Magnetic susceptibility is a fundamental property that determines how a material responds to an applied magnetic field. Diamagnetic materials exhibit weak magnetism in the opposite direction of the applied field, making them unique among different types of magnetic materials. One important aspect of diamagnetic behavior is how magnetic susceptibility varies with temperature.
This topic explores the variation of susceptibility with temperature for diamagnetic materials, the underlying principles, and its real-world applications.
Understanding Magnetic Susceptibility
What is Magnetic Susceptibility?
Magnetic susceptibility (χ) is a dimensionless quantity that measures how much a material becomes magnetized in response to an applied magnetic field (H). It is given by:
where:
- M is the magnetization (magnetic moment per unit volume)
- chi is the magnetic susceptibility
- H is the applied magnetic field
A positive susceptibility means the material is attracted to the magnetic field (paramagnetic or ferromagnetic), while a negative susceptibility means the material is repelled (diamagnetic).
What are Diamagnetic Materials?
Diamagnetic materials are substances in which all electrons are paired, and there are no permanent magnetic moments. When exposed to a magnetic field, these materials develop a weak, negative magnetization in the opposite direction of the field.
Examples of Diamagnetic Materials
- Copper (Cu)
- Gold (Au)
- Bismuth (Bi)
- Silicon (Si)
- Water (H₂O)
- Graphite
These materials show a small but negative susceptibility and are repelled by strong magnetic fields.
How Susceptibility Varies with Temperature in Diamagnetic Materials
1. Temperature Dependence of Diamagnetism
Unlike paramagnetic and ferromagnetic materials, which show a strong temperature dependence, diamagnetic susceptibility is almost independent of temperature.
This behavior is due to the fact that diamagnetism arises from the orbital motion of electrons, which does not rely on thermal energy. Since the electron pairing remains unchanged with temperature, the magnetic response stays nearly constant.
2. Mathematical Representation
For diamagnetic materials, the magnetic susceptibility is negative and nearly constant with temperature:
This is different from paramagnetic materials, which follow Curie’s Law:
where ** C ** is the Curie constant, and ** T ** is the absolute temperature.
Since diamagnetism does not arise from unpaired electrons, its susceptibility does not follow Curie’s Law and remains largely unaffected by temperature.
Graph of Susceptibility vs. Temperature for Diamagnetic Materials
If we plot magnetic susceptibility ( chi ) against temperature ( T ), we observe:
- A nearly flat line, indicating constant negative susceptibility.
- No significant change with increasing or decreasing temperature.
This contrasts with paramagnetic and ferromagnetic materials, where susceptibility varies significantly with temperature.
Why Diamagnetic Susceptibility is Constant with Temperature
1. Lack of Permanent Magnetic Moments
Diamagnetic materials do not have unpaired electrons or intrinsic magnetic moments. Instead, their magnetism results from the induced currents within electron orbitals, which remain largely unchanged with temperature.
2. Weak Interaction with Thermal Energy
Since diamagnetism is caused by Lenz’s Law, where an external field induces opposing currents, it is not dependent on thermal agitation. This is why increasing or decreasing the temperature does not significantly affect the material’s response to a magnetic field.
3. Independence from Electron Spin Alignment
Unlike paramagnetic and ferromagnetic materials, where electron spin alignment changes with temperature, diamagnetic materials do not rely on spin interactions. This makes their susceptibility nearly temperature-independent.
Factors That Slightly Affect Diamagnetic Susceptibility
Although diamagnetic susceptibility remains mostly constant, some factors can cause minor variations:
1. Extreme Temperature Changes
At very high temperatures, the atomic structure of some diamagnetic materials may change slightly, affecting their electron cloud distribution. This could lead to minute variations in susceptibility.
2. Structural Changes in Certain Materials
Some complex diamagnetic materials (such as certain organic compounds) may show slight susceptibility variations due to phase transitions or structural rearrangements at extreme temperatures.
3. External Pressure or Strain
Applying high pressure to some diamagnetic materials can modify their electronic structure, slightly altering their magnetic response.
However, in most practical conditions, these effects are negligible, and diamagnetic susceptibility remains nearly constant.
Comparison with Other Types of Magnetic Materials
| Property | Diamagnetic | Paramagnetic | Ferromagnetic |
|---|---|---|---|
| Susceptibility ( chi ) | Small & Negative | Small & Positive | Large & Positive |
| Temperature Dependence | Almost Constant | chi propto frac{1}{T} (Curie’s Law) | Strongly Varies (Curie Temperature) |
| Effect in Magnetic Field | Repelled | Weakly Attracted | Strongly Attracted |
| Electron Configuration | All Electrons Paired | Some Unpaired Electrons | Aligned Magnetic Domains |
This table highlights how diamagnetic materials differ significantly from paramagnetic and ferromagnetic materials, especially in terms of their temperature dependence.
Applications of Diamagnetic Materials
1. Magnetic Levitation (MagLev Trains)
Diamagnetic materials like graphite and bismuth can be used to levitate objects using strong magnetic fields. This principle is used in MagLev trains, which reduce friction by floating above tracks.
2. Superconductors
Superconducting materials exhibit perfect diamagnetism (Meissner Effect), which is crucial in MRI machines, maglev technology, and ptopic accelerators.
3. Biomedical Imaging
Diamagnetic substances are used in MRI scans, where their weak magnetic properties help in high-resolution imaging of soft tissues.
4. Quantum Computing
Certain diamagnetic materials are being explored for quantum computing applications due to their stable magnetic properties.
5. Shielding from Magnetic Fields
Diamagnetic materials are used in electromagnetic shielding to protect sensitive instruments from unwanted magnetic interference.
Common Misconceptions About Diamagnetic Susceptibility
1. ‘Diamagnetic Susceptibility Increases with Temperature’
Incorrect. Unlike paramagnetic materials, diamagnetic susceptibility remains almost constant with temperature.
2. ‘Diamagnetic Materials Are Strongly Repelled by Magnets’
False. The repulsion is extremely weak compared to ferromagnetic materials. It can only be observed under strong magnetic fields.
3. ‘All Metals Are Ferromagnetic’
Incorrect. Many metals, such as copper, silver, and gold, are actually diamagnetic.
The variation of susceptibility with temperature for diamagnetic materials shows that:
- Diamagnetic susceptibility remains nearly constant with temperature because it does not depend on electron spin alignment.
- Unlike paramagnetic and ferromagnetic materials, diamagnetism is not significantly influenced by thermal energy.
- Diamagnetic materials are weakly repelled by magnetic fields and have various applications in technology, medicine, and engineering.
Understanding this concept is essential for applications in quantum physics, superconductivity, and electromagnetic shielding technologies.
