Atomic spin

Magnetic fields are always created by the movement of charges. The magnetic properties of matter, namely ferro-, para- and diamagnetism, are also determined by the states of motion of the charged elementary particles in the material’s atoms. This results in magnetic effects that behave like small elementary magnets, or in physics jargon, like magnetic moments, at the location of the individual atoms.

Contributions to atomic spin: electron spin, orbital moment and nuclear spin

The electron spin provides the biggest contribution to the magnetic moment of atoms. In contrast to the orbital motion of the electrons around the atomic nucleus (the so-called orbital moment), the electron spin is a property of the electrons themselves, which can be imagined in a way as the rotation of a charged sphere around its own axis, even if physics can prove that this idea is not entirely correct.

But electron spin is not the only elementary magnet. The so-called orbital angular momentum, i.e. the movement of the electrons around the atomic nucleus, also contributes to the total magnetic moment of the atoms. Which contribution is greatest depends very much on the type of magnetic material. In common ferromagnetic materials (iron, cobalt, nickel), the electron spin dominates.
However, there are many compounds and alloys (e.g. samarium cobalt) that also have a magnetic orbital moment of the electrons around the nucleus, which contributes strongly to magnetism. In addition, there is also nuclear spin, which is about a factor of 1000 weaker than electron spin. The atomic nuclei can have very different spins, as their total spin is composed of the spin of all protons and neutrons in the atomic nucleus. The spin of the protons and neutrons, in turn, is formed by the spin of the quarks which are the elementary particles that make up the atomic nucleus.

Atomic spin here refers to the total magnetic moment of the atoms, which determines the magnetic properties of the material. It can be calculated by vectorial summation of the individual contributions (electron spin, nuclear spin, orbital moment) (see illustration).

In ferromagnetic materials, aligned electron spins are additionally stabilised by the so-called exchange interaction. As a result, the contribution of the electron spin to magnetisation is very large and ferromagnetic materials are very easy to magnetise. They increase the magnetic forces in magnetic fields by a factor known as 'magnetic permeability' μ, which can be up to μ=150 000.