Magnetisation

When matter is exposed to a magnetic field, magnetisation always occurs. This magnetisation creates an additional magnetic field within the matter, which overlaps with the external magnetic field. Materials are classified as havingdiamagnetic, mit paramagnetic and ferromagnetic properties.

If the magnetisation is aligned with the external magnetic field, it is called paramagnetism. In ferromagnetic objects, the magnetisation is also aligned with the external magnetic field but is much more stable than in simple paramagnets due to a special interaction known as exchange interaction.

In diamagnetic materials, the magnetisation is directed opposite to the external magnetic field. Strong magnetisation is particularly noticeable in ferromagnetic materials (e.g. iron). This can be easily verified in an experiment.

Experiment for the magnetisation of ferromagnetic materials

When a ferrous object (e.g. a pair of scissors) is exposed to the strong magnetic field of a magnet, it is observed that the scissors can attract iron-containing pins, for example, even though the magnet has already been removed from the scissors. This remaining magnetisation is called remanence.

Quantification of magnetisation by magnetic permeability

The magnetisation M, that occurs with a certain external magnetic field is quantified by the magnetic permeability μ.

It can be conceived, in simplified terms, that the permeability μ indicates how strongly the magnetic field H changes due to the influence of matter when an external magnetic field H₀ is applied. Here, H=μ•H₀. The magnetic field H, in turn, is the sum of the externally applied magnetic field H₀ and the magnetisation of the object M: H=H₀+M. The following therefore applies to magnetisation:

M=H-H₀=μ•H₀-H₀=(μ-1)•H₀
The permeability of a vacuum is μ=1, which means that a vacuum cannot be magnetised. The magnetisation M of the vacuum is M=0.

Paramagnetic materials have a permeability that is slightly greater than 1. The permeability of diamagnetic materials is slightly less than 1. It makes the magnetisation negative. This means that it is directed in the opposite direction to the incident external field H₀. The permeability of a superconductor is μ=0. The magnetisation of a superconductor is therefore directed in the opposite direction to the external field and its magnitude is the same as the external field. As a result, the inside of the superconductor is field-free and the superconductor floats in the magnetic field.

Ferromagnets can have very high permeability numbers. With iron, μ can reach values of up to 10 000; special ferromagnetic, so-called amorphous metals, reach values of μ = 150 000. With such large permeabilities, the magnetisation in an external magnetic field H₀ is approximately M ≈ μ•H₀.

The physical cause of magnetisation

To understand the physical cause of magnetisation, one can conceptualise that every substance consists of atoms with atomic nuclei and electrons. It is the electrons, that are primarily responsible for the magnetisation effects.

When an external magnetic field is applied, movements of the electrons, i.e. currents, are induced under the influence of this magnetic field. This causes diamagnetism (see illustration). According to Lenz's law, these currents are directed in such a way that they counteract their cause. The magnetisation in the material is therefore directed in the opposite direction to the external field. However, it is possible that additional paramagnetic or ferromagnetic properties may override the diamagnetism of the material. This is because the electrons have a so-called electron spin, which has magnetic properties. The electron spins form elementary magnets in the material. The spin has a fixed magnetic moment. If not all electron spins on each individual atom are compensated by an electron with opposite spin (usually in materials with an even number of electrons per atom), then the magnetic moments of these spins can align themselves in the external magnetic field. In this case, the sample itself behaves like a magnet. The magnetisation is aligned with the external field.

A ferromagnet is said to exist when the alignment of the atomic spins is stabilised. This is caused by the exchange interaction. Each of the tiny elementary magnets is stabilised in its alignment. The object then remains noticeably magnetic as a whole when the external magnetic field is switched off and remanence is observed. With paramagnets, on the other hand, the magnetisation disappears immediately when the external field is switched off.

Demagnetisation of a magnetised ferromagnetic object can be achieved if the aligned electron spins are jumbled again.

This can be done with heat (heating above the so-called Curie temperature), through heavy impacts or with a magnetic field with reversed polarisation.