Demagnetising: How a magnet loses its power

The term demagnetisation describes the process in which a magnet loses its magnetic properties. This can be caused by various external influences such as heat, mechanical shocks or the effect of an opposing magnetic field. In technology, demagnetisation is a targeted process that is used, for example, to remove unwanted magnetic fields.

How does demagnetisation work?

Magnets consist of materials whose magnetic domains - small areas with uniform magnetisation - are aligned. This alignment provides the magnetic force. During demagnetisation, these domains are distributed in random directions so that their magnetic effects cancel each other out.

Demagnetisation can take place in various ways:

• Heat: If a magnet is heated above its Curie temperature, the magnetic domains lose their alignment. The magnet can be irreversibly demagnetised.
• Counter-magnetic fields: A strong, oppositely directed magnetic field can disturb the alignment of the domains and demagnetise the magnet.
• Mechanical shock: Shocks or vibrations can disrupt the magnetic order in some materials, especially brittle magnets such as ferrites.
• Alternating magnetic fields: Applying an alternating current with decreasing amplitude slowly reduces the magnetisation to zero (alternating current demagnetisation).

When is demagnetisation necessary?

In many technical and industrial applications, demagnetisation is a targeted and useful process:

• Removal of residual magnetism: After processing metal parts, unwanted magnetic fields can arise that interfere with the function of machines or sensors, for example.
• Quality control: Demagnetisation is used to ensure that materials no longer have any magnetic properties.
• Tool care: Tools such as screwdrivers are demagnetised to prevent them from attracting metal shavings or small parts.

Methods of demagnetisation

• Demagnetising coils: A coil through which alternating current flows generates an oscillating magnetic field that gradually decreases. This ensures that the material is demagnetised.
• Inductive demagnetisation: Metal parts are passed through a demagnetising device that generates an oscillating magnetic field.
• Thermal demagnetisation: The magnet is heated above its Curie temperature to destroy the magnetic domains.

Did you know?

Did you know that even the earth generates a weak magnetic field that can lead to the demagnetisation of magnets in the long term? This happens particularly with magnets that are stored in unfavourable positions, for example parallel to the earth's magnetic field.