlike ferromagnetic materials, diamagnets do not retain magnetization once the external field is removed.
3. Weak Magnetic Response
The diamagnetic effect is very weak and is often overshadowed by paramagnetism or ferromagnetism in mixed materials.
4. Temperature Independence
Unlike paramagnetic and ferromagnetic materials, the diamagnetic effect is nearly independent of temperature. This is because it arises from fundamental quantum mechanical principles rather than thermal energy effects.
5. Opposes External Magnetic Fields
When placed in a magnetic field, diamagnetic materials experience a weak repulsion, as their induced field is in the opposite direction.
6. Applies to All Materials
All substances exhibit some degree of diamagnetism, but it is only noticeable in materials without strong paramagnetic or ferromagnetic effects.
Examples of Diamagnetic Materials
Diamagnetic behavior can be found in various elements, compounds, and everyday materials. Some of the most well-known diamagnetic substances include:
1. Elements
- Bismuth (one of the strongest diamagnetic materials)
- Copper
- Silver
- Gold
- Carbon (Graphite, Diamond, and Graphene)
2. Organic Compounds
- Water (H₂O)
- Benzene (C₆H₆)
- Proteins and Biological Molecules
3. Superconductors
Superconductors are perfect diamagnets with χ = -1, meaning they completely expel magnetic fields due to the Meissner effect.
Applications of Diamagnetic Materials
Diamagnetic materials have a range of applications in various scientific and technological fields.
1. Magnetic Levitation (Diamagnetic Levitation)
- Bismuth and graphite can be used to levitate small magnets due to their strong diamagnetic properties.
- Superconductors can achieve complete magnetic levitation, enabling applications like maglev trains.
2. MRI (Magnetic Resonance Imaging)
Water, a diamagnetic substance, is present in biological tissues. The diamagnetic properties of water molecules influence MRI scans, helping to create detailed body images.
3. Quantum Computing
Superconductors, which exhibit perfect diamagnetism, are essential in developing quantum computers by enabling the creation of superconducting qubits.
4. Electromagnetic Shielding
Diamagnetic materials can be used for shielding sensitive instruments from weak magnetic interference.
5. Biology and Chemistry
The diamagnetic properties of biomolecules help scientists study molecular structures using techniques like nuclear magnetic resonance (NMR) spectroscopy.
How Diamagnetic Susceptibility Works
Lenz’s Law and Induced Currents
The behavior of diamagnetic materials can be explained using Lenz’s Law. When an external magnetic field is applied, the electrons in the material adjust their motion to oppose the change in the magnetic field. This creates a small induced magnetic moment that repels the applied field.
Quantum Mechanical Explanation
In quantum mechanics, the motion of electrons in atoms contributes to diamagnetic effects. Electrons in atoms create tiny current loops, which generate opposing magnetic fields when exposed to an external field.
Comparison of Diamagnetism with Other Magnetic Properties
| Property | Diamagnetism | Paramagnetism | Ferromagnetism |
|---|---|---|---|
| Susceptibility (χ) | Negative (χ < 0) | Positive (χ > 0) | Very large positive (χ >> 0) |
| Effect on External Magnetic Field | Weak repulsion | Weak attraction | Strong attraction |
| Retains Magnetization? | No | No | Yes |
| Temperature Dependence? | No | Yes | Yes |
| Examples | Bismuth, Copper, Water | Aluminum, Platinum, Oxygen | Iron, Nickel, Cobalt |
Factors Affecting Diamagnetic Susceptibility
While diamagnetism is a universal property, certain factors influence the strength of diamagnetic susceptibility in a material:
1. Electron Configuration
Materials with fully filled electron shells exhibit stronger diamagnetism because their electrons form closed-loop currents that oppose magnetic fields.
2. Bonding and Molecular Structure
The arrangement of atoms in a material affects how electrons respond to an external field. Delocalized electrons (as in graphene) can enhance diamagnetic effects.
3. Presence of Other Magnetic Effects
In mixed materials, diamagnetic properties can be overshadowed by stronger paramagnetic or ferromagnetic effects.
4. External Magnetic Field Strength
The strength of the applied magnetic field affects the induced diamagnetic response, though the effect remains weak compared to other magnetic phenomena.
The susceptibility of a diamagnetic material is always negative, meaning it generates a weak magnetic field in the opposite direction of an applied field. Though diamagnetism is weak, it plays an essential role in various applications, from magnetic levitation to medical imaging.
By understanding diamagnetic susceptibility, we can appreciate its significance in physics, chemistry, and engineering, and continue to develop innovative technologies based on magnetic principles.