Coercive field strength: Resistance to demagnetisation

The coercive field strength is a measure of the resistance of a magnetic material to demagnetisation. It indicates the strength of the opposing magnetic field required to reduce the magnetisation of the material to zero. The coercive field strength is represented by the symbol H_c and is measured in units of amperes per metre (A/m).

How is the coercive field strength defined?

Mathematically, the coercive field strength describes the point on the hysteresis curve at which the magnetic flux density ( B ) becomes zero. This point is crucial for assessing the stability of a magnet against external fields.

The higher the coercive field strength, the more difficult it is to demagnetise the magnet. Materials with a high coercive field strength are referred to as hard magnetic, while those with a low coercive field strength are soft magnetic.

Properties of the coercivity

• Material dependence: The coercivity varies greatly between materials. For example, neodymium magnets have a high coercive field strength, while iron has lower values.
• Temperature dependence: At higher temperatures, the coercive field strength often decreases, which increases the susceptibility to demagnetisation.
• Hysteresis: The coercive field strength is a key parameter of the hysteresis curve and characterises the magnetisation and demagnetisation processes of a material.

Examples of coercive field strengths

• Soft magnetic materials: Ferrites and irons typically have a coercive field strength of less than 10 A/m, which makes them ideal for electromagnetic applications (electromagnets).
• Hard magnetic materials: Neodymium magnets have values of up to 1,000 kA/m, making them used in permanent magnet applications.

Applications of coercivity

Coercivity plays an important role in the selection of materials for various technical applications:

• Permanent magnets: Magnets with high coercive field strength, such as neodymium or samarium-cobalt, are used in electric motors and generators.
• Transformers: Soft magnetic materials with low coercivity minimise energy losses.
• Magnetic data storage: Media such as hard drives use materials with specific coercivity values to reliably store data.

Interesting facts about coercivity

Did you know that the coercivity of a magnet plays a decisive role in high-temperature applications? Neodymium magnets lose coercivity at temperatures above 80 °C, while samarium-cobalt magnets remain stable up to 350 °C. This property makes them indispensable for extreme environments.