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Atomic spin

What is an atomic spin?

Atomic spin is a measurable magnetic moment of atoms that behaves like a small elementary magnet. The atomic spin is attributed to the spins of the particles that make up the atoms. Those are the elementary particles. Each fundamental particle has a spin. The electron, for example, has the electron spin. The superposition of the spins of all the elementary particles in an atom causes the resulting atomic spin that determines the magnetic properties of the material.
Table of Contents
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).
Atomic spin: the measurable magnetic moment of atoms
The illustration shows an atom (in this case oxygen), which consists of an atomic nucleus and an electron shell. The states of motion of these charged building blocks create a magnetic field which consists of the sum of all the individually addable magnetic moments of the atoms (shown here very large for clarity).
The magnetic moments of the atoms are composed of the contributions of the electron spin, the magnetic orbital moment of the electrons (due to the movement around the atomic nucleus) and the nuclear spin.
However, in common ferromagnetic materials (iron, cobalt, nickel), the electron spin contribution dominates.
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.



Portrait of Dr Franz-Josef Schmitt
Author:
Dr Franz-Josef Schmitt


Dr Franz-Josef Schmitt is a physicist and academic director of the advanced practicum in physics at Martin Luther University Halle-Wittenberg. He worked at the Technical University from 2011-2019, heading various teaching projects and the chemistry project laboratory. His research focus is time-resolved fluorescence spectroscopy in biologically active macromolecules. He is also the Managing Director of Sensoik Technologies GmbH.

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