Coercivity: The resistance of a magnet to demagnetisation

The coercivity is a central concept in magnetism and describes the ability of a magnetic material to resist demagnetisation. More specifically, it is the strength of the opposing magnetic field required to reduce the magnetisation of a material to zero.

Coercivity is an important parameter for assessing the stability and durability of a magnet in various applications.

What does coercivity mean?

In a magnetised material, the magnetic moments of the atoms are aligned in a preferred direction. If an external opposing magnetic field is applied, this attempts to reduce the magnetisation of the material. The coercivity is the measure of the strength of this field that is required to completely cancel the magnetisation.

• High coercivity: Materials with high coercivity withstand strong demagnetising fields and are referred to as hard magnetic (e.g. neodymium or ferrite).
• Low coercivity: Materials with low coercivity can be easily demagnetised and are considered soft magnetic (e.g. iron or silicon steel).

How is coercivity measured?

The coercivity of a material is usually specified in kilo units per metre (kA/m) or Oersted (Oe) and shown in a hysteresis loop diagram.

• Hysteresis loop: The hysteresis loop shows the relationship between the magnetisation of a material (M) and the applied magnetic field (H). The coercivity (H_c) is the point on the axis H at which the magnetisation of the material reaches zero.
• Calculation: The coercivity can be determined experimentally using a vibrating magnetometer or a hysteresis loop analyser.

Types of coercivity

1. Intrinsic coercivity (H_ci):
   • Measures the strength of the magnetic field required to completely destroy the magnetisation in the material.
   • Relevant for the evaluation of permanent magnets.
2. Technical coercivity (H_c):
   • Refers to the strength of the field required to bring the net magnetisation of a material to zero without destroying the structure.
   • This is often measured for soft magnetic materials.

Coercivity and material classes

Coercivity distinguishes materials into two main groups:

1. Hard magnetic materials:
   • High coercivity (>100 kA/m).
   • Stay magnetically stable even when exposed to strong external fields or high temperatures.
   • Examples:
     • Neodymium magnets (H_c > 800 kA/m)
     • Samarium-cobalt (H_c ≈ 500-600 kA/m)
     • Ferrite magnets (H_c ≈ 150-300 kA/m)
2. Soft magnetic materials:
   • Low coercivity (<10 kA/m).
   • Are easily magnetised and demagnetised, making them ideal for electromagnetic applications.
   • Examples:
     • Iron
     • Silicon steel
     • Permalloy

Factors influencing coercivity

1. Crystal structure: Materials with a pronounced crystalline orientation have a higher coercivity.
2. Temperature: The coercivity generally decreases with increasing temperatures, as the thermal energy disrupts the magnetic order.
3. Material purity: Impurities can reduce the coercivity by disturbing the interactions between the magnetic moments.
4. Microstructure: Defects, grain boundaries and other structural properties influence the stability of the magnetisation direction.

Technical applications of coercivity

1. Permanent magnets: Materials with high coercivity (e.g. neodymium, samarium-cobalt) are used in permanent magnets as they maintain their magnetisation over long periods of time, even under extreme conditions.
2. Transformers and electric motors: Soft magnetic materials with low coercivity are ideal as they minimise energy losses during magnetisation changes.
3. Magnetic data storage: Coercivity is used in hard drives and other storage technologies to securely store information and protect it from external magnetic influences.
4. Sensor and actuator technology: In sensors and actuators, coercivity plays a role in sensitivity and stability.

Conclusion

Coercivity is a key parameter for selecting the right magnetic material. Materials with high coercivity offer stability and longevity, while those with low coercivity are optimised for dynamic applications such as transformers and electric motors. Knowing the coercivity helps to precisely match magnets to their areas of application and optimise their performance.