Elementary magnet

The magnetic properties of matter, i.e. diamagnetism, paramagnetism and ferromagnetism, are explained by elementary magnets that can be found on every atom of a paramagnetic and ferromagnetic solid (see illustration below). These are predominantly electron spins or nuclear spins which, as atomic spins, have a magnetic effect like small elementary magnets. In physics, such elementary magnets are referred to as magnetic moments.

Alignment of elementary magnets

The magnetic forces of permanent magnets are explained by the fact that the elementary magnets on the individual atoms of the material are aligned parallel. In fact, this parallel alignment of atomic spins is found in ferromagnetic materials, although not necessarily in the entire material, but at least in the so-called Weiss domains. This alignment can even be made visible in experiments.

The aligned magnetic moments of the elementary magnets enable permanent magnetisation precisely when the forces that stabilise the parallel alignment of the elementary magnets are large enough. The most important force here is the exchange interaction of the electron spins. In the ferromagnets, it means that aligned atomic spins cannot intermix again through thermal motion and therefore a permanent magnetisation remains.

However, the alignment of the elementary magnets can be destroyed by applying thermal energy, i.e. by intense heating. Other possibilities for demagnetisation are strong blows to the magnet or the application of an external, oppositely-directed magnetic field. In those instances, the individual elementary magnets intermix again and the measurable field on the outside of the material disappears.

A good visualisation of the effect of aligned elementary magnets can be obtained by using a set of magnetic compass needles that are mounted on a board so they can freely rotate. The compass needles arrange themselves parallel in certain areas due to the mutual interaction. This is similar to Weiss domains in non-magnetised ferromagnets. When a permanent magnet is passed over the plate from the outside, various Weiss domains combine as the compass needles all arrange themselves parallel to each other. This also happens with the elementary magnets in ferromagnetic and paramagnetic materials when they are placed in an external magnetic field. The difference between a paramagnet and a ferromagnet is that the alignment of the elementary magnets in the paramagnet is not stable, but it is in the ferromagnet. Diamagnets do not have any elementary magnets that could be aligned.

Most electromagnets have a ferromagnetic iron core whose elementary magnets are aligned by the electromagnet's magnetic field and amplify it severalfold.