Magnetism is a fundamental property of materials that influences various applications, from electronics to medicine. One crucial aspect of magnetism is magnetic susceptibility, which describes how a material responds to an external magnetic field. But what exactly is the unit of magnetic susceptibility, and how is it measured?
In this topic, we will explore the definition, formula, unit, and applications of magnetic susceptibility in a simple and understandable way.
What Is Magnetic Susceptibility?
Definition of Magnetic Susceptibility
Magnetic susceptibility ( chi ) is a dimensionless quantity that measures how much a material becomes magnetized when exposed to an external magnetic field.
It determines whether a substance is:
- Diamagnetic (repelled by a magnetic field)
- Paramagnetic (weakly attracted to a magnetic field)
- Ferromagnetic (strongly attracted to a magnetic field)
Formula for Magnetic Susceptibility
The mathematical expression for magnetic susceptibility is:
where:
- chi = Magnetic susceptibility (dimensionless)
- M = Magnetization (A/m)
- H = Applied magnetic field strength (A/m)
Since both M and H have the same unit (amperes per meter, A/m), their ratio is unitless.
What Is the Unit of Magnetic Susceptibility?
Dimensionless Nature of Magnetic Susceptibility
The unit of magnetic susceptibility is dimensionless because it is a ratio of two quantities with the same unit.
This makes it easier to compare materials without needing to convert units. However, in some specific cases, different forms of susceptibility may have associated units.
Volume, Mass, and Molar Susceptibility
There are three types of magnetic susceptibility:
- Volume Susceptibility ( chi_v ) – Dimensionless
- Mass Susceptibility ( chi_m ) – Measured in m³/kg
- Molar Susceptibility ( chi_M ) – Measured in m³/mol
These variations allow scientists to analyze susceptibility based on mass or the number of moles of a substance.
Types of Magnetic Materials Based on Susceptibility
1. Diamagnetic Materials
- Definition: Materials that generate a weak opposing magnetic field when placed in an external field.
- Susceptibility Value: Negative ( chi < 0 )
- Examples: Copper, gold, bismuth, water
- Behavior: Slightly repelled by magnets
2. Paramagnetic Materials
- Definition: Materials that are weakly attracted to a magnetic field.
- Susceptibility Value: Small positive ( $0 < chi < 1$ )
- Examples: Aluminum, platinum, oxygen
- Behavior: Temporary magnetization, disappears when the field is removed
3. Ferromagnetic Materials
- Definition: Strongly attracted to a magnetic field and retain magnetization even after the field is removed.
- Susceptibility Value: Very high ( chi >> 1 )
- Examples: Iron, nickel, cobalt
- Behavior: Permanent magnetization possible
Measurement of Magnetic Susceptibility
1. Gouy Balance Method
- Measures the force on a sample in a magnetic field.
- Used for powdered and liquid materials.
2. Faraday’s Method
- Measures the change in weight of a sample due to a magnetic field.
- Useful for small, solid samples.
3. SQUID (Superconducting Quantum Interference Device)
- Highly sensitive method for detecting very weak magnetic responses.
- Used in physics and medical applications.
Applications of Magnetic Susceptibility
1. Material Science and Engineering
- Determines suitability of materials for magnetic applications.
- Helps design magnetic shielding and transformers.
2. Medical Imaging (MRI)
- Magnetic susceptibility is crucial in Magnetic Resonance Imaging (MRI) to enhance contrast in body tissues.
3. Environmental Science
- Used to detect metal pollution in soil and water.
- Helps in archaeological studies to find buried artifacts.
4. Geophysics and Earth Sciences
- Helps classify rock types based on magnetic properties.
- Used in oil and mineral exploration.
5. Chemistry and Physics Research
- Determines the magnetic behavior of chemical compounds.
- Essential in studying molecular structures.
Comparison of Magnetic Susceptibility in Different Materials
| Material | Type | Susceptibility ( chi ) |
|---|---|---|
| Copper | Diamagnetic | -0.00001 |
| Oxygen | Paramagnetic | 0.0002 |
| Iron | Ferromagnetic | > 10^3 |
| Platinum | Paramagnetic | 0.0003 |
| Gold | Diamagnetic | -0.00001 |
Ferromagnetic materials have the highest susceptibility, while diamagnetic materials have negative susceptibility values.
Key Takeaways
- Magnetic susceptibility measures how a material responds to an external magnetic field.
- The unit of magnetic susceptibility is dimensionless, as it is a ratio of magnetization and field strength.
- There are three types of magnetic materials: diamagnetic (weak repulsion), paramagnetic (weak attraction), and ferromagnetic (strong attraction).
- Magnetic susceptibility has applications in medicine, engineering, environmental science, and physics.
- Different methods, such as the Gouy balance and SQUID, are used to measure susceptibility.
Understanding magnetic susceptibility is essential for many scientific and industrial applications, from medical imaging to geophysics.