Demagnetisation

A ferromagnetic material in an external magnetic field will itself be magnetised. Non-ferromagnetic materials (e.g. diamagnets and paramagnets) can also be magnetised, but the effect is significantly weaker in these materials. And, with dia- and paramagnets, the magnetisation will disappear when the external magnetic field is switched off. In ferromagnetic materials, one observes a residual magnetisation known as remanence.

The magnetisation of ferromagnets does not disappear on its own. However, it can be removed by demagnetisation. Demagnetisation can be achieved by strong impacts, heat or a strong magnetic field with reversed polarisation.

To demonstrate this in an experiment, you simply need to expose a ferrous material to a strong external magnetic field. After the external magnet has been removed, the material, such as a pair of scissors, for example, will remain slightly magnetic and attract lightweight ferrous objects, such as pins. However, if you hit these scissors hard against a hard edge, the magnetisation disappears. The scissors become demagnetised again.

In physics terms, the electrons in ferromagnets and paramagnets have magnetic moments of the electron spins on each atom, which behave like tiny elementary magnets in matter. In addition, the electron spins in ferromagnets are stabilised among themselves by a strong interaction, the so-called exchange interaction. When a ferromagnetic material is magnetised, the electron spins align in parallel and stabilise each other through the exchange interaction. This interaction prevents the aligned electron spins from intermixing and the magnetisation from being lost.

How can you demagnetise a magnet?

Demagnetisation can be achieved by various means.

• Strong vibration of the material: This will destroy the stabilisation of the aligned electron spins.
• Intense heating: Heat increases the kinetic energy of the electrons, and the motion causes the aligned spins to intermix when the temperature rises above the so-called Curie temperature.
• Application of a strong opposing field, which is polarised opposite to the aligned elementary magnets (electron spins): This is achieved when a certain field strength typical for the ferromagnetic material, the so-called coercive field strength, is exceeded.